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Title: Through the Telescope
Author: James Baikie
Release Date: March 17, 2017 [EBook #54378]
Language: English
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*** START OF THIS PROJECT GUTENBERG EBOOK THROUGH THE TELESCOPE ***
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THROUGH THE TELESCOPE
AGENTS
AMERICA . . THE MACMILLAN COMPANY
64 & 66 Fifth Avenue, NEW YORK
CANADA . . THE MACMILLAN COMPANY OF CANADA, LTD.
27 Richmond Street, TORONTO
INDIA . . . MACMILLAN & COMPANY, LTD.
12 Bank Street, BOMBAY
7 New China Bazaar Street, CALCUTTA
PLATE I.
The 40-inch Refractor of the Yerkes Observatory.
THROUGH
THE TELESCOPE
BY
JAMES BAIKIE, F.R.A.S.
WITH 32 FULL-PAGE ILLUSTRATIONS FROM PHOTOGRAPHS
AND 26 SMALLER FIGURES IN THE TEXT
LONDON
ADAM AND CHARLES BLACK
1906
[pg vii]
PREFACE
The main object of the following chapters is to give
a brief and simple description of the most important
and interesting facts concerning the heavenly bodies,
and to suggest to the general reader how much of
the ground thus covered lies open to his personal
survey on very easy conditions. Many people who
are more or less interested in astronomy are deterred
from making practical acquaintance with the wonders
of the heavens by the idea that these are only
disclosed to the possessors of large and costly
instruments. In reality there is probably no science
which offers to those whose opportunities and means
of observation are restricted greater stores of knowledge
and pleasure than astronomy; and the possibility
of that quickening of interest which can only
be gained by practical study is, in these days, denied
to very few indeed.
Accordingly, I have endeavoured, while recounting
the great triumphs of astronomical discovery, to
give some practical help to those who are inclined
to the study of the heavens, but do not know how
to begin. My excuse for venturing on such a task
must be that, in the course of nearly twenty years
[pg viii]
of observation with telescopes of all sorts and sizes,
I have made most of the mistakes against which
others need to be warned.
The book has no pretensions to being a complete
manual; it is merely descriptive of things seen and
learned. Nor has it any claim to originality. On
the contrary, one of its chief purposes has been to
gather into short compass the results of the work of
others. I have therefore to acknowledge my indebtedness
to other writers, and notably to Miss
Agnes Clerke, Professor Young, Professor Newcomb,
the late Rev. T. W. Webb, and Mr. W. F.
Denning. I have also found much help in the
Monthly Notices and Memoirs of the Royal Astronomical
Society, and the Journal and Memoirs of the
British Astronomical Association.
The illustrations have been mainly chosen with
the view of representing to the general reader some
of the results of the best modern observers and
instruments; but I have ventured to reproduce a
few specimens of more commonplace work done
with small telescopes. I desire to offer my cordial
thanks to those who have so kindly granted me
permission to reproduce illustrations from their
published works, or have lent photographs or drawings
for reproduction—to Miss Agnes Clerke for
Plates XXV.-XXVIII. and XXX.-XXXII. inclusive;
to Mrs. Maunder for Plate VIII.; to
M. Loewy, Director of the Paris Observatory, for
Plates XI.-XIV. and Plate XVII.; to Professor
E. B. Frost, Director of the Yerkes Observatory,
[pg ix]
for Plates I., VII., XV., and XVI.; to M. Deslandres,
of the Meudon Observatory, for Plate IX., and
the gift of several of his own solar memoirs; to the
Astronomer Royal for England, Sir W. Mahony
Christie, for Plate V.; to Mr. H. MacEwen for the
drawings of Venus, Plate X.; to the Rev. T. E. R.
Phillips for those of Mars and Jupiter, Plates XX.
and XXII.; to Professor Barnard for that of Saturn,
Plate XXIV., reproduced by permission from the
Monthly Notices of the Royal Astronomical Society;
to Mr. W. E. Wilson for Plates XXIX. and XXXII.;
to Mr. John Murray for Plates XVIII. and XIX.;
to the proprietors of Knowledge for Plate VI.; to
Mr. Denning and Messrs. Taylor and Francis for
Plate III. and Figs. 6 and 20; to the British
Astronomical Association for the chart of Mars,
Plate XXI., reproduced from the Memoirs; and to
Messrs. T. Cooke and Sons for Plate II. For those
who wish to see for themselves some of the wonders
and beauties of the starry heavens the two Appendices
furnish a few specimens chosen from an
innumerable company; while readers who have no
desire to engage in practical work are invited to
skip Chapters I. and II.
[pg x]
CONTENTS
[pg xiii]
LIST OF ILLUSTRATIONS
PRINTED SEPARATELY FROM THE TEXT
PLATE |
To face page |
I. |
The 40⁃inch Refractor of the Yerkes Observatory |
Frontispiece |
II. |
Six-inch Photo-Visual Refractor, equatorially mounted |
31 |
III. |
Twenty-inch Reflector, Stanmore Observatory |
36 |
IV. |
Telescope House and 8½-inch 'With' Reflector |
38 |
V. |
The Sun, February 3, 1905. Royal Observatory, Greenwich |
49 |
VI. |
Photograph of Bridged Sunspot (Janssen). Knowledge, February, 1890 |
50 |
VII. |
Solar Surface with Faculæ. Yerkes Observatory |
60 |
VIII. |
Coronal Streamers: Eclipse of 1898. From Photographs
by Mrs. Maunder |
71 |
IX. |
The Chromosphere and Prominences, April 11, 1894.
Photographed by M. H. Deslandres |
74 |
X. |
Venus. H. MacEwen. Five-inch Refractor |
94 |
XI. |
The Moon, April 5, 1900. Paris Observatory |
102 |
XII. |
The Moon, November 13, 1902. Paris Observatory |
108 |
XIII. |
The Moon, September 12, 1903. Paris Observatory |
110 |
XIV. |
Region of Maginus: Overlapping Craters. Paris
Observatory |
112 |
XV. |
Clavius, Tycho, and Mare Nubium. Yerkes Observatory |
114 |
XVI. |
Region of Theophilus and Altai Mountains. Yerkes
Observatory |
117 |
XVII. |
Apennines, Alps, and Caucasus. Paris Observatory |
119 |
XVIII. |
Chart of the Moon. Nasmyth and Carpenter |
125 |
XIX. |
Key to Chart of Moon. Nasmyth and Carpenter |
125 |
XX. |
Mars: Drawing 1, January 30, 1899—12 hours.
Drawing 2, April 22, 1903—10 hours[pg xiv] |
135 |
XXI. |
Chart of Mars. Memoirs of the British Astronomical
Association, Vol. XI., Part III., Plate VI. |
139 |
XXII. |
Jupiter, January 6, 1906—8 hours 20 minutes. Instrument, 9¼-inch Reflector |
159 |
XXIII. |
Jupiter, February 17, 1906. J. Baikie, 18-inch Reflector |
167 |
XXIV. |
Saturn, July 2, 1894. E. E. Barnard, 36-inch Equatorial |
172 |
XXV. |
Great Comet. Photographed May 5, 1901, with the
13-inch Astrographic Refractor of the Royal
Observatory, Cape of Good Hope |
211 |
XXVI. |
Photographs of Swift's Comet. By Professor E. E. Barnard |
220 |
XXVII. |
Region of the Milky Way in Sagittarius, showing
a Double Black Aperture. Photographed by Professor E. E. Barnard |
233 |
XXVIII. |
Irregular Star Clusters. Photographed by E. E. Barnard |
256 |
XXIX. |
Cluster M. 13 Herculis. Photographed by Mr. W. E. Wilson |
259 |
XXX. |
Photograph of the Orion Nebula (W. H. Pickering) |
263 |
XXXI. |
Photographs of Spiral Nebulæ. By Dr. Max Wolf |
265 |
XXXII. |
Photograph of Whirlpool Nebula (M. 51). Taken by
Mr. W. E. Wilson, March 6, 1897 |
265 |
[pg xv]
LIST OF ILLUSTRATIONS
PRINTED IN THE TEXT
[pg 1]
THROUGH THE TELESCOPE
CHAPTER I
THE TELESCOPE—HISTORICAL
The claim of priority in the invention of this
wonderful instrument, which has so enlarged our
ideas of the scale and variety of the universe, has
been warmly asserted on behalf of a number of individuals.
Holland maintains the rights of Jansen,
Lippershey, and Metius; while our own country
produces evidence that Roger Bacon had, in the
thirteenth century, 'arrived at theoretical proof of
the possibility of constructing a telescope and a
microscope' and that Leonard Digges 'had a method
of discovering, by perspective glasses set at due
angles, all objects pretty far distant that the sun
shone on, which lay in the country round about.'
All these claims, however, whether well or ill
founded, are very little to the point. The man to
whom the human race owes a debt of gratitude in
connection with any great invention is not necessarily
he who, perhaps by mere accident, may
stumble on the principle of it, but he who takes up
[pg 2]
the raw material of the invention and shows the full
powers and possibilities which are latent in it. In
the present case there is one such man to whom,
beyond all question, we owe the telescope as a
practical astronomical instrument, and that man is
Galileo Galilei. He himself admits that it was only
after hearing, in 1609, that a Dutchman had succeeded
in making such an instrument, that he set
himself to investigate the matter, and produced
telescopes ranging from one magnifying but three
diameters up to the one with a power of thirty-three
with which he made his famous discoveries; but
this fact cannot deprive the great Italian of the
credit which is undoubtedly his due. Others may
have anticipated him in theory, or even to a small
extent in practice, but Galileo first gave to the world
the telescope as an instrument of real value in
research.
The telescope with which he made his great
discoveries was constructed on a principle which,
except in the case of binoculars, is now discarded.
It consisted of a double convex lens converging the
rays of light from a distant object, and of a double
concave lens, intercepting the convergent rays before
they reach a focus, and rendering them parallel
again (Fig. 1). His largest instrument, as already
mentioned, had a power of only thirty-three diameters,
and the field of view was very small. A
more powerful one can now be obtained for a few
shillings, or constructed, one might almost say, for
a few pence; yet, as Proctor has observed: 'If we
[pg 3]
regard the absolute importance of the discoveries
effected by different telescopes, few, perhaps, will
rank higher than the little tube now lying in the
Tribune of Galileo at Florence.'
FIG. 1.—PRINCIPLE OF GALILEAN TELESCOPE.
Galileo's first discoveries with this instrument
were made in 1610, and it was not till nearly half
a century later that any great improvement in telescopic
construction was effected. In the middle of
the seventeenth century Scheiner and Huygens
made telescopes on the principle, suggested by
Kepler, of using two double convex lenses instead
of a convex and a concave, and the modern refracting
telescope is still constructed on essentially the
same principle, though, of course, with many minor
modifications (Fig. 2).
FIG. 2.—PRINCIPLE OF COMMON REFRACTOR.
The latter part of the seventeenth century witnessed
the introduction of telescopes on this principle
of the most amazing length, the increase in length
being designed to minimize the imperfections which
a simple lens exhibits both in definition and in colour.
[pg 4]
Huygens constructed one such telescope of 123 feet
focal length, which he presented to the Royal Society
of London; Cassini, at Paris, used instruments of
100 and 136 feet; while Bradley, in 1722, measured
the diameter of Venus with a glass whose focal
length was 212¼ feet. Auzout is said to have made
glasses of lengths varying from 300 to 600 feet, but,
as might have been expected, there is no record of
any useful observations having ever been made with
these monstrosities. Of course, these instruments
differed widely from the compact and handy telescopes
with which we are now familiar. They were
entirely without tubes. The object-glass was fastened
to a tall pole or to some high building, and was
painfully manœuvred into line with the eye-piece,
which was placed on a support near the ground, by
means of an arrangement of cords. The difficulties
of observation with these unwieldy monsters must
have been of the most exasperating type, while their
magnifying power did not exceed that of an ordinary
modern achromatic of, perhaps, 36 inches focal
length. Cassini, for instance, seems never to have
gone beyond a power of 150 diameters, which
might be quite usefully employed on a good modern
3-inch refractor in good air. Yet with such tools
he was able to discover four of the satellites of
Saturn and that division in Saturn's ring which
still bears his name. Such facts speak volumes
for the quality of the observer. Those who are
the most accustomed to use the almost perfect
products of modern optical skill will have the best
[pg 5]
conception of, and the profoundest admiration for,
the limitless patience and the wonderful ability
which enabled him to achieve such results with the
very imperfect means at his disposal.
The clumsiness and unmanageableness of these
aerial telescopes quickly reached a point which made
it evident that nothing more was to be expected of
them; and attempts were made to find a method of
combining lenses, which might result in an instrument
capable of bearing equal or greater magnifying
powers on a much shorter length. The chief
hindrance to the efficiency of the refracting telescope
lies in the fact that the rays of different colours
which collectively compose white light cannot be
brought to one focus by any single lens. The red
rays, for example, have a different focal length from
the blue, and so any lens which brings the one set to
a focus leaves a fringe of the other outstanding
around any bright object.
In 1729 Mr. Chester Moor Hall discovered a
means of conquering this difficulty, but his results
were not followed up, and it was left for the optician
John Dollond to rediscover the principle some
twenty-five years later. By making the object-glass
of the telescope double, the one lens being of crown
and the other of flint glass, he succeeded in obtaining
a telescope which gave a virtually colourless
image.
This great discovery of the achromatic form of
construction at once revolutionized the art of telescope-making.
It was found that instruments of not
[pg 6]
more than 5 feet focal length could be constructed,
which infinitely surpassed in efficiency, as well as in
handiness, the cumbrous tools which Cassini had
used; and Dollond's 5-foot achromatics, generally
with object-glasses of 3¾ inches diameter, represented
for a considerable time the acme of optical excellence.
Since the time of Dollond, the record of the
achromatic refractor has been one of continual, and,
latterly, of very rapid progress. For a time much
hindrance was experienced from the fact that it
proved exceedingly difficult to obtain glass discs of
any size whose purity and uniformity were sufficient
to enable them to pass the stringent test of optical
performance. In the latter part of the eighteenth
century, a 6-inch glass was considered with feelings
of admiration, somewhat similar to those with which
we regard the Yerkes 40-inch to-day; and when, in
1823, the Dorpat refractor of 96⁄10 inches was
mounted (Fig. 3), the astronomical world seemed
to have the idea that something very like finality
had been reached. The Dorpat telescope proved,
however, to be only a milestone on the path of
progress. Before very long it was surpassed by
a glass of 12 inches diameter, which Sir James
South obtained from Cauchoix of Paris, and which
is now mounted in the Dunsink Observatory, Dublin.
This, in its turn, had to give place to the fine instruments
of 14·9 inches which were figured by Merz
of Munich for the Pulkowa and Cambridge
(U.S.A.) Observatories; and then there came a
pause of a few years, which was broken by Alvan
[pg 7-8]
Clark's completion of an 18½-inch, an instrument
which earned its diploma, before ever it left the
workshop of its constructor, by the discovery of the
companion to Sirius.
FIG. 3.—DORPAT REFRACTOR.
The next step was made on our side of the
Atlantic, and proved to be a long and notable one,
in a sense definitely marking out the boundary line
of the modern era of giant refractors. This was
the completion, by Thomas Cooke, of York, of a
25-inch instrument for the late Mr. Newall. It
did not retain for long its pride of place. The
palm was speedily taken back to America by Alvan
Clark's construction of the 26-inch of the Washington
Naval Observatory, with which Professor Asaph
Hall discovered in 1877 the two satellites of Mars.
Then came Grubb's 27-inch for Vienna; the pair of
30-inch instruments, by Clark and Henry respectively,
for Pulkowa (Fig. 4) and Nice; and at last
the instrument which has for a number of years
been regarded as the finest example of optical skill
in the world, the 36-inch Clark refractor of the Lick
Observatory, California. Placed at an elevation of
over 4,000 feet, and in a climate exceptionally well
suited for astronomical work, this fine instrument
has had the advantage of being handled by a very
remarkable succession of brilliant observers, and has,
since its completion, been looked to as a sort of
court of final appeal in disputed questions. But
America has not been satisfied even with such an
instrument, and the 40-inch Clark refractor of the
Yerkes Observatory is at present the last word of
[pg 9]
optical skill so far as achromatics are concerned
(Frontispiece). It is not improbable that it may
also be the last word so far as size goes, for the
late Professor Keeler's report upon its performance
implies that in this splendid telescope the limit of
practicable size for object-glasses is being approached.
The star images formed by the great lens show
indications of slight flexure of the glass under its
own weight as it is turned from one part of the sky
[pg 10]
to another. It would be rash, however, to say that
even this difficulty will not be overcome. So many
obstacles, seemingly insuperable, have vanished
before the astronomer's imperious demand for 'more
light,' and so many great telescopes, believed in
their day to represent the absolute culmination of
the optical art, are now mere commoners in the
ranks where once they were supreme, that it may
quite conceivably prove that the great Yerkes refractor,
like so many of its predecessors, represents
only a stage and not the end of the journey.
FIG. 4.—30-INCH REFRACTOR, PULKOWA OBSERVATORY.
Meanwhile, Sir Isaac Newton, considering,
wrongly as the sequel showed, that 'the case of
the refractor was desperate,' set about the attempt
to find out whether the reflection of light by means
of suitably-shaped mirrors might not afford a substitute
for the refractor. In this attempt he was
successful, and in 1671 presented to the Royal
Society the first specimen, constructed by his own
hands, of that form of reflecting telescope which
has since borne his name. The principle of the
Newtonian reflector will be easily grasped from
Fig. 5. The rays of light from the object under
inspection enter the open mouth of the instrument,
and passing down the tube are converged by the
concave mirror AA towards a focus, before reaching
which they are intercepted by the small flat mirror
BB, placed at an angle of 45 degrees to the axis of
the tube, and are by it reflected into the eye-piece E
which is placed at the side of the instrument. In
this construction, therefore, the observer actually
[pg 11]
looks in a direction at right angles to that of the
object which he is viewing, a condition which seems
strange to the uninitiated, but which presents no
difficulties in practice, and is found to have several
advantages, chief among them the fact that there
is no breaking of one's neck in the attempt to
observe objects near the zenith, the line of vision
being always horizontal, no matter what may be the
altitude of the object under inspection. Other forms
of reflector have been devised, and go by the names
of the Gregorian, the Cassegrain, and the Herschelian;
but the Newtonian has proved itself the
superior, and has practically driven its rivals out of
the field, though the Cassegrain form has been
revived in a few instances of late years, and is particularly
suited to certain forms of research.
FIG. 5.—PRINCIPLE OF NEWTONIAN REFLECTOR.
FIG. 6.—LORD ROSSE'S TELESCOPE.
At first the mirrors of reflecting telescopes were
made of an alloy known as speculum metal, which
consisted of practically 4 parts of copper to 1 of
tin; but during the last half-century this metal has
been entirely superseded by mirrors made of glass
ground to the proper figure, and then polished and
[pg 12]
silvered on the face by a chemical process. To the
reflecting form of construction belong some of the
largest telescopes in the world, such as the Rosse
6-foot (metal mirrors), Fig. 6, the Common 5-foot
(silver on glass), the Melbourne 4-foot (metal mirrors,
Cassegrain form), and the 5-foot constructed by
Mr. Ritchey for the Yerkes Observatory. Probably
[pg 13]
the most celebrated, as it was also the first of these
monsters, was the 4-foot telescope of Sir William
Herschel, made by himself on the principle which
goes by his name. It was used by him to some
extent in the discoveries which have made his name
famous, and nearly everyone who has ever opened
an astronomical book is familiar with the engraving
of the huge 40-foot tube, with its cumbrous staging,
which Oliver Wendell Holmes has so quaintly
celebrated in 'The Poet at the Breakfast Table'
(Fig. 7).
FIG. 7.—HERSCHEL'S 4-FOOT REFLECTOR.
[pg 14]
CHAPTER II
THE TELESCOPE—PRACTICAL
Having thus briefly sketched the history of the
telescope, we turn now to consider the optical means
which are most likely to be in the hands or within
the reach of the beginner in astronomical observation.
Let us, first of all, make the statement that
any telescope, good, bad, or indifferent, is better
than no telescope. There are some purists who
would demur to such a statement, who make the
beginner's heart heavy with the verdict that it is
better to have no telescope at all than one that is
not of the utmost perfection, and, of course, of
corresponding costliness, and who seem to believe
that the performance of an inferior glass may breed
disgust at astronomy altogether. This is surely
mere nonsense. For most amateurs at the beginning
of their astronomical work the question is not
between a good telescope and an inferior one, it is
between a telescope and no telescope. Of course,
no one would be so foolish as willingly to observe
with an inferior instrument if a better could be had;
but even a comparatively poor glass will reveal
much that is of great interest and beauty, and its
[pg 15]
defects must even be put up with sometimes for the
sake of its advantages until something more satisfactory
can be obtained. An instrument which will
show fifty stars where the naked eye sees five is not
to be despised, even though it may show wings to
Sirius that have no business there, or a brilliant
fringe of colours round Venus to which even that
beautiful planet can lay no real claim. Galileo's
telescope would be considered a shockingly bad
instrument nowadays; still, it had its own little
influence upon the history of astronomy, and the
wonders which it first revealed are easily within the
reach of anyone who has the command of a shilling
or two, and, what is perhaps still more important,
of a little patience. The writer has still in his
possession an object-glass made out of a simple
single eyeglass, such as is worn by Mr. Joseph
Chamberlain. This, mounted in a cardboard tube
with another single lens in a sliding tube as an
eye-piece, proved competent to reveal the more
prominent lunar craters, a number of sunspots, the
phases of Venus, and the existence, though not the
true form, of Saturn's ring. Its total cost, if memory
serve, was one shilling and a penny. Of course it
showed, in addition, a number of things which
should not have been seen, such as a lovely border
of colour round every bright object; but, at the
same time, it gave a great deal more than thirteen
pence worth of pleasure and instruction.
Furthermore, there is this to be said in favour of
beginning with a cheap and inferior instrument, that
[pg 16]
experience may thus be gained in the least costly
fashion. The budding astronomer is by nature
insatiably curious. He wants to know the why and
how of all the things that his telescope does or does
not do. Now this curiosity, while eminently laudable
in itself, is apt in the end to be rather hard
upon his instrument. A fine telescope, whatever its
size may be, is an instrument that requires and should
receive careful handling; it is easily damaged, and
costly to replace. And therefore it may be better
that the beginner should make his earlier experiments,
and find out the more conspicuous and immediately
fatal of the many ways of damaging a telescope,
upon an instrument whose injury, or even
whose total destruction, need not cause him many
pangs or much financial loss.
It is not suggested that a beginning should necessarily
be made on such a humble footing as that just
indicated. Telescopes of the sizes mainly referred
to in these pages—i.e., refractors of 2 or 3 inches
aperture, and reflectors of 4½ to 6 inches—may
frequently be picked up second-hand at a very
moderate figure indeed. Of course, in these circumstances
the purchaser has to take his chance of
defects in the instrument, unless he can arrange for
a trial of it, either by himself, or, preferably, by
a friend who has some experience; yet even should
the glass turn out far from perfect, the chances are
that it will at least be worth the small sum paid for
it. Nor is it in the least probable, as some writers
seem to believe, that the use of an inferior instrument
[pg 17]
will disgust the student and hinder him from
prosecuting his studies. The chances are that it
will merely create a desire for more satisfactory
optical means. Even a skilled observer like the late
Rev. T. W. Webb had to confess of one of his telescopes
that 'much of its light went the wrong way';
and yet he was able to get both use and pleasure
out of it. The words of a well-known English
amateur observer may be quoted. After detailing
his essays with glasses of various degrees of imperfection
Mr. Mee remarks: 'For the intending
amateur I could wish no other experience than my
own. To commence with a large and perfect instrument
is a mistake; its owner cannot properly
appreciate it, and in gaining experience is pretty
sure to do the glass irreparable injury.'
Should the beginner not be willing or able to face
the purchase of even a comparatively humble instrument,
his case is by no means desperate, for he will
find facilities at hand, such as were not thought of
a few years ago, for the construction of his own telescope.
Two-inch achromatic object-glasses, with
suitable lenses for the making up of the requisite
eye-pieces, are to be had for a few shillings, together
with cardboard tubes of sizes suitable for fitting up
the instrument; and such a volume as Fowler's
'Telescopic Astronomy' gives complete directions
for the construction of a glass which is capable of
a wonderful amount of work in proportion to its
cost. The substitution of metal tubes for the cardboard
ones is desirable, as metal will be found to be
[pg 18]
much more satisfactory if the instrument is to be
much used. The observer, however, will not long
be satisfied with such tools as these, useful though
they may be. The natural history of amateur
astronomers may be summed up briefly in the words
'they go from strength to strength.' The possessor
of a small telescope naturally and inevitably covets
a bigger one; and when the bigger one has been
secured it represents only a stage in the search for
one bigger still, while along with the desire for
increased size goes that for increased optical perfection.
No properly constituted amateur will be satisfied
until he has got the largest and best instrument
that he has money to buy, space to house, and time
to use.
Let us suppose, then, that the telescope has been
acquired, and that it is such an instrument as may
very commonly be found in the hands of a beginner—a
refractor, say, of 2, 2½, or 3 inches aperture
(diameter of object-glass). The question of reflectors
will fall to be considered later. Human nature suggests
that the first thing to do with it is to unscrew
all the screws and take the new acquisition to pieces,
so far as possible, in order to examine into its construction.
Hence many glasses whose career of
usefulness is cut short before it has well begun.
'In most cases,' says Webb, 'a screw-driver is a
dangerous tool in inexperienced hands'; and Smyth,
in the Prolegomena to his 'Celestial Cycle,' utters
words of solemn warning to the 'over-handy gentlemen
who, in their feverish anxiety for meddling with
[pg 19]
and making instruments, are continually tormenting
them with screw-drivers, files, and what-not.' Unfortunately,
it is not only the screw-driver that is
dangerous; the most deadly danger to the most
delicate part of the telescope lies in the unarmed
but inexperienced hands themselves. You may do
more irreparable damage to the object-glass of your
telescope in five minutes with your fingers than you
are likely to do to the rest of the instrument in
a month with a screw-driver. Remember that an
object-glass is a work of art, sometimes as costly as,
and always much more remarkable than, the finest
piece of jewellery. It may be unscrewed, carefully,
from the end of its tube and examined. Should the
examination lead to the detection of bubbles or even
scratches in the glass (quite likely the latter if the
instrument be second-hand), these need not unduly
vex its owner's soul. They do not necessarily mean
bad performance, and the amount of light which
they obstruct is very small, unless the case be an
extreme one. But on no account should the two
lenses of the object-glass itself be separated, for this
will only result in making a good objective bad and
a bad one worse. The lenses were presumably
placed in their proper adjustment to one another by
an optician before being sent out; and should their
performance be so unsatisfactory as to suggest that
this adjustment has been disturbed, it is to an
optician that they should be returned for inspection.
The glass may, of course, be carefully and gently
cleaned, using either soft chamois leather, or preferably
[pg 20]
an old silk handkerchief, studiously kept from
dust; but the cleaning should never amount to more
than a gentle sweeping away of any dust which may
have gathered on the surface. Rubbing is not to be
thought of, and the man whose telescope has been
so neglected that its object-glass needs rubbing
should turn to some other and less reprehensible
form of mischief. For cleaning the small lenses of
the eye-pieces, the same silk may be employed;
Webb recommends a piece of blotting-paper, rolled
to a point and aided by breathing, for the edges
which are awkward to get at. Care must, of course,
be taken to replace these lenses in their original
positions, and the easiest way to ensure this is to
take out only one at a time. In replacing them,
see that the finger does not touch the surface of the
glass, or the cleaning will be all to do over again.
FIG. 8.
a, O.G. in perfect adjustment; b, O.G. defectively centred.
Next comes the question of testing the quality of
the objective. (The stand is meanwhile assumed,
but will be spoken of later.) Point the telescope to
a star of about the third magnitude, and employ the
eye-piece of highest power, if more than one goes
with the instrument—this will be the shortest eye-piece
of the set. If the glass be of high quality,
the image of the star will be a neat round disc of
small size, surrounded by one or two thin bright
rings (Fig. 8, a). Should the image be elliptical and
the rings be thrown to the one side (Fig. 8, b), the
glass may still be quite a good one, but is out of
square, and should be readjusted by an optician.
[pg 21]
Should the image be irregular and the rings broken,
the glass is of inferior quality, though it may still be
serviceable enough for many purposes. Next throw
the image of the star out of focus by racking the
eye-piece in towards the objective, and then repeat
the process by racking it again out of focus away
from the objective. The image will, in either case,
expand into a number of rings of light, and these
rings should be truly circular, and should present
precisely the same appearance at equal distances
within and without the focus. A further conception
of the objective's quality may be gained by observing
whether the image of a star or the detail of
the moon or of the planets comes sharply to a focus
when the milled head for focussing is turned. Should
it be possible to rack the eye-tube in or out for any
distance without disturbing the distinctness of the
picture to any extent, then the glass is defective.
A good objective will admit of no such range, but
will come sharply up to focus, and as sharply away
from it, with any motion of the focussing screw. A
good glass will also show the details of a planet like
Saturn, such as are within its reach, that is, with
[pg 22]
clearness of definition, while an inferior one will
soften all the outlines, and impart a general haziness
to them. The observer may now proceed to test
the colour correction of his objective. No achromatic,
its name notwithstanding, ever gives an
absolutely colourless image; all that can be expected
is that the colour aberration should have been so far
eliminated as not to be unpleasant. In a good
instrument a fringe of violet or blue will be seen
around any bright object, such as Venus, on a dark
sky; a poor glass will show red or yellow. It is
well to make sure, however, should bad colour be
seen, that the eye-piece is not causing it; and, therefore,
more than one eye-piece should be tried before
an opinion is formed. Probably more colour will be
seen at first than was expected, more particularly
with an object so brilliant as Venus. But the observer
need not worry overmuch about this. He
will find that the eye gets so accustomed to it as
almost to forget that it is there, so that something of
a shock may be experienced when a casual star-gazing
friend, on looking at some bright object,
remarks, as friends always do, 'What beautiful
colours!' Denning records a somewhat extreme
case in which a friend, who had been accustomed to
observe with a refractor, absolutely resented the
absence of the familiar colour fringe in the picture
given by a reflector, which is the true achromatic
in nature, though not in name. The beginner is
recommended to read the article 'The Adjustment
of a Small Equatorial,' by Mr. E. W. Maunder, in
[pg 23]
the Journal of the British Astronomical Association,
vol. ii., p. 219, where he will find the process of
testing described at length and with great clearness.
In making these tests, allowance has, of course, to
be made for the state of the atmosphere. A good
telescope can only do its best on a good night, and
it is not fair to any instrument to condemn it until
it has been tested under favourable conditions. The
ideal test would be to have its performance tried
along with that of another instrument of known
good quality and of as nearly the same size as
possible. If this cannot be arranged for, the tests
must be made on a succession of nights, and good
performance on one of these is sufficient to vindicate
the reputation of the glass, and to show that any
deficiency on other occasions was due to the state
of the air, and not to the instrument. Should his
telescope pass the above tests satisfactorily, the
observer ought to count himself a happy man, and
will until he begins to hanker after a bigger instrument.
The mention of the pointing of the telescope to
a star brings up the question of how this is to be
done. It seems a simple thing; as a matter of fact,
with anything like a high magnifying power it is
next to impossible; and there are few things more
exasperating than to see a star or a planet shining
brightly before your eyes, and yet to find yourself
quite unable to get it into the field of view. The
simple remedy is the addition of a finder to the
telescope. This is a small telescope of low magnifying
[pg 24]
power which is fastened to the larger instrument
by means of collars bearing adjusting screws,
which enable it to be laid accurately parallel with
the large tube (Fig. 10). Its eye-piece is furnished
with cross-threads, and a star brought to the intersection
of these threads will be in the field of the
large telescope. In place of the two threads crossing
at right angles there may be substituted three threads
interlacing to form a little triangle in the centre of
the finder's field. By this device the star can always
be seen when the glass is being pointed instead of
being hidden, as in the other case, behind the intersection
of the two threads. A fine needle-point
fixed in the eye-piece will also be found an efficient
substitute for the cross-threads. In the absence of
a finder the telescope may be pointed by using the
lowest power eye-piece and substituting a higher
one when the object is in the field; but beyond
question the finder is well worth the small addition
which it makes to the cost of an instrument. A
little care in adjusting the finder now and again will
often save trouble and annoyance on a working
evening.
The question of a stand on which to mount the
telescope now falls to be considered, and is one of
great importance, though apt to be rather neglected
at first. It will soon be found that little satisfaction
or comfort can be had in observing unless the stand
adopted is steady. A shaky mounting will spoil the
performance of the best telescope that ever was
made, and will only tantalize the observer with
[pg 25]
occasional glimpses of what might be seen under
better conditions. Better have a little less aperture
to the object-glass, and a good steady mounting,
than an extra inch of objective and a mounting
which robs you of all comfort in the using of your
telescope. Beginners are indeed rather apt to be
misled into the idea that the only matters of importance
are the objective and its tube, and that money
spent on the stand is money wasted. Hence many
fearful and wonderful contrivances for doing badly
what a little saved in the size of the telescope and
expended on the stand would have enabled them to
do well. It is very interesting, no doubt, to get a
view of Jupiter or Saturn for one field's-breadth, and
then to find, on attempting to readjust the instrument
for another look, that the mounting has
obligingly taken your star-gazing into its own hands,
and is now directing your telescope to a different
object altogether; but repetition of this form of
amusement is apt to pall. A radically weak stand can
never be made into a good one; the best plan is to
get a properly proportioned mounting at once, and
be done with it.
FIG. 9.—SMALL TELESCOPE ON PILLAR AND CLAW STAND.
For small instruments, such as we are dealing
with, the mounting generally adopted is that known
as the Altazimuth, from its giving two motions, one
in altitude and one in azimuth, or, to use more
familiar terms, one vertical and the other horizontal.
There are various types of the Altazimuth. If the
instrument be of not more than 3 feet focal length,
the ordinary stand known as the 'pillar and claw'
[pg 26]
(Fig. 9) will meet all the requirements of this form
of motion. Should the focal length be greater than
3 feet, it is advisable to have the instrument mounted
on a tripod stand, such as is shown in Fig. 10. In
the simpler forms of both these mountings the two
motions requisite to follow an object must be given
by hand, and it is practically impossible to do this
without conveying a certain amount of tremor to
the telescope, which disturbs clearness of vision until
it subsides, by which time the object to be viewed
is generally getting ready to go out of the field
again. To obviate this inconvenience as far as
[pg 27]
possible, the star or planet when found should be
placed just outside the field of view, and allowed
[pg 28]
to enter it by the diurnal motion of the earth. The
tremors will thus have time to subside before the
object reaches the centre of the field, and this
process must be repeated as long as the observation
continues. In making this adjustment attention
must be paid to the direction of the object's motion
through the field, which, of course, varies according
to its position in the sky. If it be remembered that
a star's motion through the telescopic field is the
exact reverse of its true direction across the sky,
little difficulty will be found, and use will soon
render the matter so familiar that the adjustment
will be made almost automatically.
FIG. 10.—TELESCOPE ON TRIPOD, WITH FINDER AND SLOW MOTIONS.
A much more convenient way of imparting the
requisite motions is by the employment of tangent
screws connected with Hooke's joint-handles, which
are brought conveniently near to the hands of the
observer as he sits at the eye-end. These screws
clamp into circles or portions of circles, which have
teeth cut on them to fit the pitch of the screw, and
by means of them a slow and steady motion may be
imparted to the telescope. When it is required to
move the instrument more rapidly, or over a large
expanse of sky, the clamps which connect the screws
with the circles are slackened, and the motion is
given by hand. Fig. 10 shows an instrument provided
with these adjuncts, which, though not absolutely
necessary, and adding somewhat to the cost
of the mounting, are certainly a great addition to the
ease and comfort of observation.
FIG. 11.—EQUATORIAL MOUNTING FOR SMALL TELESCOPE.
The Altazimuth mounting, from its simplicity and
[pg 29]
comparative cheapness, has all along been, and will
probably continue to be, the form most used by
amateurs. It is, however, decidedly inferior in every
respect to the equatorial form of mount. In this
form (Fig. 11) the telescope is carried by means of
two axes, one of which—the Polar axis—is so adjusted
as to be parallel to the pole of the earth's
rotation, its degree of inclination being therefore
dependent upon the latitude of the place for which
[pg 30]
it is designed. At the equator it will be horizontal,
will lie at an angle of 45 degrees half-way between
the equator and either pole, and will be vertical at
the poles. At its upper end it carries a cross-head
with bearings through which there passes another
axis at right angles to the first (the declination axis).
Both these axes are free to rotate in their respective
bearings, and thus the telescope is capable of two
motions, one of which—that of the declination axis—enables
the instrument to be set to the elevation of
the object to be observed, while the other—that of the
polar axis—enables the observer to follow the object,
when found, from its rising to its setting by means
of a single movement, the telescope sweeping out
circles on the sky corresponding to those which the
stars themselves describe in their journey across the
heavens. This single movement may be given by
means of a tangent screw such as has already been
described, and the use of a telescope thus equipped
is certainly much easier and more convenient than
that of an Altazimuth, where two motions have
constantly to be imparted. To gain the full advantage
of the equatorial form of mounting, the
polar axis must be placed exactly in the North and
South line, and unless the mounting can be adjusted
properly and left in adjustment, it is robbed of much
of its superiority. For large fixed instruments it is,
of course, almost universally used; and in observatories
the motion in Right Ascension, as it is called,
which follows the star across the sky, is communicated
to the driving-wheel of the polar axis by
[pg 31]
means of a clock which turns the rod carrying the
tangent screw (Plate II.). These are matters which
in most circumstances are outside the sphere of the
amateur; it may be interesting for him, however, to
see examples of the way in which large instruments
are mounted. The frontispiece, accordingly, shows
the largest and most perfect instrument at present in
existence, while Plate II., with Figs. 4 and 12, give
further examples of fine modern work. The student
can scarcely fail to be struck by the extreme solidity
of the modern mountings, and by the way in which all
the mechanical parts of the instrument are so contrived
as to give the greatest convenience and ease
in working. Comparing, for instance, Plate II., a
6-inch refractor by Messrs. Cooke, of York, available
either for visual or photographic work, with the
Dorpat refractor (Fig. 3), it is seen that the modern
maker uses for a 6-inch telescope a stand much more
solid and steady than was deemed sufficient eighty
years ago for an instrument of 96⁄10 inches. Attention
is particularly directed to the way in which
nowadays all the motions are brought to the eye-end
so as to be most convenient for the observer,
and frequently, as in this case, accomplished by
electric power, while the declination circle is read
by means of a small telescope so that the large
instrument can be directed upon any object with the
minimum of trouble. The driving clock, well shown
on the right of the supporting pillar, is automatically
controlled by electric current from the sidereal clock
of the observatory.
PLATE II.
6-inch Photo-Visual Refractor, equatorially mounted. Messrs. T. Cooke & Sons.
[pg 32]
We have now to consider the reflecting form of
telescope, which, especially in this country, has
deservedly gained much favour, and has come to be
regarded as in some sense the amateur's particular
tool.
FIG. 12.—8-INCH REFRACTOR ON EQUATORIAL MOUNTING.
[pg 33]
As a matter of policy, one can scarcely advise the
beginner to make his first essay with a reflector.
Its adjustments, though simple enough, are apt to
be troublesome at the time when everything has
to be learned by experience; and its silver films,
though much more durable than is commonly supposed,
are easily destroyed by careless or unskilful
handling, and require more careful nursing than the
objective of a refractor. But, having once paid his
first fees to experience, the observer, if he feel so
inclined, may venture upon a reflector, which has
probably more than sufficient advantages to make
up for its weaker points. First and foremost of
these advantages stands the not inconsiderable one
of cheapness. A 10½-inch reflector may be purchased
new for rather less than the sum which will
buy a 4-inch refractor. True, the reflector has not
the same command of light inch for inch as the
refractor, but a reflector of 10½ inches should at
least be the match of an 8-inch refractor in this
respect, and will be immeasurably more powerful
than the 4-inch refractor, which comes nearest to
it in price. Second stands the ease and comfort
so conspicuous in observing with a Newtonian.
Instead of having almost to break his neck craning
under the eye-piece of a telescope pointed to near
the zenith, the observer with a Newtonian looks
always straight in front of him, as the eye-piece of a
reflector mounted as an altazimuth is always horizontal,
and when the instrument is mounted equatorially,
the tube, or its eye-end, is made to rotate
[pg 34]
so that the line of vision may be kept horizontal.
Third is the absence of colour. Colour is not conspicuous
in a small refractor, unless the objective be
of very bad quality; but as the aperture increases it
is apt to become somewhat painfully apparent. The
reflector, on the other hand, is truly achromatic,
and may be relied upon to show the natural tints of
all objects with which it deals. This point is of
considerable importance in connection with planetary
observation. The colouring of Jupiter, for instance,
will be seen in a reflector as a refractor can never
show it.
Against these advantages there have to be set
certain disadvantages. First, the question of adjustments.
A small refractor requires practically
none; but a reflector, whatever its size, must be
occasionally attended to, or else its mirrors will get
out of square and bad performance will be the
result. It is easy, however, to make too much of
this difficulty. The adjustments of the writer's
8½-inch With reflector have remained for months
at a time as perfect as when they had been newly
attended to. Second, the renewal of the silver
films. This may cause some trouble in the neighbourhood
of towns where the atmosphere is such
as to tarnish silver quickly; and even in the country
a film must be renewed at intervals. But these
may be long enough. The film on the mirror
above referred to has stood without serious deterioration
for five years at a time. Third, the reflector,
with its open-mouthed tube, is undoubtedly more
[pg 35]
subject to disturbance from air currents and changes
of temperature, and its mirrors take longer to settle
down into good definition after the instrument has
been moved from one point of the sky to another.
This difficulty cannot be got over, and must be put
up with; but it is not very conspicuous with the
smaller sizes of telescopes, such as are likely to be
in the hands of an amateur at the beginning of his
work. There are probably but few nights when
an 8½-inch reflector will not give quite a good
account of itself in this respect by comparison with
a refractor of anything like equal power. On the
whole, the state of the question is this: If the
observer wishes to have as much power as possible
in proportion to his expenditure, and is not afraid
to take the risk of a small amount of trouble with
the adjustments and films, the reflector is probably
the instrument best suited to him. If, on the other
hand, he is so situated that his telescope has to
be much moved, or, which is almost as bad, has
to stand unused for any considerable intervals of
time, he will be well advised to prefer a refractor.
One further advantage of the reflecting form is
that, aperture for aperture, it is very much shorter.
The average refractor will probably run to a length
of from twelve to fifteen times the diameter of its
objective. Reflectors are rarely of a greater length
than nine times the diameter of the large mirror,
and are frequently shorter still. Consequently, size
for size, they can be worked in less space, which is
often a consideration of importance.
[pg 36]
FIG. 13.—FOUR-FOOT REFLECTOR EQUATORIALLY MOUNTED.
The mountings of the reflector are in principle
precisely similar to those of the refractor already
described. The greater weight, however, and the
convenience of having the body of the instrument
kept as low as possible, owing to the fact of the
eye-piece being at the upper end of the tube, have
necessitated various modifications in the forms to
which these principles are applied. Plates III.
and IV., and Fig. 13, illustrate the altazimuth
and equatorial forms of mounting as applied to
[pg 37]
reflectors of various sizes, Fig. 13 being a representation
of Lassell's great 4-foot reflector.
PLATE III.
20-inch Reflector, Stanmore Observatory.
And now, having his telescope, whatever its size,
principle, or form of mounting, the observer has to
proceed to use it. Generally speaking, there is no
great difficulty in arriving at the manner of using
either a refractor or a reflector, and for either
instrument the details of handling must be learned
by experience, as nearly all makers have little
variations of their own in the form of clamps and
slow motions, though the principles in all instruments
are the same. With regard to these, the
only recommendation that need be made is one of
caution in the use of the glass until its ways of
working have been gradually found out. With a
knowledge of the principles of its construction and
a little application of common-sense, there is no
part of a telescope mounting which may not be
readily understood. Accordingly, what follows
must simply take the form of general hints as to
matters which every telescopist ought to know,
and which are easier learned once and for all at
the beginning than by slow experience. These
hints are of course the very commonplaces of
observation; but it is the commonplace that is
the foundation of good work in everything.
If possible, let the telescope be fixed in the open
air. Where money is no object, a few pounds will
furnish a convenient little telescope-house, with
either a rotating or sliding roof, which enables the
instrument to be pointed to any quarter of the
[pg 38]
heavens. Such houses are now much more easily
obtained than they once were, and anyone who has
tried both ways can testify how much handier it is
to have nothing to do but unlock the little observatory,
and find the telescope ready for work,
than to have to carry a heavy instrument out into
the open. Plate IV. illustrates such a shelter,
which has done duty for more than twelve years,
covering an 8½-inch With, whose tube and mounting
are almost entirely the work of a local smith;
and in the Journal of the British Astronomical
Association, vol. xiv., p. 283, Mr. Edwin Holmes
gives a simple description of a small observatory
which was put up at a cost of about £3, and has
proved efficient and durable. The telescope-house
has also the advantage of protecting the observer
and his instrument from the wind, so that observation
may often be carried on on nights which
would be quite too windy for work in the open.
PLATE IV.
Telescope House and 8½-inch 'with' Reflector.
Should it not be possible to obtain such a luxury,
however, undoubtedly the next best is fairly outside.
No one who has garden room should ever think of
observing from within doors. If the telescope be
used at an open window its definition will be
impaired by air-currents. The floor of the room
will communicate tremors to the instrument, and
every movement of the observer will be accompanied
by a corresponding movement of the object
in the field, with results that are anything but
satisfactory. In some cases no other position is
available. If this be so, Webb's advice must be
[pg 39]
followed, the window opened as widely and as long
beforehand as possible, and the telescope thrust out
as far as is convenient. But these precautions only
palliate the evils of indoor observation. The open
air is the best, and with a little care in wrapping up
the observer need run no risk.
Provide the telescope, if a refractor, with a dew-cap.
Without this precaution dew is certain to
gather upon the object-glass, with the result of
stopping all observation until it is removed, and
the accompanying risk of damage to the objective
itself. Some instruments are provided by their
makers with dew-caps, and all ought to be; but in
the absence of this provision a cap may be easily
contrived. A tube of tin three or four times as
long as the diameter of the object-glass, made so
as to slide fairly stiffly over the object end of the
tube where the ordinary cap fits, and blackened
inside to a dead black, will remove practically all
risk. The blackening may be done with lamp-black
mixed with spirit varnish. Some makers—Messrs.
Cooke, of York, for instance—line both
tube and dew-cap with black velvet. This ought
to be ideal, and might be tried in the case of the
dew-cap by the observer. Finders are rarely fitted
with dew-caps, but certainly should be; the addition
will often save trouble and inconvenience.
Be careful to cover up the objective or mirror
with its proper cap before removing it into the
house. If this is not done, dewing at once results,
the very proper punishment for carelessness. This
[pg 40]
may seem a caution so elementary as scarcely to be
worth giving; but it is easier to read and remember
a hint than to have to learn by experience, which
in the case of a reflector will almost certainly mean
a deteriorated mirror film. Should the mirror, if
you are using a reflector, become dewed in spite
of all precautions, do not attempt to touch the film
while it is moist, or you will have the pleasure
of seeing it scale off under your touch. Bring it
into a room of moderate temperature, or stand it
in a through draught of dry air until the moisture
evaporates; and should any stain be left, make
sure that the mirror is absolutely dry before
attempting to polish it off. With regard to this
matter of polishing, touch the mirror as seldom
as possible with the polishing-pad. Frequent
polishing does far more harm than good, and the
mirror, if kept carefully covered when not in use,
does not need it. A fold of cotton-wool between
the cap and the mirror will, if occasionally taken out
and dried, help greatly to preserve the film.
Next comes a caution which beginners specially
need. Almost everyone on getting his first telescope
wants to see everything as big as possible,
and consequently uses the highest powers. This
is an entire mistake. For a telescope of 2½ inches
aperture two eye-pieces, or at most three, are
amply sufficient. Of these, one may be low in
power, say 25 to 40, to take in large fields, and,
if necessary, to serve in place of a finder. Such
an eye-piece will give many star pictures of surprising
[pg 41]
beauty. Another may be of medium power,
say 80, for general work; and a third may be as
high as 120 for exceptionally fine nights and for
work on double stars. Nominally a 2½ inch, if of
very fine quality, should bear on the finest nights
and on stars a power of 100 to the inch, or 250.
Practically it will do nothing of the sort, and on
most nights the half of this power will be found
rather too high. Indeed, the use of high powers
is for many reasons undesirable. A certain proportion
of light to size must be preserved in the
image, or it will appear faint and 'clothy.' Further,
increased magnifying power means also increased
magnification of every tremor of the atmosphere;
and with high powers the object viewed passes
through the field so rapidly that constant shifting
of the telescope is required, and only a brief
glimpse can be obtained before the instrument has
to be moved again. It is infinitely more satisfactory
to see your object of a moderate size and steady
than to see it much larger, but hazy, tremulous, and
in rapid motion. 'In inquiring about the quality of
some particular instrument,' remarks Sir Howard
Grubb, 'a tyro almost invariably asks, "What is the
highest power you can use?" An experienced observer
will ask, "What is the lowest power with
which you can do so and so?"'
Do not be disappointed if your first views of
celestial objects do not come up to your expectations.
They seldom do, particularly in respect of
the size which the planets present in the field. A
[pg 42]
good deal of the discouragement so often experienced
is due to the idea that the illustrations in
text-books represent what ought to be seen by anyone
who looks through a telescope. It has to be
remembered that these pictures are, for one thing,
drawn to a large scale, in order to insure clearness
in detail, that they are in general the results of
observation with the very finest telescopes, and the
work of skilled observers making the most of
picked nights. No one would expect to rival a
trained craftsman in a first attempt at his trade;
yet most people seem to think that they ought to
be able at their first essay in telescopic work to see
and depict as much as men who have spent half a
lifetime in an apprenticeship to the delicate art of
observation. Given time, patience, and perseverance,
and the skill will come. The finest work shown in
good drawings represents, not what the beginner
may expect to see at his first view, but a standard
towards which he must try to work by steady
practice both of eye and hand. In this connection
it may be suggested that the observer should take
advantage of every opportunity of seeing through
larger and finer instruments than his own. This
will teach him two things at least. First, to respect
his own small telescope, as he sees how bravely it
stands up to the larger instrument so far as regards
the prominent features of the celestial bodies; and,
second, to notice how the superior power of the
large glass brings out nothing startlingly different
from that which is shown by his own small one, but
[pg 43]
a wealth of delicate detail which must be looked for
(compare Plate XV. with Fig. 22). A little occasional
practice with a large instrument will be found
a great encouragement and a great help to working
with a small one, and most possessors of large
glasses are more than willing to assist the owners
of small ones.
Do not be ashamed to draw what you see,
whether it be little or much, and whether you can
draw well or ill. At the worst the result will have
an interest to yourself which no representation by
another hand can ever possess; at the best your
drawings may in course of time come to be of real
scientific value. There are few observers who
cannot make some shape at a representation of
what they see, and steady practice often effects an
astonishing improvement. But draw only what you
see with certainty. Some observers are gifted with
abnormal powers of vision, others with abnormal
powers of imagination. Strange to say, the results
attained by these two classes differ widely in appearance
and in value. You may not be endowed
with faculties which will enable you to take rank in
the former class; but at least you need not descend
to the latter. It is after all a matter of conscience.
Do not be too hasty in supposing that everybody
is endowed with a zeal for astronomy equal to your
own. The average man or woman does not enjoy
being called out from a warm fireside on a winter's
night, no matter how beautiful the celestial sight to
be seen. Your friend may politely express interest,
[pg 44]
but to tempt him to this is merely to encourage a
habit of untruthfulness. The cause of astronomy is
not likely to be furthered by being associated in
any person's mind with discomfort and a boredom
which is not less real because it is veiled under
quite inadequate forms of speech. It is better to
wait until the other man's own curiosity suggests a
visit to the telescope, if you wish to gain a convert
to the science.
When observing in the open be sure to wrap up
well. A heavy ulster or its equivalent, and some
form of covering for the feet which will keep them
warm, are absolute essentials. See that you are
thoroughly warm before you go out. In all probability
you will be cold enough before work is over;
but there is no reason why you should make yourself
miserable from the beginning, and so spoil your
enjoyment of a fine evening.
Having satisfied his craving for a general survey
of everything in the heavens that comes within the
range of his glass, the beginner is strongly advised
to specialize. This is a big word to apply to the
using of a 2½- or 3-inch telescope, but it represents
the only way in which interest can be kept up. It
does no good, either to the observer or to the
science of astronomy, for him to take out his glass,
have a glance at Jupiter and another at the Orion
nebula, satisfy himself that the two stars of Castor
are still two, wander over a few bright clusters, and
then turn in, to repeat the same dreary process the
next fine night. Let him make up his mind to
[pg 45]
stick to one, or at most two, objects. Lunar work
presents an attractive field for a small instrument,
and may be followed on useful lines, as will be
pointed out later. A spell of steady work upon
Jupiter will at least prepare the way and whet the
appetite for a glass more adequate to deal with the
great planet. Should star work be preferred, a fine
field is opened up in connection with the variable
stars, the chief requirement of work in this department
being patience and regularity, a small telescope
being quite competent to deal with a very
large number of interesting objects.
The following comments in Smyth's usual pungent
style are worth remembering: 'The furor of a
green astronomer is to possess himself of all sorts of
instruments—to make observations upon everything—and
attempt the determination of quantities which
have been again and again determined by competent
persons, with better means, and more
practical acquaintance with the subject. He starts
with an enthusiastic admiration of the science, and
the anticipation of new discoveries therein; and all
the errors consequent upon the momentary impulses
of what Bacon terms "affected dispatch" crowd
upon him. Under this course—as soon as the more
hacknied objects are "seen up"—and he can
decide whether some are greenish-blue or bluish-green—the
excitement flags, the study palls, and the
zeal evaporates in hyper-criticism on the instruments
and their manufacturers.'
This is a true sketch of the natural history, or
[pg 46]
rather, of the decline and fall, of many an amateur
observer. But there is no reason why so ignominious
an end should ever overtake any man's
pursuit of the study if he will only choose one
particular line and make it his own, and be thorough
in it. Half-study inevitably ends in weariness and
disgust; but the man who will persevere never
needs to complain of sameness in any branch of
astronomical work.
[pg 47]
CHAPTER III
THE SUN
From its comparative nearness, its brightness and
size, and its supreme importance to ourselves, the
sun commands our attention; and in the phenomena
which it presents there is found a source
of abundant and constantly varying interest.
Observation of these phenomena can only be conducted,
however, after due precautions have been
taken. Few people have any idea of the intense
glow of the solar light and heat when concentrated
by the object-glass of even a small telescope, and
care must be exercised lest irreparable damage be
done to the eye. Galileo is said to have finally
blinded himself altogether, and Sir William Herschel
to have seriously impaired his sight by solar observation.
No danger need be feared if one or
other of the common precautions be adopted, and
some of these will be shortly described; but before
we consider these and the means of applying them,
let us gather together briefly the main facts about
the sun itself.
Our sun, then, is a body of about 866,000 miles
[pg 48]
in diameter, and situated at a distance of some
92,700,000 miles from us. In bulk it equals
1,300,000 of our world, while it would take about
332,000 earths to weigh it down. Its density, as
can be seen from these figures, is very small indeed.
Bulk for bulk, it is considerably lighter than the
earth; in fact, it is not very much denser than
water, and this has very considerable bearing upon
our ideas of its constitution.
Natural operations are carried on in this immense
globe upon a scale which it is almost impossible for
us to realize. A few illustrations gathered from
Young's interesting volume, 'The Sun,' may help
to make clearer to us the scale of the ruling body of
our system. Some conception of the immensity of
its distance from us may first be gained from Professor
Mendenhall's whimsical illustration. Sensation,
according to Helmholtz's experiments, travels
at a rate of about 100 feet per second. If, then, an
infant were born with an arm long enough to reach
to the sun, and if on his birthday he were to exercise
this amazing limb by putting his finger upon
the solar surface, he would die in blissful ignorance
of the fact that he had been burned, for the sensation
of burning would take 150 years to travel along
that stupendous arm. Were the sun hollowed out
like a gigantic indiarubber ball and the earth placed
at its centre, the enclosing shell would appear like a
far distant sky to us. Our moon would have room
to circle within this shell at its present distance of
240,000 miles, and there would still be room for
[pg 49]
another satellite to move in an orbit exterior to
that of the moon at a further distance of more than
190,000 miles. The attractive power of this great
body is no less amazing than its bulk. It has been
calculated that were the attractive power which
keeps our earth coursing in its orbit round the sun
to cease, and to be replaced by a material bond
consisting of steel wires of a thickness equal to that
of the heaviest telegraph-wires, these would require
to cover the whole sunward side of our globe in the
proportion of nine to each square inch. The force
of gravity at the solar surface is such that a man
who on the earth weighs 10 stone would, if transported
to the sun, weigh nearly 2 tons, and, if he
remained of the same strength as on earth, would be
crushed by his own weight.
PLATE V.
The Sun, February 3, 1905. Royal Observatory, Greenwich.
The first telescopic view of the sun is apt, it must
be confessed, to be a disappointment. The moon is
certainly a much more attractive subject for a casual
glance. Its craters and mountain ranges catch the
eye at once, while the solar disc presents an appearance
of almost unbroken uniformity. Soon,
however, it will become evident that the uniformity
is only apparent. Generally speaking, the surface
will quickly be seen to be broken up by one or
more dark spots (Plate V.), which present an
apparently black centre and a sort of grey shading
round about this centre. The margin of the disc
will be seen to be notably less bright than its central
portions; and near the margin, and oftenest, though
not invariably, in connection with one of the dark
[pg 50]
spots, there will be markings of a brilliant white,
and often of a fantastically branched shape, which
seem elevated above the general surface; while as
the eye becomes more used to its work it will be
found that even a small telescope brings out a kind
of mottled or curdled appearance over the whole
disc. This last feature may often be more readily
seen by moving the telescope so as to cause the
solar image to sweep across the field of view, or by
gently tapping the tube so as to cause a slight vibration.
Specks of dirt which may have gathered
upon the field lens of the eye-piece will also be seen;
but these may be distinguished from the spots by
moving the telescope a little, when they will shift
their place relatively to the other features; and their
exhibition may serve to suggest the propriety of
keeping eye-pieces as clean as possible.
PLATE VI.
Photograph of Bridged Sunspot (Janssen). Knowledge, February, 1890.
The spots when more closely examined will be
found to present endless irregularities in outline and
size, as will be seen from the accompanying plates
and figures. On the whole, there is comparative
fidelity to two main features—a dark central nucleus,
known as the umbra, and a lighter border, the
penumbra; but sometimes there are umbræ which
have no penumbra, and sometimes there are spots
which can scarcely be called more than penumbral
shadings. The shape of the spot is sometimes
fairly symmetrical; at other times the most fantastic
forms appear. The umbra appears dark upon the
bright disc, but is in reality of dazzling lustre, sending
to us, according to Langley, 54 per cent. of the
[pg 51]
amount of heat received from a corresponding area
of the brilliant unspotted surface. Within the
umbra a yet darker deep, if it be a deep, has been
detected by various observers, but is scarcely likely
to be seen with the small optical means which we
are contemplating. The penumbra is very much
lighter in colour than the umbra, and invariably
presents a streaked appearance, the lines all running
in towards the umbra, and resembling very much
the edge of a thatched roof. It will be seen to be
very much lighter in colour on the edge next the
umbra, while it shades to a much darker tone on
that side which is next to the bright undisturbed
part of the surface (Figs. 14 and 15). Frequently
a spot will be seen interrupted by a bright projection
from the luminous surface surrounding it which
may even extend from side to side of the spot,
forming a bridge across it (Plate VI., and Figs. 16,
17, and 18). These are the outstanding features of
the solar spots, and almost any telescope is competent
to reveal them. But these appearances have
to be interpreted, so far as that is possible, and to
have some scale applied to them before their significance
can in the least be recognised. The
observer will do well to make some attempt at
realizing the enormous actual size of the seemingly
trifling details which his instrument shows. For
example, the spot in Figs. 14 and 15 is identical
with that measured by Mr. Denning on the day
between the dates of my rough sketches; and its
greatest diameter was then 27,143 miles. Spots
[pg 52]
such as those of 1858, of February, 1892, and
February, 1904, have approached or exceeded
140,000 miles in diameter, while others have been
frequently recorded, which, though not to be compared
to these leviathans, have yet measured from
40,000 to 50,000 miles in diameter, with umbræ of
25,000 to 30,000 miles. Of course, the accurate
measurement of the spots demands appliances which
are not likely to be in a beginner's hands; but there
are various ways of arriving at an approximation
which is quite sufficient for the purpose in view—namely,
[pg 53]
a realization of the scale of any spot as
compared with that of the sun or of our own earth.
FIG. 14.—SUN-SPOT, JUNE 18, 1889.
FIG. 15.—SUN-SPOT, JUNE 20, 1889.
Of these methods, the simplest on the whole
seems to be that given by Mr. W. F. Denning in
his admirable volume, 'Telescopic Work for Starlight
Evenings.' Fasten on the diaphragm of an
eye-piece (the blackened brass disc with a central
hole which lies between the field and eye lenses of
the eye-piece) a pair of fine wires at right angles to
one another. Bring the edge of the sun up to the
vertical wire, the eye-piece being so adjusted that
the sun's motion is along the line of the horizontal
[pg 54]
wire. This can easily be accomplished by turning
the eye-piece round until the solar motion follows
the line of the wire. Then note the number of
seconds which the whole disc of the sun takes to
cross the vertical wire. Note, in the second place,
the time which the spot to be measured takes to
cross the vertical wire; and, having these two
numbers, a simple rule of three sum enables the
diameter of the spot to be roughly ascertained. For
the sun's diameter, 866,000 miles, is known, and the
proportion which it bears to the number of seconds
which it takes to cross the wire will be the same
as that borne by the spot to its time of transit. Thus,
to take Mr. Denning's example, if the sun takes
133 seconds to cross the wire, and the spot takes
6·5, then 133 : 866,000 : : 6·5 : 42,323, which latter
number will be, roughly speaking, the diameter of
the spot in miles. This, method is only a very
rough approximation; still, it at least enables the
observer to form some conception of the scale of
what is being seen. It will answer best when the
sun is almost south, and is, of course, less and less
accurate as the spot in question is removed from the
centre of the disc; for the sun being a sphere, and
not a flat surface, foreshortening comes largely and
increasingly into play as spots near the edge (or
limb) of the disc.
Continued observation will speedily lead to the
detection of the exceedingly rapid changes which
often affect the spots and their neighbourhood.
There are instances in which a spot passes across
[pg 55]
the disc without any other apparent changes save
those which are due to perspective; and the same
spot may even accomplish a complete rotation and
appear again with but little change. But, generally
speaking, it will be noticed that the average spot
changes very considerably during the course of a
single rotation. Often, indeed, the changes are so
rapid as to be apparent within the course of a few
hours. Figs. 14 and 15 represent a spot which was
seen on June 18 and 20, 1889, and sketched by
means of a 2½-inch refractor with a power of 80.
A certain proportion of the change noticeable is due
to perspective, but there are also changes of considerable
importance in the structure of the spot
which are actual, and due to motion of its parts.
Mr. Denning's drawing ('Telescopic Work,' p. 95)
shows the spot on the day between these two representations,
and exhibits an intermediate stage of the
change. The late Professor Langley has stated
that when he was making the exquisite drawing of
a typical sun-spot which has become so familiar to
all readers of astronomical text-books and periodicals,
a portion of the spot equal in area to the continent
of South America changed visibly during the time
occupied in the execution of the drawing; and this
is only one out of many records of similar tenor.
Indeed, no one who has paid any attention to solar
observation can fail to have had frequent instances
of change on a very large scale brought under his
notice; and when the reality of such change has
been actually witnessed, it brings home to the mind,
[pg 56]
as no amount of mere statement can, the extraordinary
mobility of the solar surface, and the fact
that we are here dealing with a body where the
conditions are radically different from those with
which we are familiar on our own globe. Changes
which involve the complete alteration in appearance
of areas of many thousand square miles have to be
taken into consideration as things of common occurrence
upon the sun, and must vitally affect our ideas
of his constitution and structure (Figs. 16, 17, 18).
FIG. 16.—SUN-SPOT SEEN IN 1870.
Little more can be done by ordinary observation
with regard to the spots and the general surface.
Common instruments are not likely to have much
chance with the curious structure into which the
[pg 57]
coarse mottling of the disc breaks up when viewed
under favourable circumstances. This structure,
compared by Nasmyth to willow-leaves, and by
others to rice-grains, is beautifully seen in a number
of the photographs taken by Janssen and others;
but it is seldom that it can be seen to full advantage.
FIG. 17.—ANOTHER PHASE OF SPOT (FIG. 16).
FIG. 18.—PHASE OF SPOT (FIGS. 16 AND 17).
On the other hand, the spots afford a ready means
by which the observer may for himself determine
approximately the rotation period of the sun. A
spot will generally appear to travel across the solar
disc in about 13 days 14½ hours, and to reappear at
the eastern limb after a similar lapse of time, thus
[pg 58]
making the apparent rotation-period 27 days 5 hours.
This has to be corrected, as the earth's motion round
the sun causes an apparent slackening in the rate of
the spots, and a deduction of about 2 days has to
be made for this reason, the resulting period being
about 25 days 7 hours. It will quickly be found
that no single spot can be relied upon to give anything
like a precise determination, as many have
motions of their own independent of that due to
the sun's rotation; and, in addition, there has been
shown to be a gradual lengthening of the period in
high latitudes. Thus, spots near the equator yield
[pg 59]
a period of 25·09 days, those in latitude 15° N. or S.
one of 25·44, and those in latitude 30° one of 26·53.
This law of increase, first established by Carrington,
has been confirmed by the spectroscopic
measures of Dunér at Upsala. His periods, while
uniformly in excess of those derived from ordinary
observations, show the same progression. For 0°
his period is 25·46 days, for 15° 26·35, and for
30° 27·57. Continuing his researches up to 15°
from the solar pole, Dunér has found that at that
point the period of rotation is protracted to 38.5 days.
Reference has already been made to the bright
and fantastically branched features which diversify
the solar surface, generally appearing in connection
with the spots, and best seen near the limb, though
existing over the whole disc. These 'faculæ,' as
they are called, will be readily seen with a small
instrument—I have seen them easily with a 2-inch
finder and a power of 30. They suggest at once
to the eye the idea that they are elevations above
the general surface, and look almost like waves
thrown up by the convulsions which produce the
spots. The rotation-period given by them has also
been ascertained, and the result is shorter than that
given by the spots. In latitude 0° it is 24·66 days,
at 15° it is 25·26, at 30° 25·48. These varieties of
rotation show irresistibly that the sun cannot in
any sense of the term be called a rigid body. As
Professor Holden remarks: 'It is more like a vast
whirlpool, where the velocities of rotation depend on
the situation of the rotating masses, not only as to
[pg 60]
latitude, but also as to depth beneath the rotating
surface.' Plate VII., from a photograph of the sun
taken by Mr. Hale, in which the surface is portrayed
by the light of one single calcium ray of the solar
spectrum, presents a view of the mottled appearance
of the disc, together with several bright forms which
the author of the photograph considers to be faculæ.
M. Deslandres, of the Meudon Observatory, who
has also been very successful in this new branch of
solar photography, considers, however, that these
forms are not faculæ, but distinct phenomena, to
which he proposes to assign the name 'faculides';
and for various reasons his view appears to be the
more probable. They are, however, in any case, in
close relation with the faculæ, and, as Miss Clerke
observes, 'symptoms of the same disturbance.'
PLATE VII.
Solar Surface with Faculæ. Yerkes Observatory.
The question of the nature of the sun spots is one
that at once suggests itself; but it must be confessed
that no very satisfactory answer can yet be
given to it. None of the many theories put forward
have covered all the observed facts, and an adequate
solution seems almost as far off as ever. No one
can fail to be struck with the resemblance which the
spots present to cavities in the solar surface. Instinctively
the mind seems to regard the umbra of
the spot as being the centre of a great hollow of
which the penumbra represents the sloping sides;
and for long it was generally held that Wilson's
theory, which assumed this appearance to correspond
to an actual fact, was correct. Wilson found
by observation of certain spots that when the spot
[pg 61]
was nearest to one limb the penumbra disappeared,
either altogether or in part, on the side towards the
centre, and that this process was reversed as the
spot approached the opposite limb, the portion of
the penumbra nearest the centre of the disc being
always the narrowest.
This is the order of appearances which would
naturally follow if the spot in question were a
cavity; and if it were invariable there could scarcely
be any doubt as to its significance. But while the
Wilsonian theory has been recognised in all the
text-books for many years, there has always been a
suspicion that it was by no means adequately established,
and that it was too wide an inference from
the number of cases observed; and of late years it
has been falling more and more into discredit.
Howlett, for example, an observer of great experience,
has asserted that the appearances on which
the theory is based are not the rule, but the exception,
and that therefore it must be given up.
Numbers of spots seem to present the appearance
of elevations rather than of depressions, and altogether
it seems as though no category has yet been
attained which will embrace all the varieties of spot-form.
On this point further observation is very
much needed, and the work that has to be done is
well within the reach of even moderate instruments.
The fact that sun-spots wax and wane in numbers
in a certain definite period was first ascertained by the
amateur observer Schwabe of Dessau, whose work
is a notable example of what may be accomplished
[pg 62]
by steadfast devotion to one particular branch of
research. Without any great instrumental equipment,
Schwabe effected the discovery of this most
important fact—a discovery second to none made
in the astronomical field during the last century—simply
by the patient recording of the state of the
sun's face for a period of over thirty years, during
which he succeeded in securing an observation, on
the average, on about 300 days out of every year.
The period now accepted differs slightly from that
assigned by him, and amounts to 11·11 years.
Beginning with a minimum, when few spots or none
may be visible for some time, the spots will be
found to increase gradually in number, until, about
four and a half years from the minimum, a maximum
is reached; and from this point diminution sets in,
and results, in about 6·6 years, in a second minimum.
The period is not one of absolute regularity—a
maximum or a minimum may sometimes lag considerably
behind its proper time, owing to causes as
yet unexplained. Still, on the whole, the agreement
is satisfactory.
This variation is also accompanied by a variation
in the latitude of the spots. Generally they follow
certain definite zones, mostly lying between 10° and
35° on either side of the solar equator. As a
minimum approaches, they tend to appear nearer to
the equator than usual; and when the minimum has
passed the reappearance of the spots takes the
opposite course, beginning in high latitudes.
It has further been ascertained that a close connection
[pg 63]
exists between the activity which results in
the formation of sun-spots, and the electrical phenomena
of our earth. Instances of this connection
have been so repeatedly observed as to leave no
doubt of its reality, though the explanation of it has
still to be found. It has been suggested by Young
that there may be immediate and direct action in
this respect between the sun and the earth, an
action perhaps kindred with that solar repulsive
force which seems to drive off the material of a
comet's tail. As yet not satisfactorily accounted for
is the fact that it does not always follow that the
appearance of a great sun-spot is answered by a
magnetic storm on the earth. On the average the
connection is established; but there are many individual
instances of sun-spots occurring without any
answering magnetic thrill from the earth. To meet
this difficulty, Mr. E. W. Maunder has proposed a
view of the sun's electrical influence upon our earth,
which, whether it be proved or disproved in the
future, seems at present the most living attempt to
account for the observed facts. Briefly, he considers
it indubitably proved—
1. That our magnetic disturbances are connected
with the sun.
2. That the sun's action, of whatever nature, is
not from the sun as a whole, but from restricted
areas.
3. That the sun's action is not radiated, but restricted
in direction.
On his view, the great coronal rays or streamers
[pg 64]
seen in total eclipses (Plate VIII.) are lines of force,
and similarly the magnetic influence which the sun
exerts upon the earth acts along definite and restricted
lines. Thus a disturbance of great magnitude
upon the sun would only be followed by a
corresponding disturbance on the earth if the latter
happened to be at or near the point where it would
fall within the sweep of the line of magnetic force
emanating from the sun. In proportion as the line
of magnetic force approached to falling perpendicularly
on the earth, the magnetic disturbance
would be large: in proportion as it departed from
the perpendicular it would diminish until it vanished
finally altogether. The suggestion seems an inviting
one, and has at least revived very considerably
the interest in these phenomena.
Such, then, are the solar features which offer
themselves to direct observation by means of a small
telescope. The spots, apart from their own intrinsic
interest, are seen to furnish a fairly accurate method
by which the observer can determine for himself the
sun's rotation period. Their size may be approximately
measured, thus conveying to the mind some
idea of the enormous magnitude of the convulsions
which take place upon this vast globe. The spot
zones may be noted, together with the gradual shift
in latitude as the period approaches or recedes from
minimum; while observations of individual spots
may be conducted with a view to gathering evidence
which shall help either to confirm or to confute the
Wilsonian theory. In this latter department of
[pg 65]
observation the main requisite is that the work
should be done systematically. Irregular observation
is of little or no value; but steady work may
yield results of high importance. While, however,
systematic observation is desirable, it is not everyone
who has the time or the opportunity to give
this; and to many of us daily solar observation may
represent an unattainable ideal. Even if this be
the case, there still remains an inexhaustible fund
of beauty and interest in the sun-spots. It does
not take regular observation to enable one to be
interested in the most wonderful intricacy and
beauty of the solar detail, in its constant changes,
and in the ideas which even casual work cannot fail
to suggest as to the nature and mystery of that great
orb which is of such infinite importance to ourselves.
A small instrument, used in the infrequent intervals
which may be all that can be snatched from
the claims of other work, will give the user a far
more intelligent interest in the sun, and a far better
appreciation of its features, than can be gained by
the most careful study of books. In this, and in all
other departments of astronomy, there is nothing
like a little practical work to give life to the subject.
In the conduct of observation, however, regard
must be paid to the caution given at the beginning
of this chapter. Various methods have been adopted
for minimizing the intense glare and heat. For
small telescopes—up to 2½ inches or so—the common
device of the interposition of a coloured glass between
the eye-piece and the eye will generally be
[pg 66]
found sufficient on the score of safety, though other
arrangements may be found preferable. Such glasses
are usually supplied with small instruments, mounted
in brass caps which screw or slide on to the ends of
the various eye-pieces. Neutral tint is the best,
though a combination of green and red also does
well. Red transmits too much heat for comfort.
Should dark glasses not be supplied, it is easy to
make them by smoking a piece of glass to the
required depth, protecting it from rubbing by fastening
over it a covering glass which rests at each end
on a narrow strip of cardboard.
With anything larger than 2½ inches, dark glass
is never quite safe. A 3-inch refractor will be found
quite capable of cracking and destroying even a
fairly thick glass if observation be long continued.
The contrivance known as a polarizing eye-piece
was formerly pretty much beyond the reach of the
average amateur by reason of its costliness. Such
eye-pieces are now becoming much cheaper, and
certainly afford a most safe and pleasant way of
viewing the sun. They are so arranged that the
amount of light and heat transmitted can be reduced
at will, so as to render the use of a dark glass
unnecessary, thus enabling the observer to see all
details in their natural colouring. The ordinary
solar diagonal, in which the bulk of the rays is
rejected, leaving only a small portion to reach the
eye, is cheaper and satisfactory, though a light
screen-glass is still required with it. But unquestionably
the best general method of observing, and
[pg 67]
also the least costly, is that of projecting the sun's
image through the telescope upon a prepared white
surface, which may be of paper, or anything else
that may be found suitable.
To accomplish this a light framework may be
constructed in the shape of a truncated cone. At its
narrow end it slips or screws on to the eye-end of
the telescope, and it may be made of any length
required, in proportion to the size of solar disc which
it is desired to obtain. It should be covered with
black cloth, and its base may be a board with white
paper stretched on it to receive the image, which is
viewed through a small door in the side. In place
of the board with white paper, other expedients may
be tried. Noble recommends a surface of plaster of
Paris, smoothed while wet on plate glass, and this is
very good if you can get the plaster smooth enough.
I have found white paint, laid pretty thickly on glass
and then rubbed down to a smooth matt surface by
means of cuttle-fish bone, give very satisfactory
results. Should it be desired to exhibit the sun's
image to several people at once, this can easily be
done by projecting it upon a sheet of paper fastened
on a drawing-board, and supported at right angles
to the telescope by an easel. The framework, or
whatever takes its place, being in position, the
telescope is pointed at the sun by means of its
shadow; when this is perfectly round, or when the
shadow of the framework perfectly corresponds to
the shape of its larger end, the sun's image should
be in the field of view.
[pg 68]
CHAPTER IV
THE SUN'S SURROUNDINGS
We have now reached the point beyond which mere
telescopic power will not carry us, a point as definite
for the largest instrument as for the smallest. We
have traced what can be seen on the visible sun, but
beyond the familiar disc, and invisible at ordinary
seasons or with purely telescopic means, there lie
several solar features of the utmost interest and
beauty, the study of which very considerably
modifies our conception of the structure of our
system's ruler. These features are only revealed
in all their glory and wonder during the fleeting
moments in which a total eclipse is central to any
particular portion of the earth's surface.
A solar eclipse is caused by the fact that the moon,
in her revolution round the earth, comes at certain
periods between us and the sun, and obscures the
light of the latter body either partially or totally.
Owing to the fact that the plane of the orbit in
which the moon revolves round the earth does not
coincide with that in which the earth revolves round
the sun, the eclipse is generally only partial, the
[pg 69]
moon not occupying the exact line
between the centres of the sun and
the earth. The dark body of the
moon then appears to cut off a certain
portion, larger or smaller, of the sun's
light; but none of the extraordinary
phenomena to be presently described
are witnessed. Even during a partial
eclipse, however, the observer may
find considerable interest in watching
the outline of the dark moon, as projected
upon the bright background of
the sun. It is frequently jagged or
serrated, the projections indicating the
existence, on the margin of the lunar
globe, of lofty mountain ranges.
FIG. 19.—ECLIPSES OF THE SUN AND MOON.
Occasionally the conditions are
such that the moon comes centrally
between the earth and the sun
(Fig. 19), and then an eclipse occurs
which may be either total or annular.
The proportion between the respective
distances from us of the sun and the
moon is such that these two bodies,
so vastly different in real bulk, are
sensibly the same in apparent diameter,
so that a very slight modification of
the moon's distance is sufficient to
reduce her diameter below that of
the sun. The lunar orbit is not quite circular, but
has a small eccentricity. It may therefore happen
[pg 70]
that an eclipse occurs when the moon is nearest
the earth, at which point she will cover the sun's
disc with a little to spare; or the eclipse may occur
when she is furthest away from the earth, in which
case the lunar diameter will appear less than that of
the sun, and the eclipse will be only an annular one,
and a bright ring or 'annulus' of sunlight will be
seen surrounding the dark body of the moon at the
time when the eclipse is central.
All conditions being favourable, however—that is
to say, the eclipse being central, and the moon at
such a position in her orbit as to present a diameter
equal to, or slightly greater than, that of the sun—a
picture of extraordinary beauty and wonder reveals
itself the moment that totality has been established.
The centre of the view is the black disc of the
moon. From behind it on every side there streams
out a wonderful halo of silvery light which in some
of its furthest streamers may sometimes extend to a
distance of several million miles. In the Indian
Eclipse of 1898, for example, one streamer was
photographed by Mrs. Maunder, which extended to
nearly six diameters from the limb of the eclipsed
sun (Plate VIII.). The structure of this silvery
halo is of the most remarkable complexity, and
appears to be subject to continual variations, which
have already been ascertained to be to some extent
periodical and in sympathy with the sun-spot period.
At its inner margin this halo rests upon a ring of
crimson fire which extends completely round the
sun, and throws up here and there great jets or
[pg 71]
waves, which frequently assume the most fantastic
forms and rise to heights varying from 20,000 to
100,000 miles, or in extreme instances to a still
greater height. To these appearances astronomers
have given the names of the Corona, the Chromosphere,
and the Prominences. The halo of silvery
light is the Corona, the ring of crimson fire the
Chromosphere, and the jets or waves are the
Prominences.
PLATE VIII.
Coronal Streamers: Eclipse of 1898. From Photographs by Mrs. Maunder.
The Corona is perhaps the most mysterious of all
the sun's surroundings. As yet its nature remains
undetermined, though the observations which have
been made at every eclipse since attention was first
directed to it have been gradually suggesting and
strengthening the idea that there exists a very close
analogy between the coronal streamers and the
Aurora or the tails of comets. The extreme rarity
of its substance is conclusively proved by the fact
that such insubstantial things as comets pass
through it apparently unresisted and undelayed.
Its structure presents variations in different latitudes.
Near the poles it exhibits the appearance
of brushes of light, the rays shooting out from the
sun towards each summit of his axis, while the
equatorial rays curve over, presenting a sort of fish-tail
appearance. These variations are modified, as
already mentioned, by some cause which is at all
events coincident with the sun-spot period. At
minimum the corona presents itself with polar
brushes of light and fish-tail equatorial rays, the
latter being sometimes of the most extraordinary
[pg 72]
length, as in the case of the eclipse of July 29,
1878, when a pair of these wonderful streamers
extended east and west of the eclipsed sun to a
distance of at least 10,000,000 miles.
When an eclipse occurs at a spot-maximum, the
distribution of the coronal features is found to have
entirely changed. Instead of being sharply divided
into polar brushes and equatorial wings, the
streamers are distributed fairly evenly around the
whole solar margin, in a manner suggesting the
rays from a star, or a compass-card ornament. The
existence of this periodic change has been repeatedly
confirmed, and there can be no doubt that the
corona reflects in its structure the system of variation
which prevails upon the sun. 'The form of
the corona,' says M. Deslandres, 'undergoes
periodical variations, which follow the simultaneous
periodical variations already ascertained for spots,
faculæ, prominences, and terrestrial magnetism.'
Certainty as to its composition has not yet been
attained; nor is this to be wondered at, for the
corona is only to be seen in the all too brief
moments during which a total eclipse is central,
and then only over narrow tracts of country, and
all attempts to secure photographs of it at other
times have hitherto failed. When examined with
the spectroscope, it yields evidence that its light is
derived from three sources—from the incandescence
of solid or liquid particles, from reflected sunshine,
and from gaseous emissions. The characteristic
feature of the coronal spectrum is a bright green
[pg 73]
line belonging to an unknown element which has
been named 'coronium.'
The Chromosphere and the Prominences, unlike
the elusive corona, may now be studied continuously
by means of the spectroscope, and instruments are
now made at a comparatively moderate price, which,
in conjunction with a small telescope—3 inches will
suffice—will enable the observer to secure most
interesting and instructive views of both. The
chromosphere is, to use Miss Clerke's expression,
'a solar envelope, but not a solar atmosphere.' It
surrounds the whole globe of the sun to a depth of
probably from 3,000 to 4,000 miles, and has been
compared to an ocean of fire, but seems rather to
present the appearance of a close bristling covering
of flames which rise above the surface of the visible
sun like the blades of grass upon a lawn. Any one
of these innumerable flames may be elevated into
unusual proportions in obedience to the vast and
mysterious forces which are at work beneath, and
then becomes a prominence. On the whole the
constitution of the chromosphere is the same as that
of the prominences. Professor Young has found
that its normal constituents are hydrogen, helium,
coronium, and calcium. But whenever there is any
disturbance of its surface, the lines which indicate the
presence of these substances are at once reinforced
by numbers of metallic lines, indicating the presence
of iron, sodium, magnesium, and other substances.
The scale to which these upheavals attain in the
prominences is very remarkable. For example,
[pg 74]
Young records the observation of a prominence on
October 7, 1880. When first seen, at about
10.30 a.m., it was about 40,000 miles in height
and attracted no special attention. Half an hour
later it had doubled its height. During the next
hour it continued to soar upwards until it reached
the enormous altitude of 350,000 miles, and then
broke into filaments which gradually faded away,
until by 12.30 there was nothing left of it. On
another occasion he recorded one which darted
upwards in half an hour from a moderate elevation
to a height of 200,000 miles, and in which clouds of
hydrogen must have been hurled aloft with a speed
of at least 200 miles per second. (Plate IX. gives
a representation of the chromosphere and prominences
from a photograph by M. Deslandres.)
Between the chromosphere and the actual glowing
surface of the sun which we see lies what is known
as the 'reversing layer,' from the fact that owing to
its presence the dark lines of the solar spectrum are
reversed in the most beautiful way during the second
at the beginning and end of totality in an eclipse.
Young, who was the first to observe this phenomenon
(December 22, 1870), remarks of it that as
soon as the sun has been hidden by the advancing
moon, 'through the whole length of the spectrum,
in the red, the green, the violet, the bright lines
flash out by hundreds and thousands, almost
startlingly; as suddenly as stars from a bursting
rocket-head, and as evanescent, for the whole thing
is over within two or three seconds.'
PLATE IX.
The Chromosphere and Prominences, April 11, 1894. Photographed by
M. H. Deslandres.
[pg 75]
The spectrum of the reversing layer has since
been photographed on several occasions—first by
Shackleton, at Novaya Zemlya, on August 9, 1896—and
its bright lines have been found to be true
reversals of the dark lines of the normal solar
spectrum. This layer may be described as a thin
mantle, perhaps 500 miles deep, of glowing metallic
vapours, surrounding the whole body of the sun, and
normally, strange to say, in a state of profound
quiescence. Its presence was of course an integral
part of Kirchhoff's theory of the mode in which the
dark lines of the solar spectrum were produced.
Such a covering was necessary to stop the rays
whose absence makes the dark lines; and it was
assumed that the rays so stopped would be seen
bright, if only the splendour of the solar light could
be cut off. These assumptions have therefore been
verified in the most satisfactory manner.
Thus, then, the structure of the sun as now known
is very different from the conception of it which
would be given by mere naked-eye, or even telescopic,
observation. We have first the visible bright
surface, or photosphere, with its spots, faculæ, and
mottling, and surrounded by a kind of atmosphere
which absorbs much of its light, as is evidenced by
the fact that the solar limb is much darker than the
centre of the disc (Plate V.); next the reversing
layer, consisting of an envelope of incandescent
vapours, which by their absorption of the solar rays
corresponding to themselves give rise to the dark
lines in the spectrum. Beyond these again lies the
[pg 76]
chromosphere, rising into gigantic eruptive or cloud-like
forms in the prominences; and yet further out
the strange enigmatic corona.
It must be confessed that the reversing layer, the
chromosphere, and the corona lie somewhat beyond
the bounds and purpose of this volume; but without
mention of them any account of the sun is hopelessly
incomplete, and it is not at all improbable that a few
years may see the spectroscope so brought within
the reach of ordinary observers as to enable them in
great measure to realize for themselves the facts
connected with the complex structure of the sun.
In any case, the mere recital of these facts is fitted
to convey to the mind a sense of the utter inadequacy
of our ordinary conceptions of that great
body which governs the motions of our earth, and
supplies to it and to the other planets of our system
life and heat, light and guidance. With the unaided
eye we view the sun as a small tranquil white disc;
the telescope reveals to us that it is a vast globe
convulsed by storms which involve the upheaval or
submersion, within a few hours, of areas far greater
than our own world; the spectroscope or the total
eclipse adds to this revelation the further conception
of a sweltering ocean of flame surrounding the
whole solar surface, and rising in great jets of fire
which would dissolve our whole earth as a drop of
wax is melted in the flame of a candle; while
beyond that again the mysterious corona stretches
through unknown millions of miles its streamers of
silvery light—the great enigma of solar physics.
[pg 77]
Other bodies in the universe present us with
pictures of beautiful symmetry and vast size: some
even within our own system suggest by their
appearance the presence within their frame of
tremendous forces which are still actively moulding
them; but the sun gives us the most stupendous
demonstration of living force that the mind of man
can apprehend. Of course there are many stars
which are known to be suns on which processes
similar to those we have been considering are being
carried on on a yet vaster scale; but the nearness
of our sun brings the tremendous energy of these
processes home to us in a way that impresses the
mind with a sense almost of fear.
'Is it possible,' says Professor Newcomb, 'to
convey to the mind any adequate conception of the
scale on which natural operations are here carried
on? If we call the chromosphere an ocean of fire,
we must remember that it is an ocean hotter than
the fiercest furnace, and as deep as the Atlantic is
broad. If we call its movements hurricanes, we
must remember that our hurricanes blow only about
100 miles an hour, while those of the chromosphere
blow as far in a single second. They are such hurricanes
as, coming down upon us from the north, would,
in thirty seconds after they had crossed the St. Lawrence,
be in the Gulf of Mexico, carrying with them
the whole surface of the continent in a mass not
simply of ruin, but of glowing vapour.... When
we speak of eruptions, we call to mind Vesuvius
burying the surrounding cities in lava; but the solar
[pg 78]
eruptions, thrown 50,000 miles high, would engulf
the whole earth, and dissolve every organized being
on its surface in a moment. When the mediæval
poets sang, "Dies iræ, dies illa, solvet sæclum
in favilla," they gave rein to their wildest imagination
without reaching any conception of the magnitude
or fierceness of the flames around the
sun.'
The subject of the maintenance of the sun's light
and heat is one that scarcely falls within our scope,
and only a few words can be devoted to it. It is
practically impossible for us to attain to any adequate
conception of the enormous amount of both which
is continually being radiated into space. Our own
earth intercepts less than the two thousand millionth
part of the solar energy. It has been estimated
that if a column of ice 2¼ miles in diameter could
be erected to span the huge interval of 92,700,000
miles between the earth and the sun, and if the sun
could concentrate the whole of his heat upon it,
this gigantic pillar of ice would be dissolved in a
single second; in seven more it would be vaporized.
The amount of heat developed on each square foot
of solar surface is 'equivalent to the continuous
evolution of about 10,000 horse-power'; or, as otherwise
stated, is equal to that which would be produced
by the hourly burning of nine-tenths of a ton of
anthracite coal on the same area of 1 square
foot.
It is evident, therefore, that mere burning cannot
be the source of supply. Lord Kelvin has shown
[pg 79]
that the sun, if composed of solid coal, would burn
itself out in about 6,000 years.
Another source of heat may be sought in the
downfall of meteoric bodies upon the solar surface;
and it has been calculated that the inrush of all the
planets of our system would suffice to maintain
the present energy for 45,604 years. But to suppose
the existence near the sun of anything like
the amount of meteoric matter necessary to account,
on this theory, for the annual emission of heat
involves consequences which are quite at variance
with observed facts, though it is possible, or even
practically certain, that a small proportion of the
solar energy is derived from this source.
We are therefore driven back upon the source
afforded by the slow contraction of the sun. If
this contraction happens, as it must, an enormous
amount of heat must be developed by the process,
so much so that Helmholtz has shown that an
annual contraction of 250 feet would account for
the total present emission. This contraction is so
slow that about 9,500 years would need to elapse
before it became measurable with anything like
certainty. In the meantime, then, we may assume
as a working hypothesis that the light and heat of
the central body of our system are maintained,
speaking generally, by his steady contraction. Of
course this process cannot have gone on, and cannot
go on, indefinitely; but as the best authorities have
hitherto regarded the date when the sun shall have
shrunk so far as to be no longer able to support life
[pg 80]
on the earth as distant from us by some ten million
years, and as the latest investigations on the subject,
those of Dr. See, point in the direction of a very
large extension of this limit, we may have reasonable
comfort in the conviction that the sun will last
our time.
[pg 81]
CHAPTER V
MERCURY
The planet nearest to the sun is not one which has
proved itself particularly attractive to observers in
the past; and the reasons for its comparative unattractiveness
are sufficiently obvious. Owing to
the narrow limits of his orbit, he never departs
further from the sun either East or West than between
27° and 28°, and the longest period for which
he can be seen before sunrise or after sunset is two
hours. It follows that, when seen, he is never very
far from the horizon, and is therefore enveloped in
the denser layers of our atmosphere, and presents
the appearance sadly familiar to astronomers under
the name of 'boiling,' the outlines of the planet
being tremulous and confused. Of course, observers
who have powerful instruments provided with graduated
circles can find and follow him during the day,
and it is in daylight that nearly all the best observations
have been secured. But with humbler
appliances observation is much restricted; and, in
fact, probably many observers have never seen the
planet at all.
[pg 82]
Views of Mercury, however, such as they are, are
by no means so difficult to secure as is sometimes
supposed. Denning remarks that he has seen the
planet on about sixty-five occasions with the naked
eye—that in May, 1876, he saw it on thirteen
different evenings, and on ten occasions between
April 22 and May 11, 1890; and he states it as his
opinion that anyone who will make it a practice to
obtain naked-eye views should succeed from about
twelve to fifteen times in the year. During the
spring of 1905, to take a recent example, Mercury
was quite a conspicuous object for some time in the
Western sky, close to the horizon, and there was
no difficulty whatever in obtaining several views of
him both with the telescope and with the naked
eye, though the disc was too much disturbed by
atmospheric tremors for anything to be made of it
telescopically. In his little book, 'Half-hours with
the Telescope,' Proctor gives a method of finding the
planet which would no doubt prove quite satisfactory
in practice, but is somewhat needlessly elaborate.
Anyone who takes the pains to note those dates
when Mercury is most favourably placed for observation—dates
easily ascertained from Whitaker or
any other good almanac—and to carefully scan the
sky near the horizon after sunset either with the
naked eye, or, better, with a good binocular, will
scarcely fail to detect the little planet which an old
English writer more graphically than gracefully calls
'a squinting lacquey of the sun.'
Mercury is about 3,000 miles in diameter, and
[pg 83]
circles round the sun at a mean distance of
36,000,000 miles. His orbit is very eccentric, so
that when nearest to the sun this distance is reduced
to 28,500,000, while when furthest away from him
it rises to 43,500,000. The proportion of sunlight
which falls upon the planet must therefore vary
considerably at different points of his orbit. In
fact, when he is nearest to the sun he receives nine
times as much light and heat as would be received
by an equal area of the earth; but when the conditions
are reversed, only four times the same amount.
The bulk of the planet is about one-nineteenth that
of the earth, but its weight is only one-thirtieth,
so that its materials are proportionately less dense
than those of our own globe. It is about 3½ times
as dense as water, the corresponding figure for the
earth being rather more than 5½.
Further, it is apparent that the materials of which
Mercury's globe is composed reflect light very
feebly. It has been calculated that the planet
reflects only 17 per cent. of the light which falls
upon it, 83 per cent. being absorbed; and this fact
obviously carries with it the conclusion that the
atmosphere of this little world cannot be of any
great density. For clouds in full sunlight are
almost as brilliantly white as new-fallen snow,
and if Mercury were surrounded with a heavily
cloud-laden atmosphere, he would reflect nearly five
times the amount of light which he at present sends
out into space.
As his orbit falls entirely within that of our own
[pg 84]
earth, Mercury, like his neighbour Venus, exhibits
phases. When nearest to us the planet is 'new,'
when furthest from us it is 'full,' while at the stages
intermediate between these points it presents an
aspect like that of the moon at its first and third
quarters. It may thus be seen going through the
complete series from a thin crescent up to a completely
rounded disc. The smallness of its apparent
diameter, and the poor conditions under which it is
generally seen, make the observation of these phases
by no means so easy as in the case of Venus; yet
a small instrument will show them fairly well.
Observers seem generally to agree that the surface
has a dull rosy tint, and a few faint markings have,
by patient observation, been detected upon it
(Fig. 20); but these are far beyond the power of
small telescopes. Careful attention to them and to
[pg 85]
the rate of their apparent motion across the disc has
led to the remarkable conclusion that Mercury takes
as long to rotate upon his axis as he does to complete
his annual revolution in his orbit; in other
words, that his day and his year are of the same
length—namely, eighty-eight of our days. This
conclusion, when announced in 1882 by the well-known
Italian observer Schiaparelli, was received
with considerable hesitation. It has, however, been
confirmed by many observers, notably by Lowell at
Flagstaff Observatory, Arizona, and is now generally
received, though some eminent astronomers still
maintain that really nothing is certainly known as
to the period of rotation.
FIG. 20.—MERCURY AS A MORNING STAR. W. F. DENNING,
10-INCH REFLECTOR.
If the long period be accepted, it follows that
Mercury must always turn the same face to the sun—that
one of his hemispheres must always be
scorching under intense heat, and the other held in
the grasp of an unrelenting cold of which we can
have no conception. 'The effects of these arrangements
upon climate,' says Miss Agnes Clerke, 'must
be exceedingly peculiar.... Except in a few
favoured localities, the existence of liquid water
must be impossible in either hemisphere. Mercurian
oceans, could they ever have been formed,
should long ago have been boiled off from the hot
side, and condensed in "thick-ribbed ice" on the
cold side.'
From what has been said it will be apparent that
Mercury is scarcely so interesting a telescopic object
as some of the other planets. Small instruments
[pg 86]
are practically ruled out of the field by the diminutive
size of the disc which has to be dealt with, and
the average observer is apt to be somewhat lacking
in the patience without which satisfactory observations
of an object so elusive cannot be secured. At
the same time there is a certain amount of satisfaction
and interest in the mere detection of the little
sparkling dot of light in the Western sky after the
sun has set, or in the Eastern before it has risen;
and the revelation of the planet's phase, should the
telescope prove competent to accomplish it, gives
better demonstration than any diagram can convey
of the interior position of this little world. It is
consoling to think that even great telescopes have
made very little indeed of the surface of Mercury.
Schiaparelli detected a number of brownish stripes
and streaks, which seemed to him sufficiently permanent
to be made the groundwork of a chart, and
Lowell has made a remarkable series of observations
which reveal a globe seamed and scarred with long
narrow markings; but many observers question the
reality of these features altogether.
It is perhaps just within the range of possibility
that, even with a small instrument, there may be
detected that blunting of the South horn of the
crescent planet which has been noticed by several
reliable observers. But caution should be exercised
in concluding that such a phenomenon has been
seen, or that, if seen, it has been more than an
optical illusion. Those who have viewed Mercury
under ordinary conditions of observation will be well
[pg 87]
aware how extremely difficult it is to affirm positively
that any markings on the surface or any deformations
of the outline of the disc are real and actual
facts, and not due to the atmospheric tremors which
affect the little image.
Interesting, though of somewhat rare occurrence,
are the transits of Mercury, when the planet comes
between us and the sun, and passes as a black
circular dot across the bright solar surface. The
first occasion on which such a phenomenon was
observed was November 7, 1631. The occurrence
of this transit was predicted by Kepler four years in
advance; and the transit itself was duly observed
by Gassendi, though five hours later than Kepler's
predicted time. It gives some idea of the uncertainty
which attended astronomical calculations
in those early days to learn that Gassendi considered
it necessary to begin his observations two days in
advance of the time fixed by Kepler. If, however,
the time of a transit can now be predicted with
almost absolute accuracy, it need not be forgotten
that this result is largely due to the labours of men
who, like Kepler, by patient effort and with most
imperfect means, laid the foundations of the most
accurate of all sciences.
The next transit of Mercury available for observation
will take place on November 14, 1907. It may
be noted that during transits certain curious appearances
have been observed. The planet, for example,
instead of appearing as a black circular dot, has been
seen surrounded with a luminous halo, and marked
[pg 88]
by a bright spot upon its dark surface. No satisfactory
explanation of these appearances has been
offered, and they are now regarded as being of the
nature of optical illusions, caused by defects in the
instruments employed, or by fatigue of the eye. It
might, however, be worth the while of any who
have the opportunity of observing the transit of 1907
to take notice whether these features do or do not
present themselves. For their convenience it may
be noted that the transit will begin about eleven
o'clock on the forenoon of November 14, and end
about 12.45.
[pg 89]
CHAPTER VI
VENUS
Next in order to Mercury, proceeding outwards
from the sun, comes the planet Venus, the twin-sister,
so to speak, of the earth, and familiar more
or less to everybody as the Morning and Evening
Star. The diameter of Venus, according to
Barnard's measures with the 36-inch telescope of
the Lick Observatory, is 7,826 miles; she is therefore
a little smaller than our own world. Her
distance from the sun is a trifle more than 67,000,000
miles, and her orbit, in strong contrast with that of
Mercury, departs very slightly from the circular.
Her density is a little less than that of the
earth.
There is no doubt that, to the unaided eye,
Venus is by far the most beautiful of all the planets,
and that none of the fixed stars can for a moment
vie with her in brilliancy. In this respect she is
handicapped by her position as an inferior planet,
for she never travels further away from the sun than
48°, and, even under the most favourable circumstances,
cannot be seen for much more than four
[pg 90]
hours after sunset. Thus we never have the
opportunity of seeing her, as Mars and Jupiter can
be seen, high in the South at midnight, and far
above the mists of the horizon. Were it possible
to see her under such conditions, she would indeed
be a most glorious object. Even as it is, with all
the disadvantages of a comparatively low position
and a denser stratum of atmosphere, her brilliancy is
extremely striking, having been estimated, when at
its greatest, at about nine times that of Sirius, which
is the brightest of all the fixed stars, and five times
that of Jupiter when the giant planet is seen to the
best advantage. It is, in fact, so great that, when
approaching its maximum, the shadows cast by the
planet's light are readily seen, more especially if the
object casting the shadow have a sharply defined
edge, and the shadow be received upon a white
surface—of snow, for instance. This extreme
brilliance points to the fact that the surface of Venus
reflects a very large proportion of the sunlight which
falls upon it—a proportion estimated as being at
least 65 per cent., or not very much less than that
reflected by newly fallen snow. Such reflective
power at once suggests an atmosphere very dense
and heavily cloud-laden; and other observations
point in the same direction. So that in the very
first two planets of the system we are at once confronted
with that diversity in details which coexists
throughout with a broad general likeness as to
figure, shape of orbit, and other matters. Mercury's
reflective power is very small, that of Venus is
[pg 91]
exceedingly great; Mercury's atmosphere seems to
be very attenuated, that of Venus, to all appearance,
is much denser than that of our own earth.
Periodically, when Venus appears in all her
splendour in the Western sky, one meets with the
suggestion that we are having a re-appearance of the
Star of Bethlehem; and it seems to be a perpetual
puzzle to some people to understand how the same
body can be both the Morning and the Evening
Star. Those who have paid even the smallest
attention to the starry heavens are not, however, in
the least likely to make any mistake about the
sparkling silver radiance of Venus; and it would
seem as though the smallest application of common-sense
to the question of the apparent motion of
a body travelling round an almost circular orbit
which is viewed practically edgewise would solve
for ever the question of the planet's alternate appearance
on either side of the sun. Such an orbit must
appear practically as a straight line, with the sun at
its middle point, and along this line the planet will
appear to travel like a bead on a wire, appearing
now on one side of the sun, now on another. If the
reader will draw for himself a diagram of a circle
(sufficiently accurate in the circumstances), with the
sun in the centre, and divide it into two halves by a
line supposed to pass from his eye through the sun,
he will see at once that when this circle is viewed
edgewise, and so becomes a straight line, a planet
travelling round it is bound to appear to move back
and forward along one half of it, and then to repeat
[pg 92]
the same movement along the other half, passing
the sun in the process.
Like Mercury, and for the same reason of a
position interior to our orbit, Venus exhibits phases
to us, appearing as a fully illuminated disc when she
is furthest from the earth, as a half-moon at the two
intermediate points of her orbit, and as a new moon
when she is nearest to us. The actual proof of the
existence of these phases was one of the first-fruits
which Galileo gathered by means of his newly
invented telescope. It is said that Copernicus predicted
their discovery, and they certainly formed
one of the conclusive proofs of the correctness of
his theory of the celestial system. It was the somewhat
childish custom of the day for men of science
to put forth the statement of their discoveries in the
form of an anagram, over which their fellow-workers
might rack their brains; probably this was done
somewhat for the same reason which nowadays
makes an inventor take out a patent, lest someone
should rob the discoverer of the credit of his
discovery before he might find it convenient to
make it definitely public. Galileo's anagram, somewhat
more poetically conceived than the barbarous
alphabetic jumble in which Huygens announced his
discovery of the nature of Saturn's ring, read as
follows: 'Hæc immatura a me jam frustra leguntur
o. y.' This, when transposed into its proper order,
conveyed in poetic form the substance of the
discovery: 'Cynthiæ figuras æmulatur Mater
Amorum' (The Mother of the Loves [Venus]
[pg 93]
imitates the phases of Cynthia). It is true that two
letters hang over the end of the original sentence,
but too much is not to be expected of an anagram.
As a telescopic object, Venus is apt to be a little
disappointing. Not that her main features are
difficult to see, or are not beautiful. A 2-inch
telescope will reveal her phases with the greatest
ease, and there are few more exquisite sights than
that presented by the silvery crescent as she approaches
inferior conjunction. It is a picture which
in its way is quite unique, and always attractive
even to the most hardened telescopist.
Still, what the observer wants is not merely confirmation
of the statement that Venus exhibits
phases. The physical features of a planet are
always the most interesting, and here Venus disappoints.
That very brilliant lustre which makes
her so beautiful an object to the naked eye, and
which is even so exquisite in the telescopic view, is
a bar to any great progress in the detection of the
planet's actual features. For it means that what we
are seeing is not really the surface of Venus, but
only the sunward side of a dense atmosphere—the
'silver lining' of heavy clouds which interpose
between us and the true surface of the planet, and
render it highly improbable that anything like satisfactory
knowledge of her features will ever be
attained. Newcomb, indeed, roundly asserts that
all markings hitherto seen have been only temporary
clouds and not genuine surface markings at all;
though this seems a somewhat absolute verdict in
[pg 94]
view of the number of skilled observers who have
specially studied the planet and assert the objective
reality of the markings they have detected. The
blunting of the South horn of the planet, visible in
Mr. MacEwen's fine drawing (Plate X.), is a feature
which has been noted by so many observers that its
reality must be conceded. On the other hand, some
of the earlier observations recording considerable
irregularities of the terminator (margin of the planet
between light and darkness), and detached points of
light at one of the horns, must seemingly be given
up. Denning, one of the most careful of observers,
gives the following opinion: 'There is strong
negative evidence among modern observations as
to the existence of abnormal features, so that the
presence of very elevated mountains must be regarded
as extremely doubtful.... The detached
point at the South horn shown in Schröter's telescope
was probably a false appearance due to
atmospheric disturbances or instrumental defects.'
It will be seen, therefore, that the observer should
be very cautious in inferring the actual existence of
any abnormal features which may be shown by a
small telescope; and the more remarkable the
features shown, the more sceptical he may reasonably
be as to their reality. The chances are somewhat
heavily in favour of their disappearance under
more favourable conditions of seeing.
PLATE X.
Venus. H. MacEwen. 5-inch Refractor.
The same remark applies, with some modifications,
to the dark markings which have been detected on
the planet by all sorts of observers with all sorts of
[pg 95]
telescopes. There is no doubt that faint grey
markings, such as those shown in Plate X., are to
be seen; the observations of many skilled observers
put this beyond all question. Even Denning, who
says that personally he has sometimes regarded the
very existence of these markings as doubtful, admits
that 'the evidence affirming their reality is too
weighty and too numerously attested to allow them
to be set aside'; and Barnard, observing with the
Lick telescope, says that he has repeatedly seen
markings, but always so 'vague and ill-defined that
nothing definite could be made of them.'
The observations of Lowell and Douglass at
Flagstaff, Arizona, record quite a different class of
markings, consisting of straight, dark, well-defined
lines; as yet, however, confirmation of these remarkable
features is scanty, and it will be well for the
beginner who, with a small telescope and in ordinary
conditions of observing, imagines he has detected
such markings to be rather more than less doubtful
about their reality. The faint grey areas, which are
real features, at least of the atmospheric envelope,
if not of the actual surface, are beyond the reach of
small instruments. Mr. MacEwen's drawings,
which accompany this chapter, were made with a
5-inch Wray refractor, and represent very well the
extreme delicacy of these markings. I have
suspected their existence when observing with an
8½-inch With reflector in good air, but could never
satisfy myself that they were really seen.
Up till the year 1890 the rotation period of
[pg 96]
Venus was usually stated at twenty-three hours
twenty-one minutes, or thereby, though this figure
was only accepted with some hesitation, as in order
to arrive at it there had to be some gentle squeezing
of inconvenient observations. But in that year
Schiaparelli announced that his observations were
only consistent with a long period of rotation, which
could not be less than six months, and was not
greater than nine. The announcement naturally
excited much discussion. Schiaparelli's views were
strongly controverted, and for a time the astronomical
world seemed to be almost equally divided
in opinion. Gradually, however, the conclusion has
come to be more and more accepted that Venus, like
Mercury, rotates upon her axis in the same time as
she takes to make her journey round the sun—in
other words, that her day and her year are of the
same length, amounting to about 225 of our days.
In 1900 the controversy was to some extent reopened
by the statement of the Russian astronomer
Bélopolsky that his spectroscopic investigations
pointed to a much more rapid rotation—to a period,
indeed, considerably shorter than twenty-four hours.
It is difficult, however, to reconcile this with the
absence of polar flattening in the globe of Venus.
Lowell's spectroscopic observations are stated by
him to point to a period in accordance with his
telescopic results—namely, 225 days. The matter
can scarcely be regarded as settled in the meantime,
but the balance of evidence seems in favour of the
longer period.
[pg 97]
Another curious and unexplained feature in connection
with the planet is what is frequently termed
the 'phosphorescence' of the dark side. This is an
appearance precisely similar to that seen in the case
of the moon, and known as 'the old moon in the
young moon's arms.' The rest of the disc appears
within the bright crescent, shining with a dull rusty
light. In the case of Venus, however, an explanation
is not so easily arrived at as in that of the
moon, where, of course, earth-light accounts for the
visibility of the dark portion. Had the planet been
possessed of a satellite, the explanation might have
lain there; but Venus has no moon, and therefore
no moonlight to brighten her unilluminated portion;
and our world is too far distant for earth-shine to
afford an explanation. It has been suggested that
electrical discharges similar to the aurora may be at
the bottom of the mystery; but this seems a little
far-fetched, as does also the attribution of the
phenomenon to real phosphorescence of the oceans
of Venus. Professor Newcomb cuts the Gordian
knot by observing: 'It is more likely due to an
optical illusion.... To whatever we might
attribute the light, it ought to be seen far better
after the end of twilight in the evening than during
the daytime. The fact that it is not seen then
seems to be conclusive against its reality.' But the
appearance cannot be disposed of quite so easily as
this, for it is not accurate to say that it is only seen
in the daytime, and against Professor Newcomb's
dictum may be set the judgment of the great
[pg 98]
majority of the observers who have made a special
study of the planet.
We may, however, safely assign to the limbo of
exploded ideas that of the existence of a satellite of
Venus. For long this object was one of the most
persistent of astronomical ghosts, and refused to be
laid. Observations of a companion to the planet,
much smaller, and exhibiting a similar phase, were
frequent during the eighteenth century; but no such
object has presented itself to the far finer instruments
of modern times, and it may be concluded
that the moon of Venus has no real existence.
Venus, like Mercury, transits the sun's disc, but
at much longer intervals which render her transits
among the rarest of astronomical events. Formerly
they were also among the most important, as they
were believed to furnish the most reliable means for
determining the sun's distance; and most of the
estimates of that quantity, up to within the last
twenty-five years, were based on transit of Venus
observations. Now, however, other methods, more
reliable and more readily applicable, are coming into
use, and the transit has lost somewhat of its former
importance. The interest and beauty of the
spectacle still remain; but it is a spectacle not
likely to be seen by any reader of these pages, for
the next transit of Venus will not take place until
June, 2004.
As already indicated, Venus presents few opportunities
for useful observation to the amateur.
The best time for observing, as in the case of
[pg 99]
Mercury, is in broad daylight; and for this, unless
in exceptional circumstances, graduated circles and
a fairly powerful telescope are required. Practically
the most that can be done by the possessor of a
small instrument is to convince himself of the reality
of the phases, and of the non-existence of a satellite
of any size, and to enjoy the exquisite and varying
beauty of the spectacle which the planet presents.
Should his telescope be one of the small instruments
which show hard and definite markings on the
surface, he may also consider that he has learned a
useful lesson as to the possibility of optical illusion,
and, incidentally, that he may be well advised to
procure a better glass when the opportunity of doing
so presents itself. The 'phosphorescence' of the
dark side may be looked for, and it may be noted
whether it is not seen after dark, or whether it persists
and grows stronger. Generally speaking,
observations should be made as early in the
evening as the planet can be seen in order that
the light of the sky may diminish as much as
possible the glare which is so evident when Venus
is viewed against a dark background.
[pg 100]
CHAPTER VII
THE MOON
Our attention is next engaged by the body which is
our nearest neighbour in space and our most faithful
attendant and useful servant. The moon is an orb
of 2,163 miles in diameter, which revolves round
our earth in a slightly elliptical orbit, at a mean
distance of about 240,000 miles. The face which
she turns to us is a trifle greater in area than the
Russian Empire, while her total surface is almost
exactly equal to the areas of North and South
America, islands excluded. Her volume is about
2⁄99
of that of the earth; her materials are, however,
much less dense than those of which our world is
composed, so that it would take about eighty-one
moons to balance the earth. One result of these
relations is that the force of gravity at the lunar surface
is only about one-sixth of that at the surface of
the earth, so that a twelve-stone man, if transported
to the moon, would weigh only two stone, and would
be capable of gigantic feats in the way of leaping
and lifting weights. The fact of the diminished
force of gravity is of importance in the consideration
of the question of lunar surfacing.
[pg 101]
FIG. 21.—THE TIDES.
A, Spring Tide (New Moon); B, Neap Tide.
The most conspicuous service which our satellite
performs for us is that of raising the tides. The
complete statement of the manner in which she does
this would be too long for our pages; but the
general outline of it will be seen from the accompanying
rough diagram (Fig. 21), which, it
must be remembered, makes no attempt at representing
the scale either of the bodies concerned or
of their distances from one another, but simply
pictures their relations to one another at the times
of spring and neap tides. The moon (M in
Fig. 21, A) attracts the whole earth towards it.
Its attraction is greatest at the point nearest to it,
and therefore the water on the moonward side is
[pg 102]
drawn up, as it were, into a heap, making high tide
on that side of the earth. But there is also high
tide at the opposite side, the reason being that the
solid body of the earth, which is nearer to the moon
than the water on the further side, is more strongly
attracted, and so leaves the water behind it. Thus
there are high tides at the two opposite sides of the
earth which lie in a straight line with the moon, and
corresponding low tides at the intermediate positions.
Tides are also produced by the attraction of the sun,
but his vastly greater distance causes his tide-producing
power to be much less than that of the moon.
His influence is seen in the difference between spring
and neap tides. Spring tides occur at new or full
moon (Fig. 21, A, case of new moon). At these
two periods the sun, moon, and earth, are all in one
straight line, and the pull of the sun is therefore
added to that of the moon to produce a spring tide.
At the first and third quarters the sun and moon are
at right angles to one another; their respective pulls
therefore, to some extent, neutralize each other, and
in consequence we have neap tide at these seasons.
PLATE XI.
The Moon, April 5, 1900. Paris Observatory.
No one can fail to notice the beautiful set of
phases through which the moon passes every
month. A little after the almanac has announced
'new moon,' she begins to appear as a thin crescent
low down in the West, and setting shortly after the
sun. Night by night we can watch her moving
eastward among the stars, and showing more and
more of her illuminated surface, until at first quarter
half of her disc is bright. The reader must distinguish
[pg 103]
this real eastward movement from the
apparent east to west movement due to the daily
rotation of the earth. Its reality can readily be
seen by noting the position of the moon relatively
to any bright star. It will be observed that if she
is a little west of the star on one night, she will
have moved to a position a little east of it by the
next. Still moving farther East, she reaches full,
and is opposite to the sun, rising when he sets, and
setting when he rises. After full, her light begins
to wane, till at third quarter the opposite half of her
disc is bright, and she is seen high in the heavens in
the early morning, a pale ghost of her evening
glories. Gradually she draws nearer to the sun,
thinning down to the crescent shape again until she
is lost once more in his radiance, only to re-emerge
and begin again the same cycle of change.
The time which the moon actually takes to complete
her journey round the earth is twenty-seven
days, seven hours, and forty-three minutes; and if
the earth were fixed in space, this period, which is
called the sidereal month, would be the actual time
from new moon to new moon. While the moon has
been making her revolution, however, the earth has
also been moving onwards in its journey round the
sun, so that the moon has a little further to travel in
order to reach the 'new moon' position again, and
the time between two new moons amounts to
twenty-nine days, twelve hours, forty-four minutes.
This period is called a lunar month, and is also the
synodic period of our satellite, a term which signifies
[pg 104]
generally the period occupied by any planet or
satellite in getting back to the same position with
respect to the sun, as observed from the earth.
The fact that the moon shows phases signifies
that she shines only by reflected light; and it is
surprising to notice how little of the light that falls
upon her is really reflected by her. On an ordinarily
clear night most people would probably say that the
moon is much brighter than any terrestrial object
viewed in the daytime, when it also is lit by the sun,
as the moon is. Yet a very simple comparison will
show that this is not so. If the moon be compared
during the daytime with the clouds floating around
her, she will be seen to be certainly not brighter
than they, generally much less bright; indeed, even
an ordinary surface of sandstone will look as bright
as her disc. In fact, the reason of her great apparent
brightness at night is merely the contrast
between her and the dark background against which
she is seen; a fragment of our own world, put in
her place, would shine quite as brightly, perhaps
even more so. It is possibly rather difficult at first
to realize that our earth is shining to the moon and
to the other planets as they do to us, but anyone
who watches the moon for a few days after new will
find convincing evidence of the fact. Within the
arms of the thin crescent can be seen the whole
body of the lunar globe, shining with a dingy
coppery kind of light—'the ashen light,' as it is
called. People talk of this as 'the old moon in the
young moon's arms,' and weather-wise (or foolish)
[pg 105]
individuals pronounce it to be a sign of bad weather.
It is, of course, nothing of the sort, for it can be seen
every month when the sky is reasonably clear; but
it is the sign that our world shines to the other
worlds of space as they do to her; for this dim light
upon the part of the moon unlit by the sun is simply
the light which our own world reflects from her
surface to the moon. In amount it is thirteen times
more than that which the moon gives to us, as the
earth presents to her satellite a disc thirteen times
as large as that exhibited by the latter.
The moon's function in causing eclipses of the
sun has already been briefly alluded to. In turn
she is herself eclipsed, by passing behind the earth
and into the long cone of shadow which our world
casts behind it into space (Fig. 19). It is obvious
that such eclipses can only happen when the moon
is full. A total eclipse of the moon, though by no
means so important as a solar eclipse, is yet a very
interesting and beautiful sight. The faint shadow
or penumbra is often scarcely perceptible as the
moon passes through it; but the passage of the
dark umbra over the various lunar formations can
be readily traced, and is most impressive. Cases of
'black eclipses' have been sometimes recorded, in
which the moon at totality has seemed actually to
disappear as though blotted out of the heavens;
but in general this is not the case. The lunar disc
still remains visible, shining with a dull coppery
light, something like the ashen light, but of a redder
tone. This is due to the fact that our earth is not,
[pg 106]
like its satellite, a next to airless globe, but is possessed
of a pretty extensive atmosphere. By this
atmosphere those rays of the sun which would otherwise
have just passed the edge of the world are
caught and refracted so that they are directed upon
the face of the eclipsed moon, lighting it up feebly.
The redness of the light is due to that same atmospheric
absorption of the green and blue rays which
causes the body of the setting sun to seem red when
viewed through the dense layer of vapours near the
horizon. When the moon appears totally eclipsed
to us, the sun must appear totally eclipsed to an
observer stationed on the moon. A total solar
eclipse seen from the moon must present features
of interest differing to some extent from those which
the similar phenomenon exhibits to us. The duration
of totality will be much longer, and, in addition
to the usual display of prominences and corona,
there will be the strange and weird effect of the
black globe of our world becoming gradually
bordered with a rim of ruddy light as our atmosphere
catches and bends the solar rays inwards
upon the lunar surface.
In nine cases out of ten the moon will be the
first object to which the beginner turns his telescope,
and he will find in our satellite a never-failing source
of interest, and a sphere in which, by patient observation
and the practice of steadily recording what is
seen, he may not only amuse and instruct himself,
but actually do work that may become genuinely
useful in the furtherance of the science. The possession
[pg 107]
of powerful instrumental means is not an
absolute essential here, for the comparative nearness
of the object brings it well within the reach of
moderate glasses. The writer well remembers the
keen feeling of delight with which he first discovered
that a very humble and commonplace telescope—nothing
more, in fact, than a small ordinary spy-glass
with an object-glass of about 1 inch in aperture—was
able to reveal many of the more prominent
features of lunar scenery; and the possessor of any
telescope, no matter whether its powers be great or
small, may be assured that there is enough work
awaiting him on the moon to occupy the spare time
of many years with one of the most enthralling of
studies. The view that is given by even the
smallest instrument is one of infinite variety and
beauty; and its interest is accentuated by the fact
that the moon is a sphere where practically every
detail is new and strange.
If the moon be crescent, or near one or other of
her quarters at the time of observation, the eye will
at once be caught by a multitude of circular, or
nearly circular depressions, more clearly marked the
nearer they are to the line of division between the
illuminated and unilluminated portions of the disc.
(This line is known as the Terminator, the circular
outline, fully illuminated, being called the Limb).
The margins of some of these depressions will be
seen actually to project like rings of light into the
darkness, while their interiors are filled with black
shadow (Plates XI., XIII., XV., and XVI.). At
[pg 108]
one or two points long bright ridges will be seen,
extending for many miles across the surface, and
marking the line of one or other of the prominent
ranges of lunar mountains (Plates XI., XIII., XVI.,
XVII.); while the whole disc is mottled over with
patches of varied colour, ranging from dark grey up
to a brilliant yellow which, in some instances, nearly
approaches to white.
If observation be conducted at or near the full,
the conditions will be found to have entirely
changed. There are now very few ruggednesses
visible on the edge of the disc, which now presents
an almost smooth circular outline, nor are there any
shadows traceable on the surface. The circular depressions,
formerly so conspicuous, have now almost
entirely vanished, though the positions and outlines
of a few of them may still be traced by their contrast
in colour with the surrounding regions. The
observer's attention is now claimed by the extraordinary
brilliance and variety of the tones which
diversify the sphere, and particularly by the curious
systems of bright streaks radiating from certain well-marked
centres, one of which, the system originating
near Tycho, a prominent crater not very far from
the South Pole, is so conspicuous as to give the full
moon very much the appearance of a badly-peeled
orange (Plate XII.).
PLATE XII.
The Moon, November 13, 1902. Paris Observatory.
As soon as the moon has passed the full, the
ruggedness of its margin begins once more to
become apparent, but this time on the opposite
side; and the observer, if he have the patience to
[pg 109]
work late at night or early in the morning, has the
opportunity of seeing again all the features which
he saw on the waxing moon, but this time with the
shadows thrown the reverse way—under evening
instead of under morning illumination. In fact the
character of any formation cannot be truly appreciated
until it has been carefully studied under
the setting as well as under the rising and meridian
sun.
We must now turn our attention to the various
types of formation which are to be found upon the
moon. These may be roughly summarized as
follows: (1) The great grey plains, commonly
known as Maria, or seas; (2) the circular or approximately
circular formations, known generally as
the lunar craters, but divided by astronomers into a
number of classes to which reference will be made
later; (3) the mountain ranges, corresponding with
more or less closeness to similar features on our own
globe; (4) the clefts or rills; (5) the systems of
bright rays, to which allusion has already been
made.
1. The Great Grey Plains.—These are, of
course, the most conspicuous features of the lunar
surface. A number of them can be easily seen with
the naked eye; and, so viewed, they unite with the
brighter portions to form that resemblance to a
human face—'the man in the moon'—with which
everyone is familiar. A field-glass or small telescope
brings out their boundaries with distinctness,
and suggests a likeness to our own terrestrial oceans
[pg 110]
and seas. Hence the name Maria, which was applied
to them by the earlier astronomers, whose
telescopes were not of sufficient power to reveal
more than their broader outlines. But a comparatively
small aperture is sufficient to dispel the
idea that these plains have any right to the title of
'seas.' The smoothness which at first suggests
water proves to be only relative. They are smooth
compared with the brighter regions of the moon,
which are rugged beyond all terrestrial precedent;
but they would probably be considered no smoother
than the average of our own non-mountainous land
surfaces. A 2 or 2½-inch telescope will reveal the
fact that they are dotted over with numerous irregularities,
some of them very considerable. It is
indeed not common to find a crater of the largest
size associated with them; but, at the same time,
craters which on our earth would be considered
huge are by no means uncommon upon their surface,
and every increase of telescopic power reveals a
corresponding increase in the number of these
objects (Plates XIII., XV., XVII.).
PLATE XIII.
The Moon, September 12, 1903. Paris Observatory.
Further, the grey plains are characterized by
features of which instances may be seen with a very
small instrument, though the more delicate specimens
require considerable power—namely, the long
winding ridges which either run concentrically with
the margins of the plains, or cross their surface from
side to side. Of these the most notable is the great
serpentine ridge which traverses the Mare Serenitatis
in the north-west quadrant of the moon. As
[pg 111]
it runs, approximately, in a north and south
direction, it is well placed for observation, and
even a low power will bring out a good deal of
remarkable detail in connection with it. It rises in
some places to a height of 700 or 800 feet (Neison),
and is well shown on many of the fine lunar photographs
now so common. Another point of interest
in connection with the Maria is the existence on
their borders of a number of large crater formations
which present the appearance of having had
their walls breached and ruined on the side next
the mare by the action of some obscure agency.
From consideration of these ruined craters, and of
the 'ghost craters,' not uncommon on the plains,
which present merely a faint outline, as though
almost entirely submerged, it has been suggested,
by Elger and others, that the Maria, as we see them
represent, not the beds of ancient seas, but the consolidated
crust of some fluid or viscous substance
such as lava, which has welled forth from vents
connected with the interior of the moon, overflowing
many of the smaller formations, and partially
destroying the walls of these larger craters. Notable
instances of these half-ruined formations will be found
in Fracastorius (Plate XIX., No. 78, and Plate XI.),
and Pitatus (Plate XIX., No. 63, and Plate XV.).
The grey plains vary in size from the vast Oceanus
Procellarum, nearly 2,000,000 square miles in area,
down to the Mare Humboldtianum, whose area of
42,000 square miles is less than that of England.
[pg 112]
2. The Circular, or Approximately Circular
Formations.—These, the great distinguishing
feature of lunar scenery, have been classified according
to the characteristics, more or less marked, which
distinguish them from one another, as walled-plains,
mountain-rings, ring-plains, craters, crater-cones,
craterlets, crater-pits, and depressions. For general
purposes we may content ourselves with the single
title craters, using the more specific titles in outstanding
instances.
PLATE XIV.
Region of Maginus: Overlapping Craters. Paris Observatory.
To these strange formations we have scarcely the
faintest analogy on earth. Their multitude will at
once strike even the most casual observer. Galileo
compared them to the 'eyes' in a peacock's tail, and
the comparison is not inapt, especially when the
moon is viewed with a small telescope and low
powers. In the Southern Hemisphere particularly,
they simply swarm to such an extent that the district
near the terminator presents much the appearance of
a honeycomb with very irregular cells, or a piece of
very porous pumice (Plate XIV.). Their vast size
is not less remarkable than their number. One of
the most conspicuous, for example, is the great
walled-plain Ptolemäus, which is well-placed for
observation near the centre of the visible hemisphere.
It measures 115 miles from side to side of
its great rampart, which, in at least one peak, towers
more than 9,000 feet above the floor of the plain
within. The area of this enormous enclosure is
about equal to the combined areas of Yorkshire,
Lancashire, and Westmorland—an extent so vast
[pg 113]
that an observer stationed at its centre would see
no trace of the mountain-wall which bounds it, save
at one point towards the West, where the upper
part of the great 9,000-feet peak already referred to
would break the line of the horizon (Plate XIX.,
No. 111; Plate XIII.).
Nor is Ptolemäus by any means the largest of
these objects. Clavius, lying towards the South
Pole, measures no less than 142 miles from wall to
wall, and includes within its tremendous rampart an
area of at least 16,000 square miles. The great
wall which encloses this space, itself no mean range
of mountains, stands some 12,000 feet above the
surface of the plain within, while in one peak it rises
to a height of 17,000 feet. Clavius is remarkable
also for the number of smaller craters associated
with it. There are two conspicuous ones, one on
the north, one on the south side of its wall, each
about twenty-five miles in diameter, while the floor
is broken by a chain of four large craters and a
considerable number of smaller ones.
Though unfavourably placed for observation, there
is no lunar feature which can compare in grandeur
with Clavius when viewed either at sunrise or sunset.
At sunrise the great plain appears first as a
huge bay of black shadow, so large as distinctly to
blunt the southern horn of the moon to the naked
eye. As the sun climbs higher, a few bright points
appear within this bay of darkness—the summits of
the walls of the larger craters—these bright islands
gradually forming fine rings of light in the shadow
[pg 114]
which still covers the floor of the great plain. In
the East some star-like points mark where the peaks
of the eastern wall are beginning to catch the dawn.
Then delicate streaks of light begin to stream across
the floor, and the dark mass of shadow divides itself
into long pointed shafts, which stretch across the
plain like the spires of some great cathedral. The
whole spectacle is so magnificent and strange that
no words can do justice to it; and once seen it will
not readily be forgotten. Even a small telescope
will enable the student to detect and draw the more
important features of this great formation; and for
those whose instruments are more powerful there is
practically no limit to the work that may be done on
Clavius, which has never been studied with the
minuteness that so great and interesting an object
deserves. (Clavius is No. 13, Plate XIX. See
also Plates XIII. and XV., and Fig. 22, the latter
a rough sketch with a 2⅝-inch refractor.)
From such gigantic forms as these, the craters
range downwards in an unbroken sequence through
striking objects such as Tycho and the grand
Copernicus, both distinguished for their systems of
bright rays, as well as for their massive and regular
ramparts, to tiny pits of black shadow, a few hundred
feet across, and with no visible walls, which tax the
powers of the very finest instruments. Schmidt's
great map lays down nearly 33,000 craters, and it is
quite certain that these are not nearly all which can
be seen even with a moderate-sized telescope.
PLATE XV.
Clavius, Tycho, and Mare Nubium. Yerkes Observatory.
As to the cause which has resulted in this multitude
[pg 115]
of circular forms, there is no definite consensus
of opinion. Volcanic action is the agency generally
invoked; but, even allowing for the diminished force
of gravity upon the moon, it is difficult to conceive
of volcanic action of such intensity as to have produced
some of the great walled-plains. Indeed,
Neison remarks that such formations are much more
akin to the smaller Maria, and bear but little resemblance
to true products of volcanic action. But it
seems difficult to tell where a division is to be made,
with any pretence to accuracy, between such forms
as might certainly be thus produced and those next
[pg 116]
above them in size. The various classes of formation
shade one into the other by almost imperceptible
degrees.
FIG. 22.
Clavius, June 7, 1889, 10 p.m., 2⅝ inch.
3. The Mountain Ranges.—These are comparatively
few in number, and are never of such
magnitude as to put them, like the craters, beyond
terrestrial standards of comparison. The most
conspicuous range is that known as the Lunar
Apennines, which runs in a north-west and south-east
direction for a distance of upwards of 400 miles
along the border of the Mare Imbrium, from which
its mass rises in a steep escarpment, towering in one
instance (Mount Huygens) to a height of more
than 18,000 feet. On the western side the range
slopes gradually away in a gentle declivity. The
spectacle presented by the Apennines about first
quarter is one of indescribable grandeur. The
shadows of the great peaks are cast for many miles
over the surface of the Mare Imbrium, magnificently
contrasting with the wild tract of hill-country behind,
in which rugged summits and winding valleys are
mingled in a scene of confusion which baffles all
attempt at delineation. Two other important ranges—the
Caucasus and the Alps—lie in close proximity
to the Apennines; the latter of the two notable for
the curious Alpine Valley which runs through it in a
straight line for upwards of eighty miles. This
wonderful chasm varies in breadth from about two
miles, at its narrowest neck, to about six at its
widest point. It is closely bordered, for a considerable
portion of its length, by almost vertical cliffs
[pg 117]
thousands of feet in height, and under low magnifying
powers appears so regular as to suggest nothing
so much as the mark of a gigantic chisel, driven by
main force through the midst of the mountain mass.
The Alpine Valley is an easy object, and a power of
50 on a 2-inch telescope will show its main outlines
quite clearly. Indeed, the whole neighbourhood is
one which will well repay the student, some of the
finest of the lunar craters, such as Plato, Archimedes,
Autolycus, and Aristillus, lying in the immediate
vicinity (Plates XIII. and XVII.).
PLATE XVI.
Region of Theophilus and Altai Mountains. Yerkes Observatory.
Among the other mountain-ranges may be
mentioned the Altai Mountains, in the south-west
quadrant (Plate XVI.), the Carpathians, close to
the great crater Copernicus, and the beautiful semicircle
of hills which borders the Sinus Iridum, or
Bay of Rainbows, to the east of the Alpine range.
This bay forms one of the loveliest of lunar landscapes,
and under certain conditions of illumination
its eastern cape, the Heraclides Promontory, presents
a curious resemblance, which I have only seen once
or twice, to the head of a girl with long floating hair—'the
moon-maiden.' The Leibnitz and Doerfel
Mountains, with other ranges whose summits appear
on the edge of the moon, are seldom to be seen to great
advantage, though they are sometimes very noticeably
projected upon the bright disc of the sun during
the progress of an eclipse.* They embrace some
[pg 118]
of the loftiest lunar peaks reaching 26,000 feet in one of
or two instances, according to Schröter and Mädler.
FIG. 23.
Aristarchus and Herodotus, February 20, 1891, 6.15 p.m., 3⅞
inch.
4. The Clefts or Rills.—In these, and in the
ray-systems, we again meet with features to which
a terrestrial parallel is absolutely lacking. Schröter
of Lilienthal was the first observer to detect the
existence of these strange chasms, and since his
time the number known has been constantly increasing,
till at present it runs to upwards of a
thousand. These objects range from comparatively
coarse features, such as the Herodotus Valley
[pg 119]
(Fig. 23), and the well-known Ariadæus and
Hyginus clefts, down to the most delicate threads,
only to be seen under very favourable conditions,
and taxing the powers of the finest instruments.
They present all the appearance of cracks in a
shrinking surface, and this is the explanation of
their existence which at present seems to find most
favour. In some cases, such as that of the great
Sirsalis cleft, they extend to a length of 300 miles;
their breadth varies from half a mile, or less, to two
miles; their depth is very variously estimated,
Nasmyth putting it at ten miles, while Elger only
allows 100 to 400 yards. In a number of instances
they appear either to originate from a small crater,
or to pass through one or more craters in their
course. The student will quickly find out for himself
that they frequently affect the neighbourhood
of one or other of the mountain ranges (as, for
example, under the eastern face of the Apennines,
Plate XVII.), or of some great crater, such as
Archimedes. They are also frequently found
traversing the floor of a great walled-plain, and at
least forty have been detected in the interior of
Gassendi (Plate XIX., No. 90). Smaller instruments
are, of course, incompetent to reveal more
than a few of the larger and coarser of these strange
features. The Serpentine Valley of Herodotus, the
cleft crossing the floor of Petavius, and the Ariadæus
and Hyginus rills are among the most conspicuous,
and may all be seen with a 2½-inch telescope and a
power of 100.
PLATE XVII.
Apennines, Alps, and Caucasus. Paris Observatory.
[pg 120]
5. The Systems of Bright Rays, radiating from
certain craters, remain the most enigmatic of the
features of lunar scenery. Many of these systems
have been traced and mapped, but we need only
mention the three principal—those connected with
Tycho, Copernicus, and Kepler, all shown on
Plate XII. The Tycho system is by far the most
noteworthy, and at once attracts the eye when
even the smallest telescope is directed towards
the full moon. The rays, which are of great
brilliancy, appear to start, not exactly from the
crater itself, but from a greyish area surrounding it,
and they radiate in all directions over the surface,
passing over, and almost completely masking in
their course some of the largest of the lunar craters.
Clavius, for example, and Maginus (Plate XIV.),
become at full almost unidentifiable from this cause,
though Neison's statement that 'not the slightest
trace of these great walled-plains, with their
extremely lofty and massive walls, can be detected
in full,' is certainly exaggerated. The rays are not
well seen save under a high sun—i.e., at or near full,
though some of them can still be faintly traced under
oblique illumination.
In ordinary telescopes, and to most eyes, the
Tycho rays appear to run on uninterruptedly for
enormous distances, one of them traversing almost
the whole breadth of the moon in a north-westerly
direction, and crossing the Mare Serenitatis, on
whose dark background it is conspicuous. Professor
W. H. Pickering, who has made a special
[pg 121]
study of the subject under very favourable conditions,
maintains, however, that this appearance of
great length is an illusion, and that the Tycho rays
proper extend only for a short distance, being
reinforced at intervals by fresh rays issuing from
small craters on their track. The whole subject is
one which requires careful study with the best
optical means.
None of the other ray-systems are at all comparable
with that of Tycho, though those in connection
with Copernicus and Kepler are very striking. As
to the origin and nature of these strange features,
little is known. There are almost as many theories
as there are systems; but it cannot be said that
any particular view has commanded anything like
general acceptance. Nasmyth's well-known theory
was that they represented cracks in the lunar
surface, caused by internal pressure, through which
lava had welled forth and spread to a considerable
distance on either side of the original chasm.
Pickering suggests that they may be caused by
a deposit of white powder, pumice, perhaps, emitted
by the craters from which the rays originate.
Both ideas are ingenious, but both present grave
difficulties, and neither has commended itself to any
very great extent to observers, a remark which
applies to all other attempts at explanation.
Such are the main objects of interest upon the
visible hemisphere of our satellite. In observing
them, the beginner will do well, after the inevitable
preliminary debauch of moon-gazing, during which
[pg 122]
he may be permitted to range over the whole surface
and observe anything and everything, not to attempt
an attack on too wide a field. Let him rather confine
his energies to the detailed study of one or two
particular formations, and to the delineation of all
their features within reach of his instrument under
all aspects and illuminations. By so doing he will
learn more of the actual condition of the lunar
surface than by any amount of general and
haphazard observation; and may, indeed, render
valuable service to the study of the moon.
Neither let him think that observations made with
a small telescope are now of no account, in view of
the number of large instruments employed, and of
the great photographic atlases which are at present
being constructed. It has to be remembered that
the famous map of Beer and Mädler was the result
of observations made with a 3¾-inch telescope, and
that Lohrmann used an instrument of only 4⅘ inches,
and sometimes one of 3¼. Anyone who has seen
the maps of these observers will not fail to have a
profound respect for the work that can be done with
very moderate means. Nor have even the beautiful
photographs of the Paris, Lick, and Yerkes
Observatories superseded as yet the work of the
human eye and hand. The best of the Yerkes
photographs, taken with a 40-inch refractor, are said
to show detail 'sufficiently minute to tax the powers
of a 6-inch telescope.' But this can be said only of
a very few photographs; and, generally speaking,
a good 3-inch glass will show more detail than
[pg 123]
can be seen on any but a few exceptionally good
negatives.
In conducting his observations, the student should
be careful to outline his drawing on such a scale as
will permit of the easy inclusion of all the details
which he can see, otherwise the sketch will speedily
become so crowded as to be indistinct and valueless.
A scale of 1 inch to about 20 miles, corresponding
roughly to 100 inches to the moon's diameter,
will be found none too large in the case of formations
where much detail has to be inserted—that is
to say, in the case of the vast majority of lunar
objects. Further, only such a moderate amount of
surface should be selected for representation as can
be carefully and accurately sketched in a period of
not much over an hour at most; for, though the
lunar day is so much longer than our own, yet the
changes in aspect of the various formations due to
the increasing or diminishing height of the sun
become very apparent if observation be prolonged
unduly; and thus different portions of the sketch
represent different angles of illumination, and the
finished drawing, though true in each separate detail,
will be untrue as a whole.
Above all, care must be taken to set down only
what is seen with certainty, and nothing more. The
drawing may be good or bad, but it must be true.
A coarse or clumsy sketch which is truthful to the
facts seen is worth fifty beautiful works of art where
the artist has employed imagination or recollection
to eke out the meagre results of observation. The
[pg 124]
astronomer's primary object is to record facts, not
to make pictures. If he is skilful in recording
what he sees, his sketch will be so much the more
truthful; but the facts must come first. Such
practical falsehoods as the insertion of uncertain
details, or the practice of drawing upon one's recollection
of the work of other observers, or of
altering portions of a sketch which do not please
the eye, are to be studiously avoided. The
observer's record of what he has seen should be
above suspicion. It may be imperfect; it should
never be false. Such cautions may seem superfluous,
but a small acquaintance with the subject of astronomical
drawing will show that they are not.
The want of a good lunar chart will speedily
make itself felt. Fortunately in these days it can
be easily supplied. The great photographic atlases
now appearing are, of course, for the luxurious; and
the elaborate maps of Beer and Mädler or Schmidt
are equally out of the question for beginners. The
smaller chart of the former observers is, however,
inexpensive and good, though a little crowded.
For a start there is still nothing much better than
Webb's reduction of Beer and Mädler's large chart,
published in 'Celestial Objects for Common Telescopes.'
It can also be obtained separately; but
requires to be backed before use. Mellor's chart
is also useful, and is published in a handy form,
mounted on mill-board. Those who wish charts
between these and the more elaborate ones will find
their wants met by such books as those of Neison or
[pg 125]
Elger. Neison's volume contains a chart in twenty-two
sections on a scale of 2 feet to the moon's
diameter. It includes a great amount of detail, and
is accompanied by an elaborate description of all
the features delineated. Its chief drawbacks are the
fact that it was published thirty years ago, and that
it is an extremely awkward and clumsy volume to
handle, especially in the dim light of an observatory.
Elger's volume is, perhaps, for English students,
the handiest general guide to the moon. Its chart
is on a scale of 18 inches to the moon's diameter,
and is accompanied by a full description. With
either this or Webb's chart, the beginner will find
himself amply provided with material for many a
long and delightful evenings work.
PLATE XVIII.
Chart of the Moon. Nasmyth and Carpenter.
PLATE XIX.
Key to Chart of Moon. Nasmyth and Carpenter.
The small chart which accompanies this chapter,
and which, with its key-map, I owe to the courtesy
of Mr. John Murray, the publisher of Messrs.
Nasmyth and Carpenter's volume on the moon, is
not in any sense meant as a substitute for those
already mentioned, but merely as an introduction
to some of the more prominent features of lunar
scenery. The list of 229 named and numbered
formations will be sufficient to occupy the student
for some time; and the essential particulars with
regard to a few of the more important formations are
added in as brief a form as possible (Appendix I.).
Before we leave our satellite, something must be
said as to the conditions prevailing on her surface.
The early astronomers who devoted attention to
lunar study were drawn on in their labours largely
[pg 126]
by the hope of detecting resemblances to our own
earth, or even traces of human habitation. Schröter
and Gruithuisen imagined that they had discovered
not only indications of a lunar atmosphere, but also
evidence of change upon the surface, and traces of
the handiwork of lunarian inhabitants. Gruithuisen,
in particular, was confident that in due time it would
become possible to trace the cities and the works
of the Lunarians. Gradually these hopes have
receded into the distance. The existence of a lunar
atmosphere is, indeed, no longer positively denied
now, as it was a few years ago; but it is certain
that such atmosphere as may exist is of extreme
rarity, quite inadequate to support animal life as we
understand such a thing. Certain delicate changes
of colour which take place within some of the
craters—Plato for instance—have been referred to
vegetation; and Professor Pickering has intimated
his observation of something which he considers to
be the forming and melting of hoar-frost within
certain areas, Messier and a small crater near
Herodotus among others. But the observations at
best are very delicate and the inferences uncertain.
It cannot be denied that the moon may have an
atmosphere; but positive traces of its existence are
so faint that, even if their reality be admitted, very
little can be built upon them.
At the same time when the affirmation is made
that the moon is 'a world where there is no weather,
and where nothing ever happens,' the most careful
modern students of lunar matters would be the first
[pg 127]
to question such a statement. Even supposing it
to be true that no concrete evidence of change upon
the lunar surface can be had, this would not
necessarily mean that no change takes place. The
moon has certainly never been studied to advantage
with any power exceeding 1,000, and the average
powers employed have been much less. Nasmyth
puts 300 as about the profitable limit, and 500
would be almost an outside estimate for anything
like regular work. But even assuming the use of
a power of 1,000, that means that the moon is seen
as large as though she were only 240 miles distant
from us. The reader can judge how entirely all
but the very largest features of our world would be
lost to sight at such a distance, and how changes
involving the destruction of large areas might take
place and the observer be none the wiser. When
it is remembered that even at this long range we
are viewing our object through a sea of troubled air
of which every tremor is magnified in proportion to
the telescopic power employed, until the finer details
are necessarily blurred and indistinct, it will be seen
that the case has been understated. Indeed it may
be questioned if the moon has ever been as well
seen as though it had been situated at a distance
of 500 miles from the earth. At such a distance
nothing short of the vastest cataclysms would be
visible; and it is therefore going quite beyond the
mark to assume that nothing ever happens on the
moon simply because we do not see it happening.
Moreover, the balance of evidence does appear to
[pg 128]
be inclining, slightly perhaps, but still almost unquestionably,
towards the view that change does
occur upon the moon. Some of the observations
which seem to imply change may be explained on
other grounds; but there is a certain residuum
which appears to defy explanation, and it is very
noteworthy that while those who at once dismiss
the idea of lunar change are, generally speaking,
those who have made no special study of the
moon's surface, the contrary opinion is most strongly
maintained by eminent observers who have devoted
much time to our satellite with the best modern
instruments to aid them in their work.
The admission of the possibility of change does
not, however, imply anything like fitness for human
habitation. The moon, to use Beer and Mädler's
oft-quoted phrase, is 'no copy of the earth'; and
the conditions of her surface differ widely from
anything that we are acquainted with. The extreme
rarity of her atmosphere must render her, were
other conditions equally favourable, an ideal situation
for an observatory. From her surface the stars,
which are hidden from us in the daytime by the
diffused light in our air, would be visible at
broad noonday; while multitudes of the smaller
magnitudes which here require telescopic power
would there be plain to the unaided eye. The
lunar night would be lit by our own earth, a gigantic
moon, presenting a surface more than thirteen times
as large as that which the full moon offers us, and
hanging almost stationary in the heavens, while
[pg 129]
exhibiting all the effects of rapid rotation upon its
own axis. Those appendages of the sun, which
only the spectroscope or the fleeting total eclipse
can reveal to us, the corona, the chromosphere, and
the prominences, would there be constantly visible.
Our astronomers who are painfully wrestling with
atmospheric disturbance, and are gradually being
driven from the plains to the summits of higher and
higher hills in search of suitable sites for the giant
telescopes of to-day, may well long for a world
where atmospheric disturbance must be unknown,
or at least a negligible quantity.
[pg 130]
CHAPTER VIII
MARS
The Red Planet is our nearest neighbour on the
further, as Venus is on the hither side. He is
also in some ways the planet best situated for
our observation; for while the greatest apparent
diameter of his disc is considerably less than that
of Venus, he does not hide close to the sun's rays
like the inferior planets, but may be seen all night
when in opposition.*
Not all oppositions, however,
are equally favourable. Under the best circumstances
he may come as near to us as 35,000,000
miles; when less favourably situated, he may come
no nearer than 61,000,000. This very considerable
variation in his distance arises from the eccentricity
of the planet's orbit, which amounts to nearly one-tenth,
and, so far as we are concerned, it means
that his disc is three times larger when he comes
to opposition at his least distance from the sun than
it is when the conditions are reversed. Under the
most favourable circumstances—i.e., when opposition
[pg 131]
and perihelion†
occur together, he presents, it has been calculated, a disc of the same diameter as a half
sovereign held up 2,000 yards from the spectator.
Periods of opposition recur at intervals of about
780 days, and at the more favourable ones the
planet's brilliancy is very striking. The 1877
opposition was very notable in this respect, and in
others connected with the study of Mars, and that
which preceded the Crimean War was also marked
by great brilliancy. Readers of Tennyson will
remember how Maud
'Seem'd to divide in a dream from a band of the blest,
And spoke of a hope for the world in the coming wars—
... and pointed to Mars
As he glow'd like a ruddy shield on the Lion's breast.'
Ancient records tell us of his brightness having
been so great on some occasions as to create a
panic. Panics were evidently more easily created
by celestial phenomena then than they are now;
but possibly such statements have to be taken with
a small grain of salt.
The diameter of Mars is 4,200 miles. In volume
he is equal to one-seventh of the world; but his
density is somewhat smaller, so that nine globes
such as Mars would be required to balance the
earth. He turns upon his axis in twenty-four hours
thirty-seven minutes, and as the inclination of the
axis is not much different from that of our own world
he will experience seasonal effects somewhat similar
[pg 132]
to the changes of our own seasons. The Martian
seasons, however, will be considerably longer than
ours, as the year of Mars occupies 687 days, and
they will be further modified by the large variation
which his distance from the sun undergoes in the
course of his year—the difference between his
greatest and least distances being no less than
26,500,000 miles.
The telescopic view of Mars at once reveals
features of considerable interest. We are no longer
presented with anything like the beautiful phases of
Venus, though Mars does show a slight phase when
his position makes a right angle with the sun and
the earth. This phase, however, never amounts to
more than a dull gibbosity, like that of the moon
two or three days before or after full—the most
uninteresting of phases. But the other details
which are visible much more than atone for any
deficiency in this respect. The brilliant ruddy star
expands under telescopic power into a broad disc
whose ground tint is a warm ochre. This tint is
diversified in two ways. At the poles there are
brilliant patches of white, larger or smaller according
to the Martian season; while the whole surface of
the remaining orange-tinted portion is broken up
by patches and lines of a dark greenish-grey tone.
The analogy with Arctic and Antarctic ice and
snow-fields, and with terrestrial continents and seas,
is at once and almost irresistibly suggested, although,
as will be seen, there are strong reasons for not
pressing it too far.
[pg 133]
The dark markings, though by no means so
sharply defined as the outlines of lunar objects, are
yet evidently permanent features; at least this may
be confidently affirmed of the more prominent among
them. Some of these can be readily recognised on
drawings dating from 200 years back, and have
served to determine with very satisfactory accuracy
the planet's rotation period. In accordance with
the almost irresistible evidence which the telescope
was held to present, these features were assumed to
be seas, straits and bays, while the general ochre-tinted
portion of the planet's surface was considered
to be dry land. On this supposition the land area
of Mars amounts to 5⁄7 of the planet's surface, water
being confined to the remaining 2⁄7. But it is by no
means to be taken as an accepted fact that the dark
and light areas do represent water and land. One
fact most embarrassing to those who hold this
traditional view is that in the great wealth of detail
which observation with the huge telescopes of
to-day has accumulated the bulk belongs to the
dark areas. Gradations of shade are seen constantly
in them; delicate details are far more commonly to
be observed upon them than upon the bright portions
of the surface, and several of the 'canals' have been
traced clear through the so-called seas. Speaking
of his observations of Mars in 1894 with the 36-inch
refractor of the Lick observatory, Professor Barnard
says: 'Though much detail was shown on the
bright "continental" regions, the greater amount
was visible on the so-called "seas."... During
[pg 134]
these observations the impression seemed to force
itself upon me that I was actually looking down
from a great altitude upon just such a surface as
that in which our observatory was placed. At
these times there was no suggestion that the view
was one of far-away seas and oceans, but exactly
the reverse.' Such observations are somewhat
disconcerting to the old belief, which, nevertheless,
continues to maintain itself, though in somewhat
modified form.
It is indeed difficult, if not impossible, to explain
the observed facts with regard, for instance, to the
white polar caps, on any other supposition than that
of the existence of at least a considerable amount
of water upon the planet. These caps are observed
to be large after the Martian winter has passed
over each particular hemisphere. As the season
progresses, the polar cap diminishes, and has even
been seen to melt away altogether. In one of the
fine drawings by the Rev. T. E. R. Phillips, which
illustrate this chapter (Plate XX.), the north polar
snow will be seen accompanied by a dark circular
line, concerning which the author of the sketch
says: 'The melting cap is always girdled by a
narrow and intensely dark line. This is not seen
when the cap is forming.' It is hard to believe
that this is anything else than the result of the
melting of polar snows, and where there is melting
snow there must be water. Such results as those
obtained by Professor Pickering by photography
point in the same direction. In one of his photographs
[pg 135]
the polar cap was shown much shrunken;
in another, taken a few days later, it had very
considerably increased in dimensions—as one would
naturally conclude, from a fall of snow in the
interval. The quantity of water may not be anything
like so great as was at one time imagined;
still, to give any evidence of its presence at all at
a distance of 40,000,000 miles it must be very
considerable, and must play an important part in
the economy of the planet.
PLATE XX.
Mars: Drawing 1, January 30, 1899—12 hours. Drawing 2, April 22, 1903—10 hours.
λ = 301°, φ = +10°. λ = 200°, φ = +24°.
Rev. T. E. R. Phillips.
In 1877 Schiaparelli of Milan announced that he
had discovered that the surface of Mars was covered
with a network of lines running with perfect straightness
often for hundreds of miles across the surface,
and invariably connecting two of the dark areas.
To these markings he gave the name of 'canali,'
a word which has been responsible for a good deal
of misunderstanding. Translated into our language
by 'canals,' it suggested the work of intelligent
beings, and imagination was allowed to run riot
over the idea of a globe peopled by Martians of
superhuman intelligence and vast engineering skill.
The title 'canals' is still retained; but it should be
noted that the term is not meant to imply artificial
construction any more than the term 'rill' on the
moon implies the presence of water.
At the next opposition of Mars, Schiaparelli
not only rediscovered his canals, but made the
astonishing announcement that many of them were
double, a second streak running exactly parallel to
the first at some distance from it. His observations
[pg 136]
were received with a considerable amount of doubt
and hesitation. Skilled observers declared that they
could see nothing in the heavens the least corresponding
to the network of hard lines which the
Italian observer drew across the globe of Mars;
and therein to some extent they were right, for the
canals are not seen with that hardness of definition
with which they are sometimes represented. But,
at the same time, each successive opposition has
added fresh proof of the fact that Schiaparelli was
essentially right in his statement of what was seen.
The question of the doubling of the canals is still
under dispute, and it seems probable that it is not
a real objective fact existing upon the planet, but is
merely an optical effect due to contrast. There can
be no question, however, about the positive reality
of a great number of the canals themselves; their
existence is too well attested by observers of the
highest skill and experience. 'There is really no
doubt whatever,' says Mr. Denning, 'about the
streaked or striated configuration of the Northern
hemisphere of Mars. The canals do not appear as
narrow straight deep lines in my telescope, but as
soft streams of dusky material with frequent condensations.'
The drawings by Mr. Phillips well
represent the surface of the planet as seen with an
instrument of considerable power; and the reader
will notice that his representation of the canals
agrees remarkably well with Denning's description.
The 'soft streams with frequent condensations' are
particularly well shown on the drawing of April 22,
[pg 137]
1903, which represents the region of 200° longitude
(see Chart, Plate XXI.) on the centre of the disc.
'The main results of Professor Schiaparelli's work,'
remarks Mr. Phillips, 'are imperishable and beyond
question. During recent years some observers
have given to the so-called "canals" a hardness
and an artificiality which they do not possess, with
the result that discredit has been brought upon the
whole canal system.... But of the substantial
accuracy and truthfulness (as a basis on which to
work) of the planet's configuration as charted by the
great Italian in 1877 and subsequent years, there is
in my mind no doubt.' The question of the reality
of the canal system may almost be said to have
received a definite answer from the remarkable
photographs of Mars secured in May, 1905, by
Mr. Lampland at the Flagstaff Observatory, which
prove that, whatever may be the nature of the
canals, the principal ones at all events are actual
features of the planet's surface.
Much attention has been directed within the last
few years to the observations of Lowell, made with
a fine 24-inch refractor at the same observatory,
which is situated at an elevation of over 7,000 feet.
His conclusion as to the reality of the canals is most
positive; but in addition to his confirmation of their
existence, he has put forward other views with
regard to Mars which as yet have found comparatively
few supporters. He has pointed out that
in almost all instances the canals radiate from certain
round spots which dot the surface of the planet.
[pg 138]
These spots, which have been seen to a certain
extent by other observers, he calls 'oases,' using
the term in its ordinary terrestrial significance. His
conclusions are, briefly, as follows: That Mars has
an atmosphere; that the dark regions are not seas,
but marshy tracts of vegetation; that the polar caps
are snow and ice, and the reddish portions of the
surface desert land. The canals he holds to be
waterways, lined on either bank by vegetation, so
that we see, not the actual canal, but the green strip
of fertilized land through which it passes, while the
round dark spots or 'oases' he believes to be the
actual population centres of the planet, where the
inhabitants cluster to profit by the fertility created
by the canals. In support of this view he adduces
the observed fact that the canals and oases begin to
darken as the polar caps melt, and reasons that this
implies that the water set free by the melting of the
polar snows is conveyed by artificial means to make
the wilderness rejoice.
Lowell's theories may seem, very likely are, somewhat
fanciful. It must be remembered, however,
that the ground facts of his argument are at least
unquestionable, whatever may be thought of his
inferences. The melting of the polar caps is matter
of direct observation; nor can it be questioned that
it is followed by the darkening of the canal system.
It is probably wiser not to dogmatize upon the
reasons and purposes of these phenomena, for the
very sufficient reason that we have no means of
arriving at any certitude. Terrestrial analogies
[pg 139]
cannot safely be used in connection with a globe
whose conditions are so different from those of our
own earth. The matter is well summed up by
Miss Agnes Clerke: 'Evidently the relations of
solid and liquid in that remote orb are abnormal;
they cannot be completely explained by terrestrial
analogies. Yet a series of well-authenticated phenomena
are intelligible only on the supposition that
Mars is, in some real sense a terraqueous globe.
Where snows melt there must be water; and the
origin of the Rhone from a great glacier is scarcely
more evident to our senses than the dissolution of
the Martian ice-caps into pools and streams.'
PLATE XXI.
Chart of Mars. 'Memoirs of the British Astronomical Association,' Vol. XI., Part
III., Plate VI.
Closely linked with the question of the existence
of water on the planet, and indeed a fundamental
point in the settlement of it, is the further question
of whether there is any aqueous vapour in the
Martian atmosphere. The evidence is somewhat
conflicting. It is quite apparent that in the atmosphere
of Mars there is nothing like the volume of
water vapour which is present in that of the earth,
for if there were, his features would be much more
frequently obscured by cloud than is found to be the
case. Still there are many observations on record
which seem quite unaccountable unless the occasional
presence of clouds is allowed. Thus on
May 21, 1903, Mr. Denning records that the Syrtis
Major (see Chart, Plate XXI.) being then very dark
and sharply outlined, a very bright region crossed its
southern extremity. By May 23, the Syrtis Major,
'usually the most conspicuous object in Mars, had
[pg 140]
become extremely feeble, as if covered with highly
reflective vapours.' On May 24, Mr. Phillips
observed the region of Zephyria and Aeolis to be
also whitened, while the Syrtis Major was very
faint; and on the 25th, Mr. Denning observed the
striking whiteness of the same region observed by
Mr. Phillips the day before. Illusion, so often invoked
to explain away inconvenient observations,
seems here impossible, in view of the prominence of
the markings obscured, and the experience of the
observers; and the evidence seems strongly in
favour of real obscuration by cloud. It might have
been expected that the evidence of the spectroscope
would in such a case be decisive, but Campbell's
negative conclusion is balanced by the affirmative
result reached by Huggins and Vogel. It is safe to
say, however, that whatever be the constitution of
the Martian atmosphere, it is considerably less dense
than our own air mantle.
During the last few years the public mind has
been unusually exercised over Mars, largely by
reason of a misapprehension of the terms employed
in the discussion about his physical features. The
talk of 'canals' has suggested human, or at all
events intelligent, agency, and the expectation arose
that it might not be quite impossible to establish
communication between our world and its nearest
neighbour on the further side. The idea is, of
course, only an old one furbished up again, for
early in last century it was suggested that a huge
triangle or ellipse should be erected on the Siberian
[pg 141]
steppes to show the Lunarians or the Martians that
we were intelligent creatures who knew geometry.
In these circumstances curiosity was whetted by
the announcement, first made in 1890, and since
frequently repeated, of the appearance of bright
projections on the terminator of Mars. These were
construed, by people with vivid imaginations, as
signals from the Martians to us; while a popular
novelist suggested a more sinister interpretation,
and harrowed our feelings with weird descriptions
of the invasion of our world by Martian beings of
uncouth appearance and superhuman intelligence,
who were shot to our globe by an immense gun
whose flashes occasioned the bright projections
seen. The projections were, however, prosaically
referred by Campbell to snow-covered mountains,
while Lowell believed that one very large one
observed at Flagstaff in May, 1903, was due to
sunlight striking on a great cloud, not of water-vapour,
but of dust.
As a matter of fact, Mars is somewhat disappointing
to those who approach the study of his
surface with the hope of finding traces of anything
which might favour the idea of human habitation.
He presents an apparently enticing general resemblance
to the earth, with his polar caps and his
bright and dark markings; and his curious network
of canals may suggest intelligent agency. But the
resemblances are not nearly so striking when
examined in detail. The polar caps are the only
features that seem to hold their own beside their
[pg 142]
terrestrial analogues, and even their resemblance is
not unquestioned; the dark areas, so long thought
to be seas, are now proved to be certainly not seas,
whatever else they may be; and the canal system
presents nothing but the name of similarity to anything
that we know upon earth. It is quite probable
that were Mars to come as near to us as our own
moon, the fancied resemblances would disappear
almost entirely, and we should find that the red
planet is only another instance of the infinite variety
which seems to prevail among celestial bodies.
That being so, it need scarcely be remarked that
any talk about Martian inhabitants is, to say the
least of it, premature. There may be such creatures,
and they may be anything you like to imagine.
There is no restraint upon the fancy, for no one
knows anything about them, and no one is in the
least likely to know anything.
The moons of Mars are among the most curious
finds of modern astronomy. When the ingenious
Dr. Jonathan Swift, in editing the travels of Mr.
Lemuel Gulliver, of Wapping, wrote that the
astronomers of Laputa had discovered 'two lesser
stars, or satellites, which revolve about Mars,' the
suggestion was, no doubt, put in merely because
some detail of their skill had to be given, and as
well one unlikely thing as another. Probably no
one would have been more surprised than the Dean
of St. Patrick's, had he lived long enough, or cared
sixpence about the matter, to hear that his bow
drawn at a venture had hit the mark, and that Professor
[pg 143]
Asaph Hall had detected two satellites of
Mars. The discovery was one of the first-fruits of
the 26-inch Washington refractor, and was made in
1877, the year from which the new interest in Mars
may be said to date. The two moons have been
called Deimos and Phobos, or Fear and Panic, and
are, in all probability, among the very tiniest bodies
of our system, as their diameter can scarcely be
greater than ten miles. Deimos revolves in an orbit
which takes him thirty hours eighteen minutes to
complete, at a distance of 14,600 miles from the
centre of Mars. Phobos is much nearer the planet,
his distance from its centre being 5,800, while from
its surface he is distant only 3,700 miles. In consequence
of this nearness, he can never be seen by an
observer on Mars from any latitude higher than 69°,
the bulge of the globe permanently shutting him out
from view. His period of revolution is only seven
hours thirty-nine minutes, so that to the Martian
inhabitants, if there are any, the nearer of the
planet's moons must appear to rise in the west and
set in the east. By the combination of its own revolution
and the opposite rotation of Mars it will take
about eleven hours to cross the heavens; and during
that period it will go through all its phases and half
through a second display.
These little moons are certainly among the most
curious and interesting bodies of the solar system;
but, unfortunately, the sight of them is denied to
most observers. That they were not seen by Sir
William Herschel with his great 4-foot reflector
[pg 144]
probably only points to the superior defining power
of the 26-inch Washington refractor as compared
with Herschel's celebrated but cumbrous instrument.
Still, they were missed by many telescopes quite
competent to show them, and of as good defining
quality as the Washington instrument—a fact which
goes to add proof, if proof were needed, that the
power which makes discoveries is the product of
telescope × observer, and that of the two factors
concerned the latter is the more important. It
is said that the moons have been seen by Dr.
Wentworth Erck with a 7⅓-inch refractor. The
ordinary observer is not likely to catch even a
glimpse of them with anything much smaller than
a 12-inch instrument, and even then must use precautions
to exclude the glare of the planet, and may
count himself lucky if he succeed in the observation.
A word or two may be said as to what a beginner
may expect to see with a small instrument. It has
been stated that nothing under 6 inches can make
much of Mars; but this is a somewhat exaggerated
statement of the case. It is quite certain that the
bulk of the more prominent markings can be seen
with telescopes of much smaller aperture. Some
detail has been seen with only 1¾-inch, while Grover
has, with a 2-inch, executed drawings which show
how much can be done with but little telescopic
power. The fact is, that observers who are only in
the habit of using large telescopes are apt to be
unduly sceptical of the powers of small ones, which
are often wonderfully efficient. The fine detail of
[pg 145]
the canal system is, of course, altogether beyond
small instruments; and, generally speaking, it will
take at least a 4-inch to show even the more strongly
marked of these strange features. At the 1894
opposition, the writer, using a 3⅞-inch Dollond of
good quality, was able to detect several of the more
prominent canals, but only on occasions of the best
definition. The accompanying rough sketch (Fig. 24)
gives an idea of what may be expected to be seen,
under favourable conditions, with an instrument of
between 2 and 3 inches. It represents Mars as seen
with a glass of 2⅝-inch aperture and fair quality.
The main marking in the centre of the disc is that
formerly known as the Kaiser or Hour-glass Sea.
Its name in Schiaparelli's nomenclature, now universally
used, is the Syrtis Major. The same marking
will also be seen in Mr. Phillips's drawing of 1899,
January 30, in which it is separated by a curious
bright bridge from the Nilosyrtis to the North. The
observer need scarcely expect to see much more than
is depicted in Fig. 24, with an instrument of the
class mentioned, but Plate XX. will give a very
good idea of the appearance of the planet when
viewed with a telescope of considerable power. The
polar caps will be within reach, and sometimes
present the effect of projecting above the general
level of the planet's surface, owing, no doubt, to
irradiation.
FIG. 24.
Mars, June 25, 1890, 10 hours 15 minutes; 2⅝-inch, power 120.
To the intending observer one important caution
may be suggested. In observing and sketching the
surface of Mars, do so independently. The chart
[pg 146]
which accompanies this chapter is given for the
purpose of identifying markings which have been
already seen, not for that of enabling the observer
to see details which are beyond the power of his
glass. No planet has been the cause of more
illusion than Mars, and drawings of him are extant
which resemble nothing so much as the photograph
of an umbrella which has been turned inside out by
a gust of wind. In such cases it may reasonably be
concluded that there is something wrong, and that,
unconsciously, 'the vision and the faculty divine'
have been exercised at the expense of the more
prosaic, but in this case more useful, quality of
accuracy. By prolonged study of a modern chart
[pg 147]
of Mars, and a little gentle stretching of the
imagination, the most unskilled observer with the
smallest instrument will detect a multitude of canals
upon the planet, to which there is but one objection,
that they do not exist. There is enough genuine
interest about Mars, even when viewed with a small
glass, without the importation of anything spurious.
In observation it will be noticed that as the rotation
period of Mars nearly coincides with that of the
earth, the change in the aspect presented from night
to night will be comparatively small, the same object
coming to the meridian thirty-seven minutes later
each successive evening. Generally speaking, Mars
is an easier object to define than either Venus or
Jupiter, though perhaps scarcely bearing high powers
so well as Saturn. There is no planet more certain
to repay study and to maintain interest. He and
Jupiter may be said to be at present the 'live'
planets of the solar system in an astronomical
sense.
[pg 148]
CHAPTER IX
THE ASTEROIDS
In the year 1772 Bode of Berlin published the
statement of a curiously symmetrical relation existing
among the planets of our system. The gist of this
relation, known as Bode's law, though it was really
discovered by Titius of Wittenberg, may be summed
up briefly thus: 'The interval between the orbits of
any two planets is about twice as great as the inferior
interval, and only half the superior one.' Thus the
distance between the orbits of the earth and Venus
should, according to Bode's law, be half of that between
the earth and Mars, which again should be half of
that which separates Mars from the planet next
beyond him. Since the discovery of Neptune, this
so-called law has broken down, for Neptune is very
far within the distance which it requires; but at the
time of its promulgation it represented with considerable
accuracy the actual relative positions of
the planets, with one exception. Between Mars and
Jupiter there was a blank which should, according
to the law, have been filled by a planet, but to all
appearance was not. Noticing this blank in the
[pg 149]
sequence, Bode ventured to predict that a planet
would be found to fill it; and his foresight was not
long in being vindicated.
Several continental astronomers formed a kind of
planet-hunting society to look out for the missing
orb; but their operations were anticipated by the
discovery on January 1, 1801, of a small planet
which occupied a place closely approximating to
that indicated for the missing body by Bode's law.
The news of this discovery, made by Piazzi of
Palermo in the course of observations for his well-known
catalogue of stars, did not reach Bode till
March 20, and 'the delay just afforded time for the
publication, by a young philosopher of Jena named
Hegel, of a "Dissertation" showing, by the clearest
light of reason, that the number of the planets could
not exceed seven, and exposing the folly of certain
devotees of induction who sought a new celestial
body merely to fill a gap in a numerical series.'
The remarkable agreement of prediction and discovery
roused a considerable amount of interest,
though the planet actually found, and named Ceres
after the patron-goddess of Sicily, seemed disappointingly
small. But before very long Olbers,
one of the members of the original planet-hunting
society, surprised the astronomical world by the discovery
of a second planet which also fulfilled the
condition of Bode's law; and by the end of March,
1807, two other planets equally obedient to the
required numerical standard were found, the first by
Harding, the second by Olbers. Thus a system of
[pg 150]
four small planets, Ceres, Pallas, Juno, and Vesta,
was found to fill that gap in the series which had
originally suggested the search. To account for
their existence Olbers proposed the theory that they
were the fragments of a large planet which had been
blown to pieces either by the disruptive action of
internal forces or by collision with a comet; and
this theory remained in favour for a number of years,
though accumulating evidence against it has forced
its abandonment.
It was not till 1845 that there was any addition to
the number of the asteroids, as they had come to be
named. In that year, however, Hencke of Driessen
in Prussia, discovered a fifth, which has been named
Astræa, and in 1847 repeated his success by the
discovery of a sixth, Hebe. Since that time there
has been a steady flow of discoveries, until at the
present time the number known to exist is close
upon 700, of which 569 have received permanent
numbers as undoubtedly distinct members of the
solar system; and this total is being steadily added
to year by year, the average annual number of discoveries
for the years 1902 to 1905 inclusive, being
fifty-two. For a time the search for minor planets
was a most laborious business. The planet-hunter
had to construct careful maps of all the stars visible
in a certain small zone of the ecliptic, and to compare
these methodically with the actual face of the sky in
the same zone, as revealed by his telescope. Any
star seen in the telescope, and not found to be
marked upon the chart, became forthwith an object
[pg 151]
of grave suspicion, and was watched until its motion,
or lack of motion, relatively to the other stars either
proved or disproved its planetary nature. At present
this lengthy and wearisome process has been entirely
superseded by the photographic method, in which a
minor planet is detected by the fact that, being in
motion relatively to the fixed stars, its image will
appear upon the plate in the shape of a short line
or trail, the images of the fixed stars being round
dots. Of course the trail may be due to a planet
which has already been discovered; but should there
be no known minor planet in the position occupied
by the trail, then a new member has been added to
the system. Minor-planet hunting has always been
a highly specialized branch of astronomy, and a few
observers, such as Peters, Watson, Charlois and
Palisa, and at present Wolf, have accounted for the
great majority of the discoveries.
It was, however, becoming more and more a
matter of question what advantage was to be gained
by the continuance of the hunt, when a fresh fillip
was given to interest by the discovery in 1898 of
the anomalous asteroid named Eros. Hitherto no
minor planet had been known to have the greater
portion of its orbit within that of Mars, though
several do cross the red planet's borders; but the
mean distance of Eros from the sun proves to be
about 135,000,000, while that of Mars is 141,000,000
miles. In addition, the orbit of the new planet is such
that at intervals of sixty-seven years it comes within
15,000,000 miles of the earth, or in other words
[pg 152]
nearer to us than any other celestial body except the
moon or a chance comet. It may thus come to
afford a means of revising estimates of celestial
distances. Eros presents another peculiarity. It
has been found by E. von Oppolzer to be variable
in a period of two hours thirty-eight minutes; and
the theory has been put forward that the planet is
double, consisting of two bodies which revolve
almost in contact and mutually eclipse one another—in
short, that Eros as a planet presents the same
phenomenon which we shall find as a characteristic
of that type of variable stars known as the Algol
type. An explanation, in some respects more simple
and satisfactory, is that the variation in light is caused
by the different reflective power of various parts of
its surface; but the question is still open.
The best results for the sizes of the four asteroids
first discovered are those of Barnard, from direct
measurements with the Lick telescope in 1894. He
found the diameter of Ceres to be 485 miles, that of
Pallas 304, those of Vesta and Juno 243 and 118
miles respectively. There appears to be as great
diversity in the reflective power of these original
members of the group as in their diameters. Ceres
is large and dull, and, in Miss Clerke's words, 'must
be composed of rugged and sombre rock, unclothed
probably by any vestige of air,' while Vesta has a
surface which reflects light with four times the
intensity of that of Ceres, and is, in fact, almost as
brilliantly white as newly fallen snow.
In the place of Olbers' discredited hypothesis of
[pg 153]
an exploded planet, has now been set the theory
first suggested by Kirkwood, that instead of having
in the asteroids the remnants of a world which has
become defunct, we have the materials of one which
was never allowed to form, the overwhelming power
of Jupiter's attraction having exerted a disruptive
influence over them while their formation was still
only beginning.
So far as I am aware, they share with Mars the
distinction of being the only celestial bodies which
have been made the subjects of a testamentary disposition.
In the case of Mars, readers may
remember that some years ago a French lady
left a large sum of money to be given to the
individual who should first succeed in establishing
communication with the Planet of War; in that of
the asteroids, the late Professor Watson, a mighty
hunter of minor planets in his day, made provision
for the supervision of the twenty-two planets
captured by him, lest any of them should get lost,
stolen, or strayed.
Small telescopes are, of course, quite impotent to
deal with such diminutive bodies as the asteroids;
nor, perhaps, is it desirable that the ranks of the
minor-planet hunters should be reinforced to any
extent.
[pg 154]
CHAPTER X
JUPITER
Passing outwards from the zone of the minor
planets, we come to the greatest and most magnificent
member of the solar system, the giant planet
Jupiter. To most observers, Jupiter will probably
appear not only the largest, but also the most
interesting telescopic object which our system
affords. Some, no doubt, will put in a claim for
Mars, and some will share Sir Robert Ball's predilection
for Saturn; but the interest attaching to
Mars is of quite a different character from that
which belongs to Jupiter, and while Saturn affords
a picture of unsurpassed beauty, there is not that
interest of variety and change in his exquisite
system which is to be found in that of his neighbour
planet. Jupiter is constantly attractive by
reason of the hope, or rather the certainty, that
he will always provide something fresh to observe;
and the perpetual state of flux in which the details
of his surface present themselves to the student
offers to us the only instance which can be conveniently
inspected of the process of world-formation.
[pg 155]
Jupiter is at the very opposite end of the
scale from such a body, for example, as our own
moon. On the latter it would appear as though all
things were approaching the fixity of death; such
changes as are suspected are scarcely more than
suspected, and, even if established, are comparatively
so small as to tax the utmost resources of observation.
On the former, such a thing as fixity or
stability appears to be unknown, and changes are
constantly occurring on a scale so gigantic as not to
be beyond the reach of small instruments, at least
in their broader outlines.
The main facts relating to the planet may be
briefly given before we go on to consider the
physical features revealed to us by the telescope.
Jupiter then travels round the sun in a period of
11 years, 314·9 days, at an average distance of
almost 483,000,000 miles. According to Barnard's
measures, his polar diameter is 84,570, and his
equatorial diameter 90,190 miles. He is thus compressed
at the poles to the extent of 1⁄16, and there
is no planet which so conspicuously exhibits to the
eye the actual effect of this polar flattening, though
the compression of Saturn is really greater still. In
volume he is equal to more than 1,300 earths, but
his density is so small that only 316 of our worlds
would be needed to balance him. This low density,
not much greater than that of water, is quite in
accordance with all the other features which are
revealed by observation, and appears to be common
to all the members of that group of large exterior
[pg 156]
planets of which Jupiter is at once the first and the
chief.
The brilliancy of the great planet is exceedingly
remarkable, far exceeding that of Mars or Saturn,
and only yielding to that of Venus. In 1892 his
lustre was double that of Sirius, which is by far the
brightest of all the fixed stars; and he has been
repeatedly seen by the unaided eye even when the
sun was above the horizon. According to one
determination he reflects practically the same
amount of light as newly fallen snow; and even if
this be rejected as impossibly high, Zöllner's more
moderate estimate, which puts his reflective power
at 62 per cent. of the light received, makes him
almost as bright as white paper. Yet to the eye it
is very evident that his light has a distinct golden
tinge, and in the telescopic view this remains conspicuous,
and is further emphasized by the presence
on his disc of a considerable variety of colouring.
Under favourable circumstances Jupiter presents
to us a disc which measures as much as 50″ in
diameter. The very low magnifying power of 50
will therefore present him to the eye with a diameter
of 2,500″, which is somewhat greater than the
apparent diameter of the moon. In practice it is
somewhat difficult to realize that this is the case,
probably owing to the want of any other object in
the telescopic field with which to compare the
planet. But while there may be a little disappointment
at the seeming smallness of the disc even with
a power double that suggested, this will quickly be
[pg 157]
superseded by a growing interest in the remarkable
picture which is revealed to view.
FIG. 25.
Jupiter, October 9, 1891, 9.30 p.m.; 3⅞-inch, power 120.
Some idea of the ordinary appearance of the
planet may be gained from Fig. 25, which reproduces
a sketch made with a small telescope on
October 9, 1891. The first feature that strikes the
eye on even the most casual glance is the polar
compression. The outline of the disc is manifestly
not circular but elliptical, and this is emphasized by
the fact that nearly all the markings which are
visible run parallel to one another in the direction
of the longest diameter of the oval. A little
attention will reveal these markings as a series of
dark shadowy bands, of various breadths and
various tones, which stretch from side to side of the
disc, fading a little in intensity as they approach its
margin, and giving the whole planet the appearance
[pg 158]
of being girdled by a number of cloudy belts. The
belts may be seen with very low powers indeed, the
presence of the more conspicuous ones having
repeatedly been evident to the writer with the
rudimentary telescope mentioned in Chapter II.,
consisting of a non-achromatic double convex lens
of 1½-inch aperture, and a single lens eye-piece
giving a power of 36. Anything larger and more
perfect than this will bring them out with clearness,
and an achromatic of from 2 to 3 inches aperture
will give views of the highest beauty and interest,
and will even enable its possessor to detect some of
the more prominent evidences of the changes which
are constantly taking place.
The number of belts visible varies very considerably.
As many as thirty have sometimes been
counted; but normally the number is much smaller
than this. Speaking generally, two belts, one on
either side of a bright equatorial zone, will be found
to be conspicuous, while fainter rulings may be
traced further north and south, and the dusky hoods
which cover the poles will be almost as easily seen
as the two main belts. It will further become
apparent that this system of markings is characterized
by great variety of colouring. In this respect no
planet approaches Jupiter, and when seen under
favourable circumstances and with a good instrument,
preferably a reflector, some of the colour
effects are most exquisite. Webb remarks: 'There
is often "something rich and strange" in the
colouring of the disc. Lord Rosse describes yellow,
[pg 159]
brick-red, bluish, and even full-blue markings;
Hirst, a belt edged with crimson lake; Miss Hirst,
a small sea-green patch near one of the poles.' The
following notes of colour were made on December 26,
1905: The south equatorial belt distinct reddish-brown;
the equatorial zone very pale yellow,
almost white, with faint slaty-blue shades in the
northern portion; the north polar regions a decided
reddish-orange; while the south polar hood was of
a much colder greyish tone. But the colours are
subject to considerable change, and the variations
of the two great equatorial belts appear, according
to Stanley Williams, to be periodic, maxima and
minima of redness being separated by a period of
about twelve years, and the maximum of the one
belt coinciding with the minimum of the other.
PLATE XXII.
Jupiter, January 6, 1906—8 hours 20 minutes. Instrument, 9¼-inch Reflector.
λ = 238° (System 1); λ = 55° (System 2).
Rev. T. E. R. Phillips.
These changes in colouring bring us to the fact
that the whole system of the Jovian markings is
liable to constant and often very rapid change.
Anyone who compares drawings made a few years
ago with those made at the present time, such as
Plates XXII. and XXIII., cannot fail to notice
that while there is a general similarity, the details
have changed so much that there is scarcely one
individual feature which has not undergone some
modification. Indeed, this process of change is
sometimes so rapid that it can be actually watched
in its occurrence. Thus Mr. Denning remarks that
'on October 17, 1880, two dark spots, separated by
20° of longitude, broke out on a belt some 25°
north of the equator. Other spots quickly formed
[pg 160]
on each side of the pair alluded to, and distributed
themselves along the belt, so that by December 30
they covered three-fourths of its entire circumference.'
The dark belts, according to his observations,
'appear to be sustained in certain cases by eruptions
of dark matter, which gradually spread out into
streams.'
Even the great equatorial belts are not exempt
from the continual flux which affects all the markings
of the great planet, and the details of their structure
will be found to vary to a considerable extent at
different periods. At present the southern belt is
by far the most conspicuous feature of the surface,
over-powering all other details by its prominence,
while its northern rival has shrunk in visibility to a
mere shadow of what it appears in drawings made
in the seventies. Through all the changes of the
last thirty years, however, one very remarkable
feature of the planet has remained permanent at
least in form, though varying much in visibility.
With the exception of the canals of Mars, no feature
of any of the planets has excited so much interest
as the great red spot on Jupiter. The history of
this extraordinary phenomenon as a subject of
general study begins in 1878, though records exist
as far back as 1869 of a feature which almost
certainly was the same, and it has been suggested
that it was observed by Cassini two centuries ago.
In 1878 it began to attract general attention, which
it well deserved. In appearance it was an enormous
ellipse of a full brick-red colour, measuring some
[pg 161]
30,000 miles in length by 7,000 in breadth, and lying
immediately south of the south equatorial belt.
With this belt it appears to have some mysterious
connection. It is not actually joined to it, but seems,
as Miss Clerke observes, to be 'jammed down upon
it'; at least, in the south equatorial belt, just below
where the spot lies, there has been formed an
enormous bay, bounded on the following side (i.e.,
the right hand as the planet moves through the field),
by a sharply upstanding shoulder or cape. The
whole appearance of this bay irresistibly suggests
to the observer that it has somehow or other been
hollowed out to make room for the spot, which
floats, as it were, within it, surrounded generally by
a margin of bright material, which divides it from
the brown matter of the belt. The red spot, with
its accompanying bay and cape, is shown in Fig. 25
and in Plate XXII., which represents the planet as
seen by the Rev. T. E. R. Phillips on January 6,
1906. The spot has varied very much in colour
and in visibility, but on the whole its story has
been one of gradual decline; its tint has paled, and
its outline has become less distinct, as though it
were being obscured by an outflow of lighter-coloured
matter, though there have been occasional
recoveries both of colour and distinctness. In 1891
it was a perfectly easy object with 3⅞ inches; at the
present time the writer has never found it anything
but difficult with an 18-inch aperture, though some
observers have been able to see it steadily in 1905
and 1906 with much smaller telescopes. The continued
[pg 162]
existence of the bay already referred to seems
to indicate that it is only the colour of the spot that
has temporarily paled, and that observers may in
course of time witness a fresh development of this
most interesting Jovian feature.
The nature of the red spot remains an enigma.
It may possibly represent an opening in the upper
strata of Jupiter's dense cloud-envelope, through
which lower strata, or even the real body of the
planet, may be seen. The suggestion has also been
made that it is the glow of some volcanic fire on the
body of the planet, seen through the cloud-screen
as the light of a lamp is seen through ground-glass.
But, after all, such ideas are only conjectures, and
it is impossible to say as yet even whether the
spot is higher or lower than the average level of
the surface round it. A curious phenomenon which
was witnessed in 1891 suggested at first a hope
that this question of relative height would at least
be determined. This phenomenon was the overtaking
of the red spot by a dark spot which had
been travelling after it on the same parallel, but
with greater speed, for some months. It appeared
to be quite certain that the dark spot must either
transit the face of the red spot or else pass
behind it; and in either case interesting information
as to the relative elevations of the two features in
question would have been obtained. The dark spot,
however, disappointed expectation by drifting round
the south margin of the red one, much as the current
of a river is turned aside by the buttress of a bridge.
[pg 163]
In fact, it would almost appear as though the red
spot had the power of resisting any pressure from
other parts of the planet's surface; yet in itself it
has no fixity, for its period of rotation steadily
lengthened for several years until 1899, since when
it has begun to shorten again, so that it would
appear to float upon the surface of currents of
variable speed rather than to be an established
landmark of the globe itself. The rotation period
derived from it was, in 1902, 9 hours 55 minutes
39·3 seconds.
The mention of the changing period of rotation
of the red spot lends emphasis to the fact that no
single period of rotation can be assigned to Jupiter as
a whole. It is impossible to say of the great planet
that he rotates in such and such a period: the utmost
that can be said is that certain spots upon his surface
have certain rotation periods; but these periods are
almost all different from one another, and even the
period of an individual marking is subject, as already
seen, to variation. In fact, as Mr. Stanley Williams
has shown, no fewer than nine different periods of
rotation are found to coexist upon the surface; and
though the differences in the periods seem small
when expressed in time, amounting in the extreme
cases only to eight and half minutes, yet their
significance is very great indeed. In the case of
Mr. Williams's Zones II. and III., the difference in
speed of these two surface currents amounts to
400 miles per hour. Certain bright spots near the
equator have been found to move so much more
[pg 164]
rapidly than the great red spot as to pass it at a
speed of 260 miles an hour, and to 'lap' it in
forty-four and a half days, completing in that time
one whole rotation more than their more imposing
neighbour. It cannot, therefore, be said that
Jupiter's rotation period is known; but the average
period of his surface markings is somewhere about
nine hours fifty-two minutes.
Thus the rotation period adds its evidence to
that already afforded by the variations in colour and
in form of the planet's markings that here we are
dealing with a body in a very different condition
from that of any of the other members of our
system hitherto met with. We have here no globe
whose actual surface we can scrutinize, as we can in
the case of Mars and the moon, but one whose solid
nucleus, if it has such a thing, is perpetually veiled
from us by a mantle which seems more akin to the
photosphere of the sun than to anything else that
we are acquainted with. The obvious resemblances
may, and very probably do, mask quite as important
differences. The mere difference in scale between
the two bodies concerned must be a very important
factor, to say nothing of other causes which may be
operative in producing unlikeness. Still, there is a
considerable and suggestive general resemblance.
In the sun and in Jupiter alike we have a view,
not of the true surface, but of an envelope which
seems to represent the point of condensation of
currents of matter thrown up from depths below—an
envelope agitated in both cases, though more
[pg 165]
slowly in that of Jupiter, by disturbances which
bear witness to the operation of stupendous forces
beneath its veil. In both bodies there is a similar
small density: neither the sun nor Jupiter is much
denser than water; in both the determination of the
rotation period is complicated by the fact that the
markings of the bright envelope by which the
determinations must be made move with entirely
different speeds in different latitudes. Here, however,
there is a divergence, for while in the case
of the sun the period increases uniformly from the
equator to the poles, there is no such uniformity in
the case of Jupiter. Thus certain dark spots in
25° north latitude were found in 1880 to have a
shorter period than even the swift equatorial white
markings.
One further circumstance remains to be noted in
pursuance of these resemblances. Not only does
the disc of Jupiter shade away at its edges in a
manner somewhat similar to that of the sun, being
much more brilliant in the centre than at the limb,
but his remarkable brilliancy, already noticed, has
given rise to the suggestion that to some small
extent he may shine by his own inherent light.
There are certain difficulties, however, in the way
of such a suggestion. The satellites, for example,
disappear absolutely when they enter the shadow of
their great primary—a fact which is conclusive
against the latter being self-luminous to anything
more than a very small extent, as even a small
emission of native light from Jupiter would suffice
[pg 166]
to render them visible. But even supposing that
the idea of self-luminosity has to be abandoned,
everything points to the fact that in Jupiter we
have a body which presents much stronger analogies
to the sun than to those planets of the solar
system which we have so far considered. The late
Mr. R. A. Proctor's conclusion probably represents
the true state of the case with regard to the giant
planet: 'It may be regarded as practically proved
that Jupiter's condition rather resembles that of a
small sun which has nearly reached the dark stage
than that of a world which is within a measurable
time-interval from the stage of orb-life through
which our own Earth is passing.'
Leaving the planet itself, we turn to the beautiful
system of satellites of which it is the centre. The
four moons which, till 1892, were thought to compose
the complete retinue of Jupiter, were among the
first-fruits of Galileo's newly-invented telescope,
and were discovered in January, 1610. The names
attached to them—Io, Europa, Ganymede, and
Callisto—have now been almost discarded in favour
of the more prosaic but more convenient numbers
I., II., III., IV. The question of their visibility to
the unaided eye has been frequently discussed, but
with little result; nor is it a matter of much importance
whether or not some person exceptionally
gifted with keenness of sight may succeed in
catching a momentary glimpse of one which
happens to be favourably placed. The smallest
telescope or a field-glass will show them quite
[pg 167]
clearly. They are, in fact, bodies of considerable
size, III., which is the largest, being 3,558 miles in
diameter, while IV. is only about 200 miles less;
and a moderate magnifying power will bring out
their discs.
PLATE XXIII.
Jupiter, February 17, 1906. J. Baikie, 18-inch Reflector.
The beautiful symmetry of this miniature system
was broken in 1892 by Barnard's discovery of a
fifth satellite—so small and so close to the great
planet that very few telescopes are of power
sufficient to show it. This was followed in 1904 by
Perrine's discovery, from photographs taken at the
Lick Observatory with the Crossley reflector, of
two more members of the system, so that the train
of Jupiter as at present known numbers seven.
The fifth, sixth, and seventh satellites are, of course,
far beyond the powers of any but the very finest
instruments, their diameters being estimated at
120, 100, and 30 miles respectively. It will be a
matter of interest, however, for the observer to
follow the four larger satellites, and to watch their
rapid relative changes of position; their occultations,
when they pass behind the globe of Jupiter; their
eclipses, when they enter the great cone of shadow
which the giant planet casts behind him into
space; and, most beautiful of all, their transits. In
occultations the curious phenomenon is sometimes
witnessed of an apparent flattening of the planet's
margin as the satellite approaches it at ingress or
draws away from it at egress. This strange optical
illusion, which also occurs occasionally in the case
of transits, was witnessed by several observers on
[pg 168]
various dates during the winter of 1905-1906. It is,
of course, merely an illusion, but it is curious why
it should happen on some occasions and not on
others, when to all appearance the seeing is of very
much the same quality. The gradual fading away
of the light of the satellites as they enter into eclipse
is also a very interesting feature, but the transits
are certainly the most beautiful objects of all for a
small instrument. The times of all these events
are given in such publications as the 'Nautical
Almanac' or the 'Companion to the Observatory';
but should the student not be possessed of either
of these most useful publications, he may notice that
when a satellite is seen steadily approaching Jupiter
on the following side, a transit is impending. The
satellite will come up to the margin of the planet,
looking like a brilliant little bead of light as it joins
itself to it (a particularly exquisite sight), will glide
across the margin, and after a longer or shorter
period will become invisible, being merged in the
greater brightness of the central portions of Jupiter's
disc, unless it should happen to traverse one of the
dark belts, in which case it may be visible throughout
its entire journey. It will be followed or preceded,
according to the season, by its shadow,
which will generally appear as a dark circular dot.
In transits which occur before opposition the
shadows precede the satellites; after opposition
they travel behind them. The transit of the satellite
itself will in most cases be a pretty sharp test of
the performance of a 3-inch telescope, or anything
[pg 169]
below that aperture; but the transit of the shadow
may be readily seen with a 2½-inch, probably even
with a 2-inch. There are certain anomalies in the
behaviour of the shadows which have never been
satisfactorily explained. They have not always
been seen of a truly circular form, nor always of
the same degree of darkness, that of the second
satellite being notably lighter in most instances
than those of the others. There are few more
beautiful celestial pictures than that presented by
Jupiter with a satellite and its shadow in transit.
The swift rotation of the great planet and the rapid
motion of the shadow can be very readily observed,
and the whole affords a most picturesque illustration
of celestial mechanics.
A few notes may be added with regard to
observation. In drawing the planet regard must
first of all be paid to the fact that Jupiter's disc is
not circular, and should never be so represented.
It is easy for the student to prepare for himself a
disc of convenient size, say about 2½ inches in
diameter on the major axis, and compressed to the
proper extent (1⁄16), which may be used in outlining
all subsequent drawings. Within the outline thus
sketched the details must be drawn with as great
rapidity as is consistent with accuracy. The reason
for rapidity will soon become obvious. Jupiter's
period of rotation is so short that the aspect of his
disc will be found to change materially even in half
an hour. Indeed, twenty minutes is perhaps as long
as the observer should allow himself for any individual
[pg 170]
drawing, and a little practice will convince
him that it is quite possible to represent a good deal
of detail in that time, and that, even with rapid
work, the placing of the various markings may be
made pretty accurate. The darker and more conspicuous
features should be laid down first of all,
and the more delicate details thereafter filled in, care
being taken to secure first those near the preceding
margin of the planet before they are carried out of
view by rotation. The colours of the various
features should be carefully noted at the sides of
the original drawings, and for this work twilight
observations are to be preferred.
Different observers vary to some extent, as might
be expected, in their estimates of the planet's
colouring, but on the whole there is a broad
general agreement. No planet presents such a
fine opportunity for colour-study as Jupiter, and on
occasions of good seeing the richness of the tones
is perfectly astonishing. In showing the natural
colours of the planet the reflector has a great
advantage over the refractor, and observers using
the reflecting type of instrument should devote
particular attention to this branch of the subject, as
there is no doubt that the colour of the various
features is liable to considerable, perhaps seasonal,
variation, and systematic observation of its changes
may prove helpful in solving the mystery of Jupiter's
condition. The times of beginning and ending
observation should be carefully noted, and also the
magnifying powers employed. These should not
[pg 171]
be too high. Jupiter does not need, and will not
stand, so much enlargement as either Mars or
Saturn. It is quite easy to secure a very large
disc, but over-magnifying is a great deal worse
than useless: it is a fertile source of mistakes and
illusions. If the student be content to make reasonable
use of his means, and not to overpress either
his instrument or his imagination, he will find upon
Jupiter work full of absorbing interest, and may be
able to make his own contribution to the serious
study of the great planet.
[pg 172]
CHAPTER XI
SATURN
At nearly double the distance of Jupiter from the
sun circles the second largest planet of our system,
unique, so far as human knowledge goes, in the
character of its appendages. The orbit of Saturn
has a mean radius of 886,000,000 miles, but owing
to its eccentricity, his distance may be diminished to
841,000,000 or increased to 931,000,000. This
large variation may not play so important a part in
his economy as might have been supposed, owing
to the fact that the sun heat received by him is not
much more than 1⁄100th of that received by the earth.
The planet occupies twenty-nine and a half years in
travelling round its immense orbit. Barnard's
measures with the Lick telescope give for the
polar diameter 69,770, and for the equatorial
76,470 miles. Saturn's polar compression is accordingly
very great, amounting to about 1⁄12th.
Generally speaking, however, it is not so obvious
in the telescopic view as the smaller compression
of Jupiter, being masked by the proximity of the
rings.
PLATE XXIV.
Saturn, July 2, 1894. E. E. Barnard, 36-inch Equatorial.
[pg 173]
Saturn is the least dense of all the planets; in
fact, this enormous globe, nine times the diameter
of the earth, would float in water. This fact of
extremely low density at once suggests a state of
matters similar to that already seen to exist, in all
likelihood, in the case of Jupiter; and all the
evidence goes to support the view that Saturn, along
with the other three large exterior planets, is in the
condition of a semi-sun.
The globe presents, on the whole, similar
characteristics to those already noticed as prevailing
on Jupiter, but, as was to be expected, in
a condition enfeebled by the much greater distance
across which they are viewed and the smaller scale
on which they are exhibited. It is generally girdled
by one or two tropical belts of a grey-green tone;
the equatorial region is yellow, and sometimes, like
the corresponding region of Jupiter, bears light
spots upon it and a narrow equatorial band of a
dusky tone; the polar regions are of a cold ashy or
leaden colour. Professor Barnard's fine drawing
(Plate XXIV.) gives an admirable representation
of these features as seen with the 36-inch Lick telescope.
Altogether, whether from greater distance
or from intrinsic deficiency, the colouring of Saturn
is by no means so vivid or so interesting as that of
his larger neighbour.
The period of rotation was, till within the last few
years, thought to be definitely and satisfactorily
ascertained. Sir William Herschel fixed it, from
his observations, at ten hours sixteen minutes.
[pg 174]
Professor Asaph Hall, from observations of a white
spot near the equator, reduced this period to
ten hours fourteen minutes twenty-four seconds.
Stanley Williams and Denning, in 1891, reached
results differing only by about two seconds from
that of Hall; but the former, discussing observations
of 1893, arrived at the conclusion that there
were variations of rotation presented in different
latitudes and longitudes of the planet's surface, the
longest period being ten hours fifteen minutes, and
the shortest ten hours twelve minutes forty-five
seconds. Subsequently Keeler obtained, by spectroscopic
methods, a result exactly agreeing with that
of Hall. It appeared, therefore, that fairly satisfactory
agreement had been reached on a mean
period of ten hours fourteen minutes twenty-four
seconds.
In 1903, however, a number of bright spots
appeared in a middle north latitude which, when
observed by Barnard, Comas Solà, Denning, and
other observers, gave a period remarkably longer
than that deduced from spots in lower latitudes—namely,
about ten hours thirty-eight minutes.
Accordingly, it follows that the surface of Saturn's
equatorial regions rotates much more rapidly than
that of the regions further north—a state of affairs
which presents an obvious likeness to that prevailing
on Jupiter. But in the case of Saturn the
equatorial current must move relatively to the rest
of the surface at the enormous rate of from 800 to
900 miles an hour, a speed between three and four
[pg 175]
times greater than that of the corresponding current
on Jupiter!
The resemblance between the two great planets
is thus very marked indeed. Great size, coupled
with small density; very rapid rotation, with its
accompaniment of large polar compression; and,
even more markedly in the case of the more distant
planet than in that of Jupiter, a variety of rotation
periods for different markings, which indicates that
these features have been thrown up from different
strata of the planet's substance—such points of likeness
are too significant to be ignored. It is not at all
likely that Saturn has any solidity to speak of, any
more than Jupiter; the probabilities all point in the
direction of a comparatively small nucleus of somewhat
greater solidity than the rest, surrounded by
an immense condensation shell, where the products
of various eruptions are represented.
Were this all that can be said about Saturn, the
planet would scarcely be more than a reduced and
somewhat less interesting edition of Jupiter. As it
is, he possesses characteristics which make him
Jupiter's rival in point of interest, and, as a mere
telescopic picture, perhaps even his superior. When
Galileo turned his telescope upon Saturn, he was
presented with what seemed an insoluble enigma.
It appeared to him that, instead of being a single
globe, the planet consisted of three globes in
contact with one another; and this supposed fact
he intimated to Kepler in an anagram, which, when
rearranged, read: 'Altissimum planetam tergeminum
[pg 176]
observavi'—'I have observed the most distant
planet to be threefold.' Under better conditions of
observation, he remarked subsequently, the planet
appeared like an olive, as it still does with low
powers. This was sufficiently puzzling, but worse
was to follow. After an interval, on observing
Saturn again, he found that the appearances which
had so perplexed him had altogether disappeared;
the globe was single, like those of the other planets.
In his letter to Welser, dated December 4, 1612,
the great astronomer describes his bewilderment,
and his fear lest, after all, it should turn out that his
adversaries had been right, and that his discoveries
had been mere illusions.
Then followed a period when observers could
only command optical power sufficient to show the
puzzling nature of the planet's appendages, without
revealing their true form. It appeared that Saturn
had 'ansæ,' or handles, on either side of him,
between which and his body the sky could be seen;
and many uncouth figures are still preserved which
eloquently testify to the bewilderment of those who
drew them, though some of them are wonderfully
accurate representations of the planet's appearance
when seen with insufficient means. The bewilderment
was sometimes veiled, in amusing cuttle-fish
fashion, under an inky cloud of sesquipedalian words.
Thus Hevelius describes the aspects of Saturn in the
following blasting flight of projectiles: 'The mono-spherical,
the tri-spherical, the spherico-ansated, the
elliptico-ansated, and the spherico-cuspidated,' which
[pg 177]
is very beautiful no doubt, but scarcely so simple
as one could wish a popular explanation to be.
In the year 1659, however, Huygens, who had
been observing Saturn with a telescope of 2⅓ inches
aperture and 23 feet focal length, bearing a magnifying
power of 100, arrived at the correct solution of the
mystery, which he announced to, or rather concealed
from, the world in a barbarous jumble of letters,
which, when properly arranged, read 'annulo cingitur,
tenui, plano, nusquam cohaerente, ad eclipticam
inclinato'—'he (Saturn) is surrounded by a thin
flat ring, nowhere touching (him, and) inclined to
the ecliptic.' Huygens also discovered the first and
largest of Saturn's satellites, Titan. His discoveries
were followed by those of Cassini, who in 1676
announced his observation of that division in the
ring which now goes by his name. From Cassini's
time onwards to the middle of the nineteenth century,
nothing was observed to alter to any great extent
the conception of the Saturnian system which had
been reached; though certain observations were
made, which, though viewed with some suspicion,
seemed to indicate that there were more divisions
in the ring than that which Cassini had discovered,
and that the system was thus a multiple one. In
particular a marking on the outer ring was detected
by Encke, and named after him, though generally
seen, if seen at all, rather as a faint shading than as
a definite division. (It is not shown in Barnard's
drawing, Plate XXIV.). But in 1850 came the
last great addition to our knowledge of the ring
[pg 178]
system, W. C. Bond in America, and Dawes in
England making independently the discovery of
the faint third ring, known as the Crape Ring,
which lies between the inner bright ring and the
globe.
The extraordinary appendages thus gradually
revealed present a constantly varying aspect according
to the seasons of the long Saturnian year.
At Saturn's equinoxes they disappear, being turned
edgewise; then, reappearing, they gradually broaden
until at the solstice, 7⅓ years later, they are seen at
their widest expansion; while from this point they
narrow again to the following equinox, and repeat
the same process with the opposite side of the ring
illuminated, the whole set of changes being gone
through in 29 years 167·2 days. Barnard's measures
give for the outer diameter of the outer ring
172,310 miles; while the clear interval between
the inner margin of the Crape Ring and the ball
is about 5,800 miles, and the width of the great
division in the ring-system (Cassini's) 2,270 miles.
In sharp contrast to these enormous figures is the
fact that the rings have no measurable thickness at
all, and can only be estimated at not more than
50 miles. They disappear absolutely when seen
edgewise; even the great Lick telescope lost them
altogether for three days in October, 1891.*
The answer to the question of what may be the
[pg 179]
constitution of these remarkable features may now
be given with a moderate approach to certainty.
It has been shown successively that the rings could
not be solid, or liquid, and in 1857 Clerk-Maxwell
demonstrated that the only possible constitution for
such a body is that of an infinite number of small
satellites. The rings of Saturn thus presumably
consist of myriads of tiny moonlets, each pursuing
its own individual orbit in its individual period, and
all drawn to their present form of aggregation by
the attraction of Saturn's bulging equator. The
appearances presented by the rings are explicable
on this theory, and on no other. Thus the brightness
of the two rings A and B would arise from the
closer grouping of the satellites within these zones;
while the semi-darkness of the Crape Ring arises
from the sparser sprinkling of the moonlets, which
allows the dark sky to be seen between them.
Cassini's division corresponds to a zone which has
been deprived of satellites; and as it has been
shown that this vacant zone occupies a position
where a revolving body would be subject to disturbance
from four of Saturn's satellites, the force
which cleared this gap in the ring is obvious. It
has been urged as an objection to the satellite
theory that while the thin spreading of the moonlets
would account for the comparative darkness of
the Crape Ring when seen against the sky, it by no
means accounts for the fact that this ring is seen
as a dark stripe upon the body of the planet.
Seeliger's explanation of this is both satisfactory
[pg 180]
and obvious, when once suggested—namely, that
the darkness of the Crape Ring against the planet is
due to the fact that what we see is not the actual
transits of the satellites themselves, but the perpetual
flitting of their shadows across the ball. The final
and conclusive argument in favour of this theory of
the constitution of the rings was supplied by the
late Professor Keeler by means of the spectroscope.
It is evident that if the rings were solid, the speed
of rotation should increase from their inner to their
outer margin—i.e., the outer margin must move
faster, in miles per second, than the inner does.
If, on the contrary, the rings are composed of a
great number of satellites, the relation will be
exactly reversed, and, owing to the superior
attractive force exercised upon them by the planet
through their greater nearness to him, the inner
satellites will revolve faster than the outer ones.
Now, this point is capable of settlement by spectroscopic
methods involving the application of the
well-known Doppler's principle, that the speed of a
body's motion produces definite and regular effects
upon the pitch of the light emitted or reflected by
it. The measurements were of extreme delicacy,
but the result was to give a rate of motion of
12½ miles per second for the inner edge of ring B,
and of 10 miles for the outer edge of A, thus
affording unmistakable confirmation of the satellite
theory of the rings. Keeler's results have since
been confirmed by Campbell and others; and it
may be regarded as a demonstrated fact that the
[pg 181]
rings, as already stated, consist of a vast number of
small satellites.
It has been maintained that the ball of Saturn is
eccentrically placed within the ring, and further,
that this eccentricity is essential to the stability of
the system; while the suggestion has also been
made that the ring-system is undergoing progressive
change, and that the interval between it and the
ball is lessening. It has to be noticed, however,
that the best measures, those of Barnard, indicate
that the ball is symmetrically placed within the
rings; and the suggestion of a diminishing interval
between the ring-system and the ball receives no
countenance from comparison of the measures
which have been made at different times.
There can be no question that of all objects
presented to observation in the solar system, there
is not one, which, for mere beauty and symmetry
can be for a moment compared with Saturn, even
though, as already indicated, Mars and Jupiter
present features of more lasting interest. To quote
Proctor's words: 'The golden disc, faintly striped
with silver-tinted belts; the circling rings, with
their various shades of brilliancy and colour; and
the perfect symmetry of the system as it sweeps
across the dark background of the field of view,
combine to form a picture as charming as it is sublime
and impressive.' Fortunately the main features
of this beautiful picture are within the reach of very
humble instruments. Webb states that when the
ring system was at its greatest breadth he has seen it
[pg 182]
with a power of about twenty on only 1⅓-inch aperture.
A beginner cannot expect to do so much with such
small means; but at all events a 2-inch telescope
with powers of from 50 to 100 will reveal the main
outlines of the ring very well indeed, and, with
careful attention will show the shadow of the ring
upon the ball, and that of the ball upon the ring.
When we come to the question of the division in
the ring, we are on somewhat more doubtful ground.
Proctor affirms that 'the division in the ring
(Cassini's) can be seen in a good 2-inch aperture
in favourable weather.' One would have felt inclined
to say that the weather would require to be very
favourable indeed, were it not that Proctor's statement
is corroborated by Denning, who remarks that
'With a 2-inch refractor, power about ninety, not
only are the rings splendidly visible, but Cassini's
division is readily glimpsed, as well as the narrow
dark belt on the body of the planet.' The student
may, however, be warned against expecting that
such statements will apply to his own individual
efforts. There are comparatively few observers
whose eyes have had such systematic training as
to qualify them for work like this, and those who
begin by expecting to see all that skilled observers
see with an instrument of the same power are only
laying up for themselves stores of disappointment.
Mr. Mee's frank confession may be commended to
the notice of those who hope to see at the first
glance all that old students have learned to see by
years of hard work. 'The first time I saw Saturn
[pg 183]
through a large telescope, a fine 12-inch reflector,
I confess I could not see the division (Cassini's),
though the view of the planet was one of exquisite
beauty and long to be remembered, and notwithstanding
the fact that the much fainter division of
Encke was at the moment visible to the owner of
the instrument!' It is extremely unlikely that the
beginner will see the division with anything much
less than 3 inches, and even with that aperture he
will not see it until the rings are well opened. The
writer's experience is that it is not by any means
so readily seen as is sometimes supposed. Three
inches will show it under good conditions; with 3⅞
it can be steadily held, even when the rings are only
moderately open (steady holding is a very different
thing from 'glimpsing'), but even with larger apertures
the division becomes by no means a simple object
as the rings close up (Fig. 26). In fact, there is
nothing better fitted to fill the modern observer's
mind with a most wholesome respect for the memory
[pg 184]
of a man like Cassini, than the thought that with
his most imperfect appliances this great observer
detected the division, a much more difficult feat
than the mere seeing it when its existence and
position are already known, and discovered also
four of the Saturnian satellites. As for the minor
divisions in the ring, if they are divisions, they are
out of the question altogether for small apertures,
and are often invisible even to skilled observers
using the finest telescopes. Barnard's drawing
(Plate XXIV.), as already noted, shows no trace of
Encke's division; but nine months later the same
observer saw it faintly in both ansæ of the ring.
The conclusion from this and many similar observations
seems to be that the marking is variable, as
may very well be, from the constitution of the ring.
The Crape Ring is beyond any instrument of less
than four inches, and even with such an aperture
requires favourable circumstances.
FIG. 26.
Saturn, 3⅞-inch.
With regard to a great number of very remarkable
details which of late years have been seen and
drawn by various observers, it may be remarked
that the student need not be unduly disappointed
should his small instrument fail, as it almost certainly
will, to show these. This is a defect which his
telescope shares with an instrument of such respectable
size and undoubted optical quality as the Lick
36-inch. Writing in January, 1895, concerning the
beautiful drawing which accompanies this chapter,
Professor Barnard somewhat caustically observes:
'The black and white spots lately seen upon Saturn
[pg 185]
by various little telescopes were totally beyond the
reach of the 36-inch—as well as of the 12-inch—under
either good or bad conditions of seeing....
The inner edge (of the Crape Ring) was a uniform
curve; the serrated or saw-toothed appearance of
its inner edge which had previously been seen with
some small telescopes was also beyond the reach of
the 36-inch.' Such remarks should be consoling to
those who find themselves and their instruments
unequal to the remarkable feats which are sometimes
accomplished, or recorded.
So far as one's personal experience goes, Saturn
is generally the most easily defined of all the planets.
Of late years he has been very badly placed for
observers in the Northern Hemisphere, and this
has considerably interfered with definition. But
when well placed the planet presents a sharpness
and steadiness of outline which render him capable
of bearing higher magnifying powers than Jupiter,
and even than Mars, though a curious rippling
movement will often be noticed passing along the
rings. It can scarcely be said, however, that there
is much work for small instruments upon Saturn—the
seeing of imaginary details being excluded.
Accordingly, in spite of the undoubted beauty of
the ringed planet, Jupiter will on the whole be
found to be an object of more permanent interest.
Yet, viewed merely as a spectacle, and as an
example of extraordinary grace and symmetry,
Saturn must always command attention. The sight
of his wonderful system can hardly fail to excite
[pg 186]
speculation as to its destiny; and the question of
the permanence of the rings is one that is almost
thrust upon the spectator. With regard to this
matter it may be noted that, according to Professor
G. H. Darwin, the rings represent merely a passing
stage in the evolution of the Saturnian system. At
present they are within the limit proved by Roche,
in 1848, to be that within which no secondary body
of reasonable size could exist; and thus the discrete
character of their constituents is maintained by the
strains of unequal attraction. Professor Darwin
believes that in time the inner particles of the ring
will be drawn inwards, and will eventually fall upon
the planet's surface, while the outer ones will
disperse outwards to a point beyond Roche's limit,
where they may eventually coalesce into a satellite
or satellites—a poor compensation for the loss of
appendages so brilliant and unique as the rings.
Saturn's train of satellites is the most numerous
and remarkable in our system. As already mentioned,
Huygens, the discoverer of the true form of the
ring, discovered also the first and brightest satellite,
Titan, which is a body somewhat larger than our
own moon, having a diameter of 2,720 miles. A
few years later came Cassini's discoveries of four
other satellites, beginning in 1671 and ending in
1684. For more than 100 years discovery paused
there, and it was not until August and September,
1789, that Sir William Herschel added the sixth
and seventh to our knowledge of the Saturnian
system.
[pg 187]
In 1848 Bond in America and Lassell in England
made independently the discovery of the eighth
satellite—another of the coincidences which marked
the progress of research upon Saturn, and in both
of which Bond was concerned. Then followed
another pause of fifty years broken by the discovery,
in 1898, by Professor Pickering, of a ninth,
whose existence was not completely confirmed till
1904. The motion of this satellite has proved to
be retrograde, unlike that of the earlier discovered
members of the family, so that its discovery has
introduced us to a new and abnormal feature of the
Saturnian system. The discoverer of Phœbe, as
the ninth satellite has been named, has followed up
his success by the discovery of a tenth member of
Saturn's retinue, known provisionally as Themis.
Accordingly the system, as at present known,
consists of a triple ring and ten satellites. The
last discovered moons are very small bodies, the
diameter of Phœbe, for instance, being estimated
at 150 miles; while its distance from Saturn is
8,000,000 miles. From the surface of the planet
Phœbe would appear like a star of fifth or sixth
magnitude; to observers on our own earth its
magnitude is fifteenth or sixteenth. The ten
satellites have been named as follows: 1, Titan,
discovered by Huygens; 2, Japetus; 3, Rhea;
4, Dione; 5, Tethys, all discovered by Cassini;
6, Enceladus; and 7, Mimas, Sir William Herschel;
8, Hyperion, Bond and Lassell; 9, Phœbe; and
10, Themis, W. H. Pickering. Titan, the largest
[pg 188]
satellite, has been found to be considerably denser
than Saturn himself.
The most of these little moons are, of course,
beyond the power of small glasses; but a 2-inch
will show Titan perfectly well. Japetus also is not
a difficult object, but is much easier at his western
than at his eastern elongation, a fact which probably
points to a surface of unequal reflective power.
Rhea, Dione, and Tethys are much more difficult.
Kitchiner states that a friend of his saw them with
27⁄10-inch aperture, the planet being hidden; but
probably his friend had been amusing himself at the
quaint old gentleman's expense. Noble concludes
that with a first-class 3-inch and under favourable
circumstances four, or as a bare possibility even
five, satellites may be seen; and I have repeatedly
seen all the five with 3⅞-inches. The only particular
advantages of seeing them are the test which they
afford of the instrument used, and the accompanying
practice of the eye in picking up minute points of
light. There is a considerable interest in watching
the gradual disappearance of the brilliant disc of
Saturn behind the edge of the field, or of the thick
wire which may be placed in the eye-piece to hide
the planet, and then catching the sudden flash up
of the tiny dots of light which were previously lost
in the glare of the larger body. For purposes of
identification, recourse must be had to the 'Companion
to the Observatory,' which prints lists of the
elongations of the various satellites and a diagram
[pg 189]
of their orbits which renders it an easy matter to
identify any particular satellite seen. Transits are,
with the exception of that of Titan, beyond the
powers of such instruments as we are contemplating.
The shadow of Titan has, however, been seen in
transit with a telescope of only 2⅞-inch aperture.
[pg 190]
CHAPTER XII
URANUS AND NEPTUNE
Hitherto we have been dealing with bodies which,
from time immemorial, have been known to man as
planets. There must have been a period when one
by one the various members of our system known
to the ancients were discriminated from the fixed
stars by unknown but patient and skilful observers;
but, from the dawn of historical astronomy, up to
the night of March 13, 1781, there had been no
addition to the number of those five primary planets
the story of whose discovery is lost in the mists of
antiquity.
It may be questioned whether any one man,
Kepler and Newton being possible exceptions, has
ever done so much for the science of astronomy as was
accomplished by Sir William Herschel. Certainly
no single observer has ever done so much, or, which
is almost more important than the actual amount of
his achievement, has so completely revolutionized
methods and ideas in observing.
A Hanoverian by birth, and a member of the
band of the Hanoverian Guards, Herschel, after
[pg 191]
tasting the discomforts of war in the shape of a
night spent in a ditch on the field of Hastenbeck,
where that egregious general the Duke of Cumberland
was beaten by the French, concluded that he
was not designed by Nature for martial distinction,
and abruptly solved the problem of his immediate
destiny by recourse to the simple and unheroic
expedient of desertion. He came to England, got
employment after a time as organist of the Octagon
Chapel at Bath, and was rapidly rising into notice
as a musician, when the force of his genius, combined
with a discovery which came certainly
unsought, but was grasped as only a great man can
grasp the gifts of Fortune, again changed the
direction of his life, and gave him to the science of
astronomy.
He had for several years employed his spare time
in assiduous observation; and, finding that opticians'
prices were higher than he could well afford, had
begun to make Newtonian reflectors for himself,
and had finally succeeded in constructing one of
6½ inches aperture, and of high optical quality.
With this instrument, on the night of March 13,
1781, he was engaged in the execution of a plan
which he had formed of searching the heavens for
double stars, with a view to measuring their distance
from the earth by seeing whether the apparent
distance of the members of the double from one
another varied in any degree in the course of the
earth's journey round the sun. He was working
through the stars in the constellation Gemini, when
[pg 192]
his attention was fixed by one which presented a
different appearance from the others which had
passed his scrutiny.
In a good telescope a fixed star shows only a
very small disc, which indeed should be but a point
of light; and the finer the instrument the smaller
the disc. The disc of this object, however, was
unmistakably larger than those of the fixed stars in
its neighbourhood—unmistakably, that is, to an
observer of such skill as Herschel, though those
who have seen Uranus under ordinary powers will
find their respect considerably increased for the
skill which at once discriminated the tiny greenish
disc from that of a fixed star. Subsequent observation
revealed to Herschel that he was right in
supposing that this body was not a star, for it
proved to be in motion relatively to the stars among
which it was seen. But, in spite of poetic authority,
astronomical discoveries do not happen quite so
dramatically as the sonnet 'On First looking into
Chapman's Homer' suggests.
'Then felt I like some watcher of the skies,
When a new planet swims into his ken'
is a noble simile, were it only true to the facts. But
new planets do not swim around promiscuously in
this fashion; and in the case of Uranus, which
more nearly realizes the thought of Keats than any
other in the history of astronomy, the 'watcher of
the skies' felt probably more puzzlement than anything
else. Herschel was far from realizing that he
[pg 193]
had found a new planet. When unmistakable
evidence was forthcoming that the newly discovered
body was not a fixed star, he merely felt confirmed
in the first conjecture which had been suggested by
the size of its disc—namely, that he had discovered
a new comet; and it was as a new comet that
Uranus was first announced to the astronomical
world.
It quickly became evident, however, that the new
discovery moved in no cometary orbit, but in one
which marked it out as a regular member of the
solar system. A search was then instituted for
earlier observations of the planet, and it was found
to have been observed and mistaken for a fixed star
on twenty previous occasions! One astronomer,
Lemonnier, had actually observed it no fewer than
twelve times, several of them within a few weeks of
one another, and, had he but reduced and compared
his observations, could scarcely have failed to have
anticipated Herschel's discovery. But perhaps an
astronomer who, like Lemonnier, noted some of his
observations on a paper-bag which had formerly
contained hair-powder, and whose astronomical
papers have been described as 'the image of
Chaos,' scarcely deserved the honour of such a
discovery!
When it became known that this new addition to
our knowledge of the solar system had been made
by the self-taught astronomer at Bath, Herschel was
summoned to Court by George III., and enabled to
devote himself entirely to his favourite study by the
[pg 194]
bestowal of the not very magnificent pension of
£200 a year, probably the best investment that
has ever been made in the interests of astronomical
science. In gratitude to the penurious monarch who
had bestowed on him this meagre competence,
Herschel wished to call his planet the Georgium
Sidus—the Georgian Star, and this title, shortened
in some instances to the Georgian, is still to be
found in some ancient volumes on astronomy. The
astronomers of the Continent, however, did not feel
in the least inclined to elevate Farmer George to
the skies before his due time, and for awhile the
name of Herschel was given to the new planet,
which still bears as its symbol the first letter of its
discoverer's name with a globe attached to the
cross-bar . Finally, the name Urănus ('a' short)
prevailed, and has for long been in universal use.
Uranus revolves round the sun at a distance from
him of about 1,780,000,000 miles, in an orbit which
takes eighty-four of our years to complete. Barnard
gives his diameter at 34,900 miles, and if this
measure be correct, he is the third largest planet
of the system. Other measures give a somewhat
smaller diameter, and place Neptune above him in
point of size.
Subsequent observers have been able to see but
little more than Herschel saw upon the diminutive
disc to which even so large a body is reduced at so
vast a distance. When near opposition, Uranus
can readily be seen with the naked eye as a star of
about the sixth magnitude, and there is no difficulty
[pg 195]
in picking him up with the finder of an ordinary
telescope by means of an almanac and a good star
map, nor in raising a small disc by the application
of a moderately high power, say 200 and upwards.
(Herschel was using 227 at the time of his discovery.)
But small telescopes do little more than
give their owners the satisfaction of seeing, pretty
much as Herschel saw it, the object on which his
eye was the first to light. Nor have even the
largest instruments done very much more. Rings,
similar to those of Saturn, were once suspected, but
have long since been disposed of, and most of the
observations of spots and belts have been gravely
questioned. The Lick observers in 1890 and 1891
describe the belts as 'the merest shades on the
planet's surface.'
The spectrum of Uranus is marked by peculiarities
which distinguish it from that of the other planets. It
is crossed by six dark absorption-bands, which indicate
at all events that the medium through which the
sunlight which it reflects to us has passed is of a
constitution markedly different from that of our
own atmosphere. It was at first thought that the
spectrum gave evidence of the planet's self-luminosity;
but this has not proved to be the case,
though doubtless Uranus, like Jupiter and Saturn,
is in the condition of a semi-sun. Like the other
members of the group of large exterior planets,
his density is small, being only ⅕ greater than that
of water.
Six years after his great discovery, Herschel, with
[pg 196]
the 40-foot telescope of 4 feet in aperture which
he had now built, discovered two satellites, and
believed himself to have discovered four more.
Later observations have shown that, in the case of
the four, small stars near the planet had been mistaken
for satellites. Subsequently two more were
discovered, one by Lassell, and one by Otto Struve,
making the number of the Uranian retinue up to
four, so far as our present knowledge goes. These
four satellites, known as Ariel, Umbriel, Oberon,
and Titania, are distinguished by the fact that their
orbits are almost perpendicular to the plane of the
orbit of Uranus, and that the motions of all of them
are retrograde. Titania and Oberon, the two discovered
by Herschel, are the easiest objects; but
although they are said to have been seen with a
4·3-inch refractor, this is a feat which no ordinary
observer need hope to emulate. An 8-inch is a
more likely instrument for such a task, and a
12-inch more likely still; the average observer will
probably find the latter none too big. Accordingly,
they are quite beyond the range of such observation
as we are contemplating. The rotation period of
Uranus is not known.
In a few years after the discovery of Uranus, it
became apparent that by no possible ingenuity could
his places as determined by present observation be
satisfactorily combined with those determined by the
twenty observations available, as already mentioned,
from the period before he was recognised as a planet.
Either the old observations were bad, or else the
[pg 197]
new planet was wandering from the track which it
had formerly followed. It appeared to Bouvard,
who was constructing the tables for the motions of
Uranus, the simplest course to reject the old
observations as probably erroneous, and to confine
himself to the modern ones. Accordingly this
course was pursued, and his tables were published
in 1821, but only for it to be found that in a few
years they also began to prove unsatisfactory;
discrepancies began to appear and to increase, and
it quickly became apparent that an attempt must be
made to discover the cause of them.
Bouvard himself appears to have believed in the
existence of a planet exterior to Uranus whose
attraction was producing these disturbances, but he
died in 1843 before any progress had been made
with the solution of the enigma. In 1834 Hussey
approached Airy, the Astronomer Royal, with the
suggestion that he might sweep for the supposed
exterior planet if some mathematician would help
him as to the most likely region to investigate.
Airy, however, returned a sufficiently discouraging
answer, and Hussey apparently was deterred by it
from carrying out a search which might very possibly
have been rewarded by success. Bessel, the great
German mathematician, had marked the problem for
his own, and would doubtless have succeeded in
solving it, but shortly after he had begun the
gathering of material for his researches, he was
seized with the illness which ultimately proved fatal
to him.
[pg 198]
The question was thus practically untouched when
in 1841, John Couch Adams, then an undergraduate
of St. John's College, Cambridge, jotted down a
memorandum in which he indicated his resolve to
attack it and attempt the discovery of the perturbing
planet, 'as soon as possible after taking
my degree.' The half-sheet of notepaper on which
the memorandum was made is still extant, and forms
part of the volume of manuscripts on the subject
preserved in the library of St. John's College.
On October 21, 1845, Adams, who had taken his
degree (Senior Wrangler) in 1843, communicated to
Airy the results of his sixth and final attempt at the
solution of the problem, and furnished him with the
elements and mass of the perturbing planet, and an
indication of its approximate place in the heavens.
Airy, whose record in the matter reads very
strangely, was little more inclined to give encouragement
to Adams than to Hussey. He
replied by propounding to the young investigator a
question which he considered 'a question of vast
importance, an experimentum crucis,' which Adams
seemingly considered of so little moment, that
strangely enough he never troubled to answer it.
Then the matter dropped out of sight, though, had
the planet been sought for when Adams's results
were first communicated to the Astronomer Royal,
it would have been found within 3½ lunar diameters
of the place assigned to it.
Meanwhile, in France, another and better-known
mathematician had taken up the subject, and in
[pg 199]
three memoirs presented to the French Academy of
Sciences in 1846, Leverrier furnished data concerning
the new planet which agreed in very
remarkable fashion with those furnished by Adams
to Airy. The coincidence shook Airy's scepticism,
and he asked Dr. Challis, director of the Cambridge
Observatory, to begin a search for the planet with
the large Northumberland equatorial. Challis, who
had no complete charts of the region to be searched,
began to make observations for the construction of
a chart which would enable him to detect the planet
by means of its motion. It is more than likely that
had he adopted Hussey's suggestion of simply
sweeping in the vicinity of the spot indicated, he
would have been successful, for the Northumberland
telescope was of 11 inches aperture, and would have
borne powers sufficient to distinguish readily the
disc of Neptune from the fixed stars around it.
However, Challis chose the more thorough, but
longer method of charting; and even to that he did
not devote undivided attention. 'Some wretched
comet,' says Proctor, 'which he thought it his
more important duty to watch, prevented him from
making the reductions which would have shown him
that the exterior planet had twice been recorded in
his notes of observations.'
Indeed, a certain fatality seems to have hung over
the attempts made in Britain to realize Adams's
discovery. In 1845, the Rev. W. R. Dawes, one
of the keenest and most skilful of amateur observers,
was so much impressed by some of Adams's letters
[pg 200]
to the Astronomer Royal that he wrote to Lassell,
asking him to search for the planet. When Dawes's
letter arrived, Lassell was suffering from a sprained
ankle, and laid the letter aside till he should be able
to resume work. In the meantime the letter was
burned by an officious servant-maid, and Lassell
lost the opportunity of a discovery which would
have crowned the fine work which he accomplished
as an amateur observer.
A very different fate had attended Leverrier's
calculations. On September 23, 1846, a letter from
Leverrier was received at the Berlin Observatory,
asking that search should be made for the planet in
the position which his inquiries had pointed out.
The same night Galle made the search, and within
a degree of the spot indicated an object was found
with a measurable disc of between two and three
seconds diameter. As it was not laid down on
Bremiker's star-chart of the region, it was clearly
not a star, and by next night its planetary nature
was made manifest. The promptitude with which
Leverrier's results were acted upon by Encke and
Galle is in strong contrast to the sluggishness which
characterized the British official astronomers, who,
indeed, can scarcely be said to have come out of the
business with much credit.
A somewhat undignified controversy ensued.
The French astronomers, very naturally, were
eager to claim all the laurels for their brilliant
countryman, and were indignant when a claim was
put in on behalf of a young Englishman whose
[pg 201]
name had never previously been heard of. Airy,
however, displayed more vigour in this petty
squabble than in the search for Neptune, and
presented such evidence in support of his fellow-countryman's
right to recognition that it was impossible
to deny him the honour which, but for
official slackness, would have fallen to him as the
actual as well as the potential discoverer of the new
planet. Adams himself took no part in the strife;
spoke, indeed, no words on the matter, except to
praise the abilities of Leverrier, and gave no sign
of the annoyance which most men in like circumstances
would have displayed.
Galle suggested that the new planet should be
called Janus; but the name of the two-faced god
was felt to be rather too pointedly suitable at the
moment, and that of Neptune was finally preferred.
Neptune is about 32,900 miles in diameter, his
distance from the sun is 2,792,000,000 miles, and
he occupies 165 years in the circuit of his gigantic
orbit. The spectroscopic evidence, such as it is,
seems to point to a condition somewhat similar
to that of Uranus.
Neptune had only been discovered seventeen
days when Lassell found him to be attended by
one satellite. First seen on October 10, 1846, it
was not till the following July that the existence
of this body was verified by Lassell himself and
also by Otto Struve and Bond of Harvard. From
the fact that it is visible at such an enormous
distance, it is evident that this satellite must be
[pg 202]
of considerable size—probably at least equal to our
own moon.
Small instruments can make nothing of Neptune
beyond, perhaps, distinguishing the fact that, whatever
the tiny disc may be, it is not that of a star.
His satellite is an object reserved for the very finest
instruments alone.
Should Neptune have any inhabitants, their sky
must be somewhat barren of planets. Jupiter's
greatest elongation from the sun would be about
10°, and he would be seen under somewhat less
favourable conditions than those under which we
see Mercury; while the planets between Jupiter
and the sun would be perpetually invisible. Saturn
and Uranus, however, would be fairly conspicuous,
the latter being better seen than from the earth.
Suspicions have been entertained of the existence
of another planet beyond Neptune, and photographic
searches have been made, but hitherto
without success. So far as our present knowledge
goes, Neptune is the utmost sentinel of the regular
army of the solar system.
[pg 203]
CHAPTER XIII
COMETS AND METEORS
There is one type of celestial object which seldom
fails to stir up the mind of even the most sluggishly
unastronomical member of the community and to
inspire him with an interest in the science—an interest
which is usually conspicuous for a picturesque
inaccuracy in the details which it accumulates, for
a pathetic faith in the most extraordinary fibs which
may be told in the name of science, and for a subsidence
which is as rapid as the changes in the
object which gave the inspiration. The sun may
go on shining, a perpetual mystery and miracle,
without attracting any attention, save when a wet
spring brings on the usual talk of sun-spots and
the weather; Jupiter and Venus excite only sufficient
interest to suggest an occasional question as
to whether that bright star is the Star of Bethlehem;
but when a great comet spreads its fiery tail
across the skies everybody turns astronomer for the
nonce, and normally slumber-loving people are found
willing, or at least able, to desert their beds at the
most unholy hours to catch a glimpse of the strange
and mysterious visitant. And, when the comet
[pg 204]
eventually withdraws from view again, as much
inaccurate information has been disseminated among
the public as would fill an encyclopædia, and require
another to correct.
Comets are, however, really among the most
interesting of celestial objects. Though we no
longer imagine them to foretell wars, famines, and
plagues, or complacently to indicate the approbation
of heaven upon some illustrious person deceased or
about to decease, and have almost ceased to shiver
at the possibilities of a collision between a comet
and the earth, they have within the last half century
taken on a new and growing interest of a more
legitimate kind, and there are few departments of
science in which the advance of knowledge has
been more rapid or which promise more in the
immediate future, given material to work upon.
The popular idea of a comet is that it is a kind
of bright wandering star with a long tail. Indeed,
the star part of the conception is quite subsidiary
to the tail part. The tail is the thing, and a comet
without a tail is not worthy of attention, if it is not
rather guilty of claiming notice on false pretences.
As a matter of fact, the tail is absent in many
comets and quite inconspicuous in many more;
and a comet may be a body with any degree of
resemblance or want of resemblance to the popular
idea, from the faint globular stain of haze, scarcely
perceptible in the telescopic field against the dark
background of the sky, up to a magnificent object,
which, like the dragon in the Revelation, seems to
[pg 205]
draw the third part of the stars of heaven after it—an
object like the Donati comet of 1858, with a
nucleus brighter than a first-magnitude star, and a
tail like a great feathery plume of light fifty millions
of miles in length. It seems as impossible to set
limits to the variety of form of which comets are
capable as it is to set limits to their number.
Generally speaking, however, a comet consists of
three parts: The nucleus—which appears as a more
or less clearly defined star-like point, and is the only
part of the comet which will bear any magnification
to speak of—the coma, and the tail. In many
telescopic comets the nucleus is entirely absent,
and, in the comets in which it is present, it is
of very varied size, and often presents curious
irregularities in shape, and even occasionally the
appearance of internal motions. It frequently
changes very much in size during the period of the
comet's visibility. The nucleus is the only part of
a comet's structure which has even the most
distant claim to solidity; but even so the evidence
which has been gradually accumulated all goes to
show that while it may be solid in the sense of
being composed of particles which have some
substance, it is not solid in the sense of being one
coherent mass, but rather consists of something like
a swarm of small meteoric bodies. Surrounding
the nucleus is the coma, from which the comet
derives its name. This is a sort of misty cloud
through which the nucleus seems to shine like a
star in a nebula or a gas-lamp in a fog. Its
[pg 206]
boundaries are difficult to trace, as it appears to
fade away gradually on every side into the background;
but generally its appearance is more or less
of a globular shape except where the tail streams
away from it behind. Sometimes the coma is of
enormous extent—the Great Comet of 1811 showed
a nucleus of 428 miles diameter, enclosed within a
nebulous globe 127,000 miles across, which in its
turn was wrapped in a luminous atmosphere of four
times greater diameter, with an outside envelope
covering all, and extending backwards to form the
tail. But it is also of the most extraordinary
tenuity, the light of the very faintest stars having
been frequently observed to shine undimmed through
several millions of miles of coma. Finally, there is
the tail, which may be so short as to be barely
distinguishable; or may extend, as in the case of
Comet 1811 (ii.), to 130,000,000 miles; or, as in
that of Comet 1843 (i.), to 200,000,000. The most
tenuous substances with which we are acquainted
seem to be solidity itself compared with the material
of a comet's tail. It is 'such stuff as dreams are
made of.'
Comets fall into two classes. There are those
whose orbits follow curves that are not closed, like
the circle or the ellipse, but appear to extend indefinitely
into space. A comet following such an
orbit (parabolic or hyperbolic) seems to come
wandering in from the depths of space, passes round
the sun, and then gradually recedes into the space
from which it came, never again to be seen of
[pg 207]
human eye. It is now becoming questionable,
however, whether any comet can really be said to
come in from infinite space; and the view is being
more generally held that orbits which to us appear
portions of unclosed curves may in reality be only
portions of immensely elongated ellipses, and that
all comets are really members of the solar system,
travelling away, indeed, to distances that are
immense compared with even the largest planetary
orbit, but yet infinitely small compared with the
distances of the fixed stars.
Second, there are those comets whose orbits form
ellipses with a greater or less departure from the
circular form. Such comets must always return
again, sooner or later, to the neighbourhood of the
sun, which occupies one of the foci of the ellipse,
and they are known as Periodic Comets. The
orbits which they follow may have any degree of
departure from the circular form, from one which
does not differ very notably from that of such a
planet as Eros, up to one which may be scarcely
distinguishable from a parabola. Thus we have
Periodic Comets again divided into comets of short
and comets of long period. In the former class,
the period ranges from that of Encke's comet which
never travels beyond the orbit of Jupiter, and only
takes 3·29 years to complete its journey, up to that
of the famous comet whose periodicity was first
discovered by Halley, whose extreme distance from
the sun is upwards of 3,200,000,000 miles, and
whose period is 76·78 years. Comets of long
[pg 208]
period range from bodies which only require a
paltry two or three centuries to complete their
revolution, up to others whose journey has to be
timed by thousands of years. In the case of these
latter bodies, there is scarcely any distinction to be
made between them and those comets which are
not supposed to be periodic; the ellipse of a
comet which takes three or four thousand years to
complete its orbit is scarcely to be distinguished, in
the small portion of it that can be traced, from a
parabola.
Several comets have been found to be short period
bodies, which, though bright enough to have been
easily seen, have yet never been noticed at any
previous appearance. It is known that some at
least of these owe their present orbits to the fact
that having come near to one or other of the planets
they have been, so to speak, captured, and diverted
from the track which they formerly pursued. Several
of the planets have more or less numerous flocks of
comets associated with them which they have thus
captured and introduced into a short period career.
Jupiter has more than a score in his group, while
Saturn, Uranus and Neptune have smaller retinues.
There can be no question that a comet of first-class
splendour, such as that of 1811, that of 1858, or that
of 1861, is one of the most impressive spectacles
that the heavens have to offer. Unfortunately it is
one which the present generation, at least in the
northern hemisphere, has had but little opportunity
of witnessing. Chambers notices 'that it may be
[pg 209]
taken as a fact that a bright and conspicuous comet
comes about once in ten years, and a very remarkable
comet once every thirty years;' and adds, 'tested
then by either standard of words "bright and
conspicuous," or "specially celebrated," it may be
affirmed that a good comet is now due.' It is eleven
years since that hopeful anticipation was penned,
and we are still waiting, not only for the 'specially
celebrated,' but even for the 'bright and conspicuous'
comet; so that on the whole we may be said to have
a grievance. Still, there is no saying when the
grievance may be removed, as comets have a knack
of being unexpected in their developments; and it
may be that some unconsidered little patch of haze
is even now drawing in from the depths which may
yet develop into a portent as wonderful as those
that astonished the generation before us in 1858
and 1861.
The multitude of comets is, in all probability,
enormous. Between the beginning of the Christian
era and 1888 the number recorded was, according
to Chambers, 850; but the real number for that
period must have been indefinitely greater, as, for
upwards of 1600 out of the 1888 years, only those
comets which were visible to the naked eye could
have been recorded—a very small proportion of the
whole. The period 1801 to 1888 shows 270, so
that in less than one century there has been recorded
almost one-third of the total for nineteen centuries.
At present no year goes by without the discovery
of several comets; but very few of them become at
[pg 210]
all conspicuous. For example, in 1904, six comets
were seen—three of these being returns of comets
previously observed, and three new discoveries; but
none of these proved at all notable objects in the
ordinary sense, though Comet 1904 (a), discovered
by Brooks, was pretty generally observed.
It would serve no useful purpose to repeat here
the stories of any of the great comets. These may
be found in considerable detail in such volumes as
Chambers's 'Handbook of Astronomy,' vol. i., or
Miss Agnes Clerke's 'History of Astronomy.'
Attention must rather be turned to the question,
'What are comets?' It is a question to which no
answer of a satisfactory character could be given
till within the last fifty years. Even the great comet
of 1858, the Donati, which made so deep an impression
on the public mind, and was so closely
followed and studied by astronomers, was not the
medium of any great advance in the knowledge of
cometary nature. The many memoirs which it
elicited disclosed nothing fundamentally new, and
broke out no new lines of inquiry. Two things
have since then revolutionized the study of the
subject—the application of the spectroscope to the
various comets that have appeared in the closing years
of the nineteenth century, and the discovery of the
intimate connection between comets and meteors.
It was in 1864, a year further made memorable
astronomically by Sir William Huggins's discovery
of the gaseous nature of some of the nebulæ, that
the spectroscope was first applied to the study of
[pg 211]
a comet. The celestial visitor thus put to the
question, a comet discovered by Tempel, was in
nowise a distinguished object, appearing like a star
of the second magnitude, or less, with a feeble
though fairly long tail. When analyzed by Donati,
it was found to yield a spectrum consisting of three
bright bands, yellow, green, and blue, separated by
dark spaces. This observation at once modified
ideas as to cometary structure. Hitherto it had
been supposed that comets shone by reflected light;
but Donati's observation revealed beyond question
that the light of the 1864 comet at all events was
inherent, and that, so far as the observation went,
the comet consisted of glowing gas.
PLATE XXV.
Great Comet. Photographed May 5, 1901, with the 13-inch Astrographic Refractor
of the Royal Observatory, Cape of Good Hope.
In 1868 Sir William Huggins carried the matter
one step further by showing that the spectrum of
Winnecke's comet of that year agreed with that of
olefiant gas rendered luminous by electricity; and
the presence of the hydrocarbon spectrum has since
been detected in a large number of comets. The
first really brilliant comet to be analyzed by the
spectroscope was Coggia's (1874), and it presented
not only the three bright bands that had been
already seen, but the whole range of five bands
characteristic of the hydrocarbon spectrum. In
certain cases, however—notably, that of Holmes's
comet of 1892 and that of the great southern comet of
1901 (Plate XXV.)—the spectrum has not exhibited
the usual bright band type, but has instead shown
merely a continuous ribbon of colour. From these
analyses certain facts emerge. First, that the
[pg 212]
gaseous surroundings of comets consist mainly of
hydrogen and carbon, and that in all probability
their luminosity is due, not to mere solar heat, but
to the effect of some electric process acting upon
them during their approach to the sun; and second,
that, along with these indications of the presence
of luminous hydrocarbon compounds, there is also
evidence of the existence of solid particles, mainly
in the nucleus, but also to some extent in the rest
of the comet, which shine by reflected sunlight. It
is further almost certain, from the observation by
Elkin and Finlay of the beginning of the transit
of Comet 1882 (iii.) across the sun's face, that this
solid matter is not in any sense a solid mass. The
comet referred to disappeared absolutely as soon as
it began to pass the sun's edge. Had it been a
solid mass or even a closely compacted collection
of small bodies it would have appeared as a black
spot upon the solar surface. The conclusion, then,
is obvious that the solid matter must be very thinly
and widely spread, while its individual particles may
have any size from that of grains of sand up to that
of the large meteoric bodies which sometimes reach
our earth.
Thus the state of the case as regards the constitution
of comets is, roughly speaking, this: They
consist of a nucleus of solid matter, held together,
but with a very slack bond, by the power of gravitation.
From this nucleus, as the comet approaches
perihelion, the electric action of the sun, working in
a manner at present unknown, drives off volumes
[pg 213]
of luminous gas, which form the tail; and in some
comets the waves of this vapour have been actually
seen rising slowly in successive pulses from the
nucleus, and then being driven backwards much
as the smoke of a steamer is driven. It has been
found also by investigation of Comet Wells 1882
and the Great Comet of 1882 that in some at least
of these bodies sodium and iron are present.
The question next arises, What becomes of
comets in the end? Kepler long ago asserted his
belief that they perished, as silkworms perish by
spinning their own thread, exhausting themselves
by the very efforts of tail-production which render
them sometimes so brilliant to observation; and
this seems to be pretty much the case. Thus
Halley's comet, which was once so brilliant and
excited so much attention, was at its last visit a
very inconspicuous object indeed. At its apparition
in 1845-1846 Biela's comet was found to have split
into two separate bodies, which were found at their
return in 1852 to have parted company widely.
Since that year it has never been observed again
in the form of a comet, though, as we shall see, it
has presented itself in a different guise. The same
fate has overtaken the comets of De Vico (1844),
and Brorsen (1846). The former should have
returned in 1850, but failed to keep its appointment;
and the latter, after having established a
character for regularity by returning to observation
on four occasions, failed to appear in 1890, and has
never since been seen.
[pg 214]
The mystery of such disappearances has been
at least partially dispelled by the discovery, due to
Schiaparelli and other workers in the same field,
that various prominent meteor-showers travel in
orbits precisely the same as those of certain comets.
Thus the shower of meteors which takes place with
greater or less brilliancy every year from a point in
the constellation Perseus has been proved to follow
the orbit of the bright comet of 1862; while the great
periodic shower of the Leonids follows the track of
the comet of 1866; the orbit of the star-shower of
April 20—the Lyrids—corresponds with that of a
comet seen in 1861; and the disappearance of
Biela's comet appears to be accounted for by the
other November shower whose radiant point is in
the constellation Andromeda. In fact, the state of
the matter is well summed up by Kirkwood's question:
'May not our periodic meteors be the débris
of ancient but now disintegrated comets, whose
matter has become distributed round their orbits?'
The loosely compacted mass which forms the
nucleus of the comet appears to gradually lose its
cohesion under the force of solar tidal action, and
its fragments come to revolve independently in their
orbit, for a time in a loosely gathered swarm, and
then gradually, as the laggards drop behind, in the
form of a complete ring of meteoric bodies, which
are distributed over the whole orbit. The Leonid
shower is in the first condition, or, rather, was when
it was last seen, for it seems to be now lost to us;
the Perseid shower is in the second. The shower
[pg 215]
of the Andromedes has since confirmed its identity
with the lost comet of Biela by displays in 1872,
1885, and 1892, at the seasons when that comet
should have returned to the neighbourhood of the
sun. It appears to be experiencing the usual fate
of such showers, and becoming more widely distributed
round its orbit, and the return in 1905 was
very disappointing, the reason apparently being
that the dense group in close attendance on the
comet has suffered disturbance from Jupiter and
Saturn, and now passes more than a million miles
outside the earth's orbit.
In 1843 there appeared one of the most remarkable
of recorded comets. It was not only of
conspicuous brilliancy and size, though its tail
at one stage reached the enormous length of
200,000,000 miles, but was remarkable for the
extraordinarily close approach which it made to the
sun. Its centre came as near to the sun as
78,000 miles, leaving no more than 32,000 miles
between the surfaces of the two bodies; it must,
therefore, have passed clear through the corona,
and very probably through some of the prominences.
Its enormous tail was whirled, or rather appeared to
be whirled, right round the sun in a little over two
hours, thus affording conclusive proof that the tail
of a comet cannot possibly be an appendage, but
must consist of perpetually renewed emanations from
the nucleus. But in addition to these wonders, the
comet of 1843 proved the precursor of a series of
fine comets travelling in orbits which were practically
[pg 216]
identical. The great southern comet of 1880 proved,
when its orbit had been computed, to follow a path
almost exactly the same as that of its predecessor of
thirty-seven years before. It seemed inconceivable
that a body so remarkable as the 1843 comet should
have a period of only thirty-seven years, and yet
never previously have attracted attention. Before
the question had been fairly discussed, it was
accentuated by the discovery, in 1881, of a comet
whose orbit was almost indistinguishable from that
of the comet of 1807. But the 1807 comet was not
due to return till A.D. 3346. Further, the comet of
1881 proved to have a period, not of seventy-four
years, as would have been the case had it been a
return of that of 1807, but of 2,429 years. The
only possible conclusion was that here were two
comets which were really fragments of one great
comet which had suffered disruption, as Biela's
comet visibly did, and that one fragment followed
in the other's wake with an interval of seventy-four
years.
Meanwhile, the question of the 1843 and 1880
comets was still unsettled, and it received a fresh
complication by the appearance of the remarkable
comet of 1882, whose transit of the sun has been
already alluded to, for the orbit of this new body
proved to be a reproduction, almost, but not quite
exact, of those of the previous two. Astronomers
were at a greater loss than ever, for if this were a
return of the 1880 comet, then the conclusion
followed that something was so influencing its orbit
[pg 217]
as to have shortened its period from thirty-seven to
two years. The idea of the existence of some
medium round the sun, capable of resisting bodies
which passed through it, and thus causing them to
draw closer to their centre of attraction and
shortening their periods, was now revived, and it
seemed as though, at its next return, this wonderful
visitant must make the final plunge into the photosphere,
with what consequences none could foretell.
These forebodings proved to be quite baseless.
The comet passed so close to the sun (within
300,000 miles of his surface), that it must have been
sensibly retarded at its passage by the resisting
medium, had such a thing existed; but not the
slightest retardation was discernible. The comet
suffered no check in its plunge through the solar
surroundings, and consequently the theory of the
resisting medium may be said to have received its
quietus.
Computation showed that the 1882 comet
followed nearly the same orbit as its predecessors;
and thus we are faced by the fact of families of
comets, travelling in orbits that are practically
identical, and succeeding one another at longer or
shorter intervals. The idea that these families
have each sprung from the disruption of some
much larger body seems to be most probable, and
it appears to be confirmed by the fact that in the
1882 comet the process of further disruption was
actually witnessed. Schmidt of Athens detected
one small offshoot of the great comet, which remained
[pg 218]
visible for several days. Barnard a few
days later saw at least six small nebulous bodies
close to their parent, and a little later Brooks
observed another. 'Thus,' as Miss Agnes Clerke
remarks, 'space appeared to be strewn with the
filmy débris of this beautiful but fragile structure all
along the track of its retreat from the sun.'
The state of our knowledge with regard to comets
may be roughly summed up. We have extreme
tenuity in the whole body, even the nucleus being
apparently not solid, but a comparatively loose
swarm of solid particles. The nucleus, in all likelihood,
shines by reflected sunlight—in part, at all
events. The nebulous surroundings and tail are
produced by solar action upon the matter of which
the comet is composed, this action being almost
certainly electrical, though heat may play some part
in it. The nebulous matter appears to proceed in
waves from the nucleus, and to be swept backward
along the comet's track by some repellent force,
probably electrical, exerted by the sun. This part
of the comet's structure consists mainly of self-luminous
gases, generally of the hydrocarbon type,
though sodium and iron have also been traced.
Comets, certainly in many cases, probably in all,
suffer gradual degradation into swarms of meteors.
The existence of groups of comets, each group
probably the outcome of the disruption of a much
larger body, is demonstrated by the fact of successive
comets travelling in almost identically
similar orbits. Finally, comets are all connected
[pg 219]
with the solar system, so far, at least, that
they accompany that system in its journey of
400,000,000 miles a year through space. Our system
does not, as it were, pick up the comets as it sweeps
along upon its great journey; it carries them along
with it.
A few words may be added as to cometary
observation. It is scarcely likely that any very
great number of amateur observers will ever be
attracted by the branch of comet-hunting. The
work is somewhat monotonous and laborious, and
seems to require special aptitudes, and, above all, an
enormous endowment of patience. Probably the
true comet-hunter, like the poet, is born, not made;
and it is not likely that there are, nor desirable
that there should be, many individuals of the type
of Messier, the 'comet-ferret.' 'Messier,' writes a
contemporary, 'is at all events a very good man,
and simple as a child. He lost his wife some
years ago, and his attendance upon her death-bed
prevented his being the discoverer of a comet for
which he had been lying in wait, and which was
snatched from him by Montaigne de Limoges.
This made him desperate. A visitor began to offer
him consolation for his recent bereavement, when
Messier, thinking only of the comet, answered, "I
had discovered twelve; alas! to be robbed of the
thirteenth by that Montaigne!" and his eyes filled with
tears. Then, recollecting that it was necessary to
deplore his wife, he exclaimed, "Ah! cette pauvre
femme!" and again wept for his comet.' In addition
[pg 220]
to the fact that few have reached such a degree of
scientific detachment as to put a higher value upon
a comet than upon the nearest of relatives, there
is the further fact that the future of cometary
discovery, and of the record of cometary change
seems to lie almost entirely with photography, which
is wonderfully adapted for the work (Plate XXVI.).
PLATE XXVI.
1 2
Photographs of Swift's Comet. By Professor E. E. Barnard.
1. Taken April 4, 1892; exposure 1 hour. 2. Taken April 6, 1892; exposure 1 hour
5 minutes.
Anyone who desires to become a comet-hunter
must, in addition to the possession of the supreme
requisites, patience and perseverance, provide himself
with an instrument of at least 4 inches aperture,
together with a good and comprehensive set of star-charts
and the New General Catalogue of nebulæ
with the additions which have been made to it.
The reason for this latter item of equipment is the
fact that many telescopic comets are scarcely to be
distinguished from nebulæ, and that an accurate
knowledge of the nebulous objects in the regions
to be searched for comets, or at least a means of
quickly identifying such objects, is therefore indispensable.
The portions of the heavens which
afford the most likely fields for discovery will
naturally be those in the vicinity of where the sun
has set at evening, or where he is about to rise in
the early morning, all comets having of necessity
to approach the sun more or less closely at their
perihelion passage. Other parts of the heavens
should not be neglected; but these are the most
likely neighbourhoods.
Most of us, however, will be content to discover
our comets in the columns of the daily newspaper,
[pg 221]
or by means of a post-card from some obliging
friend. The intimation, in whatever way received,
will generally contain the position of the comet at
a certain date, given in right ascension and declination,
and either a statement of its apparent daily
motion, or else a provisional set of places for several
days ahead. Having either of these, the comet's
position must be marked down on the star-map, and
the course which it is likely to pursue must be traced
out in pencil by means of the data—a perfectly
simple matter of marking down the position for
each day by its celestial longitude and latitude as
given. The observer will next note carefully the
alignment of the comet with the most conspicuous
stars in the neighbourhood of the particular position
for the day of his observation; and, guiding his
telescope by means of these, will point it as nearly
as possible to that position. He may be lucky
enough to hit upon his object at once, especially if
it be a comparatively bright one. More probably,
he will have to 'sweep' for it. In this case the
telescope must be pointed some little distance below
and to one side of the probable position of the comet,
and moved slowly and gently along, careful watch
being kept upon the objects which pass through the
field, until a similar distance on the opposite side
of the position has been reached. Then raise the
instrument by not more than half a field's breadth,
estimating this by the stars in the field, and repeat
the process in the opposite direction, going on until
the comet appears in the field, or until it is obvious
[pg 222]
that it has been missed. A low power should be
used at first, which may be changed for a somewhat
higher one when the object has been found. But
in no case will the use of really high magnifiers be
found advisable. It is, of course, simply impossible
with the tail, for which the naked eye is the best
instrument, nor can the coma bear any degree of
magnification, though occasionally the nucleus may
be sufficiently sharply defined to bear moderate
powers. The structure of the latter should be
carefully observed, with particular attention to the
question of whether any change can be seen in it,
or whether there seem any tendency to such a
multiplication of nuclei as characterized the great
comet of 1882. It is possible that the pulses of
vapour sunwards from the nucleus may also be
observed.
Appearance of motion, wavy or otherwise, in the
tail, should also be looked for, and carefully watched
if seen. Beyond this there is not very much that
the ordinary observer can do; the determination of
positions requires more elaborate appliances, and the
spectroscope is necessary for any study of cometary
constitution. It only remains to express a wish for
the speedy advent of a worthy subject for operations.
We turn now to those bodies which, as has been
pointed out, appear to be the débris of comets which
have exhausted their cometary destiny, and ceased
to have a corporate existence. Everyone is familiar
with the phenomenon known as a meteor, or shooting-star,
[pg 223]
and there are few clear nights on which an
observer who is much in the open will not see one
or more of these bodies. Generally they become
visible in the form of a bright point of light which
traverses in a straight line a longer or shorter path
across the heavens, and then vanishes, sometimes
leaving behind it for a second or two a faintly
luminous train. The shooting-stars are of all
degrees of brightness, from the extremely faint
streaks which sometimes flash across the field of
the telescope, up to brilliant objects, brighter than
any of the planets or fixed stars, and sometimes
lighting up the whole landscape with a light like
that of the full moon.
The prevailing opinion, down to a comparatively
late date, was that shooting-stars were mere exhalations
in the earth's atmosphere, arising as one author
expressed it, 'from the fermentation of acid and
alkaline bodies, which float in the atmosphere'; and
it was also suggested by eminent astronomers that
they were the products of terrestrial volcanoes,
returning, after long wanderings, to their native
home.
The true study of meteoric astronomy may be said
to date from the year 1833, when a shower of most
extraordinary splendour was witnessed. The magnificence
of this display was the means of turning
greater attention to the subject; and it was observed
as a fact, though the importance of the observation
was scarcely realized, that the meteors all appeared
to come from nearly the one point in the constellation
[pg 224]
Leo. The fact of there being a single radiant
point implied that the meteors were all moving in
parallel lines, and had entered our atmosphere from
a vast distance. Humboldt, who had witnessed a
previous appearance of this shower in 1799, suggested
that it might be a periodic phenomenon; and
his suggestion was amply confirmed when in 1866
the shower made its appearance again in scarcely
diminished splendour. Gradually other showers
came to be recognised, and their radiant points
fixed; and meteoric astronomy began to be established
upon a scientific basis.
In 1866 Schiaparelli announced that the shower
which radiates in August from the constellation
Perseus follows the same track as that of Swift's
comet (1862 iii.); and in the following year the
great November shower from Leo, already alluded
to, was proved to have a similar connection with
Tempel's comet (1866 i.). The shower which
comes from the constellation Lyra, about April 20,
describes the same orbit as that of Comet 1861 i.;
while, as already mentioned, the mysterious disappearance
of Biela's comet received a reasonable
explanation by its association with the other great
November shower—that which radiates from the
constellation Andromeda. With regard to the last-named
shower, it has not only been shown that the
meteors are associated with Biela's comet, but also
that they separated from it subsequent to 1841, in
which year the comet's orbit was modified by perturbations
from Jupiter. The Andromeda meteors
[pg 225]
follow the modified orbit, and hence must have
been in close association with the comet when the
perturbation was exercised.
The four outstanding meteor radiants are those
named, but there are very many others. Mr.
Denning, to whom this branch of science owes so
much, estimates the number of distinct radiants
known at about 4,400; and it seems likely that
every one of these showers, some of them, of course
very feeble, represents some comet deceased. The
history of a meteor shower would appear to be
something like this: When the comet, whose
executor it is, has but recently deceased, it will
appear as a very brilliant periodic shower, occurring
on only one or two nights exactly at the point
where the comet in its journeying would have
crossed the earth's track, and appearing only at the
time when the comet itself would have been there.
Gradually the meteors get more and more tailed out
along the orbit, as runners of unequal staying powers
get strung out over a track in a long race, until the
displays may be repeated, with somewhat diminished
splendour, year after year for several years before
and after the time when the parent comet is due.
At last they get thinly spread out over the whole
orbit, and the shower becomes an annual one, happening
each year when the earth crosses the orbit
of the comet. This has already happened to the
Perseid shower; at least 500,000,000 miles of the
orbit of Biela's comet are studded with representatives
of the Andromedes; and the Leonid shower
[pg 226]
had already begun to show symptoms of the same
process at its appearance in 1866. Readers will
remember the disappointment caused by the failure
of the Leonid shower to come up to time in 1899,
and it seems probable that the action of some perturbing
cause has so altered the orbit of this shower
that it now passes almost clear of the earth's path,
so that we shall not have the opportunity of
witnessing another great display of the Leonid
meteors.
So far as is known, no member of one of these
great showers has ever fallen to the earth. There
are two possible exceptions to this statement, as in
1095 a meteor fell to the ground during the progress
of a shower of Lyrids, and in 1885 another fell
during a display of the Andromedes. In neither
case, however, was the radiant point noted, and
unless it was the same as that of the shower the
fall of the meteor was a mere coincidence. It seems
probable that this is the case, and the absence of
any evidence that a specimen from a cometary
shower has reached the earth points to the extreme
smallness of the various members of the shower
and also to the fine division of the matter of the
original comet.
In addition to the meteors originating from
systematic showers, there are also to be noted
frequent and sometimes very brilliant single
meteors. Specimens of these have in many instances
been obtained. They fall into three classes—'Those
in which iron is found in considerable
[pg 227]
amount are termed siderites; those containing an
admixture of iron and stone, siderolites; and those
consisting almost entirely of stone are known as
aerolites' (Denning). The mass of some of these
bodies is very considerable. Swords have been
forged out of their iron, one of which is in the
possession of President Diaz of Mexico, while
diamonds have been found in meteoric irons which
fell in Arizona. It may be interesting to know
that, according to a grave decision of the American
courts, a meteor is 'real estate,' and belongs to the
person on whose ground it has fallen; the alternative—that
it is 'wild game,' and the property of its
captor—having been rejected by the court. So far
as I am aware, the legal status of these interesting
flying creatures has not yet been determined in
Britain.
The department of meteoric astronomy is one in
which useful work can be done with the minimum
of appliances. The chief requisites are a good set
of star-maps, a sound knowledge of the constellations,
a straight wand, and, above all, patience.
The student must make himself familiar with the
constellations (a pleasant task, which should be
part of everyone's education), so that when a meteor
crosses his field of view he may be able to identify
at once with an approach to accuracy its points of
appearance and disappearance. It is here that the
straight wand comes into play. Mr. Denning advises
the use of it as a means of guiding the eye.
It is held so as to coincide with the path of the
[pg 228]
meteor just seen, and will thus help the eye to
estimate the position and slope of the track relatively
to the stars of the constellations which it has
crossed. This track should be marked as quickly
as possible on the charts. Mere descriptions of the
appearance of meteors, however beautiful, are quite
valueless. It is very interesting to be told that a
meteor when first seen was 'of the size and colour
of an orange,' but later 'of the apparent size of the
full moon, and surrounded by a mass of glowing
vapour which further increased its size to that of
the head of a flour-barrel'; but the description is
scarcely marked by sufficient precision of statement
for scientific purposes. The observer must note
certain definite points, of which the following is a
summary: (1) Date, hour, and minute of appearance.
(2) Brightness, compared with some well-known
star, planet, or, if exceptionally bright, with
the moon. (3) Right ascension and declination of
point of first appearance. (4) The same of point
of disappearance. (5) Length of track. (6) Duration
of visibility. (7) Colour, presence of streak or
train, and any other notable features. (8) Radiant
point.
When these have been given with a reasonable
approach to accuracy, the observer has done his best
to provide a real, though small, contribution to the
sum of human knowledge; nor is the determination
of these points so difficult as would at first appear
from their number. The fixing of the points of
appearance and disappearance and of the radiant
[pg 229]
will present a little difficulty to start with; but in
this, as in all other matters, practice will bring
efficiency. It may be mentioned that the efforts
of those who take up this subject would be greatly
increased in usefulness by their establishing a connection
with the Meteor Section of the British
Astronomical Association.
One curious anomaly has been established by
Mr. Denning's patient labour—the existence,
namely, of what are termed 'stationary radiants.'
It is obvious that if meteors have the cometary
connection already indicated, their radiant point
should never remain fixed; as the showers move
onwards in their orbit they should leave the original
radiant behind. Mr. Denning has conclusively
proved, however, that there are showers which do
not follow the rule in this respect, but proceed from
a radiant which remains the same night after night,
some feeble showers maintaining the same radiant
for several months. It is not easy to see how this
fact is to be reconciled with the theory of cometary
origin; but the fact itself is undeniable.
[pg 230]
CHAPTER XIV
THE STARRY HEAVENS
We now leave the bounds of our own system, and
pass outwards towards the almost infinite spaces
and multitudes of the fixed stars. In doing so we
are at once confronted with a wealth and profusion
of beauty and a vastness of scale which are almost
overwhelming. Hitherto we have been dealing
almost exclusively with bodies which, though sometimes
considerably larger than our world, were yet,
with the exception of the sun, of the same class and
comparable with it; and with distances which,
though very great indeed, were still not absolutely
beyond the power of apprehension. But now all
former scales and standards have to be left behind,
for even the vast orbit of Neptune, 5,600,000,000
of miles in diameter, shrinks into a point when
compared with the smallest of the stellar distances.
Even our unit of measurement has to be changed,
for miles, though counted in hundreds of millions,
are inadequate; and, accordingly, the unit in which
our distance from the stars is expressed is the 'light
year,' or the distance travelled by a ray of light in
a year.
[pg 231]
Light travels at the rate of about 186,000 miles a
second, and therefore leaps the great gulf between
our earth and the sun in about eight minutes. But
even the nearest of the fixed stars—Alpha Centauri,
a star of the first magnitude in the Southern Hemisphere—is
so incredibly distant that light takes four
years and four months to travel to us from it; while
the next nearest, a small star in Ursa Major, is
about seven light-years distant, and the star 61
of the constellation Cygnus, the first northern star
whose distance was measured, is separated from us
by two years more still.
At present the distances of about 100 stars are
known approximately; but it must be remembered
that the approximation is a somewhat rough one.
The late Mr. Cowper Ranyard once remarked of
measures of star-distances that they would be considered
rough by a cook who was in the habit of
measuring her salt by the cupful and her pepper by
the pinch. And the remark has some truth—not
because of any carelessness in the measurements,
for they are the results of the most minute and
scrupulous work with the most refined instrumental
means that modern skill can devise and construct—but
because the quantities to be measured are almost
infinitely small.
It is at present considered that the average
distance from the earth of stars of the first magnitude
is thirty-three light years, that of stars of the
second fifty-two, and of the third eighty-two. In
other words, when we look at such stars on any
[pg 232]
particular evening, we are seeing them, not as they
are at the moment of observation, but as they were
thirty-three, fifty-two, or eighty-two years ago, when
the rays of light which render them visible to us
started on their almost inconceivable journey. The
fact of the average distance of first-magnitude stars
being less than that of second, and that of second
in turn less than that of third, is not to be held as
implying that there are not comparatively small
stars nearer to us than some very bright ones.
Several insignificant stars are considerably nearer
to us than some of the most brilliant objects in the
heavens—e.g., 61 Cygni, which is of magnitude 4·8,
is almost infinitely nearer to us than the very
brilliant first magnitude star Rigel in Orion. The
rule holds only on the average.
The number of the stars is not less amazing than
their distance. It is true that the number visible to
the unaided eye is not by any means so great as
might be imagined on a casual survey. On a clear
night the eye receives the impression that the multitude
of stars is so great as to be utterly beyond
counting; but this is not the case. The naked-eye,
or 'lucid,' stars have frequently been counted, and
it has been found that the number visible to a good
average eye in both hemispheres together is about
6,000. This would give for each hemisphere 3,000,
and making allowance for those lost to sight in the
denser air near the horizon, or invisible by reason
of restricted horizon, it is probable that the number
of stars visible at any one time to any single observer
[pg 233]
in either hemisphere does not exceed 2,500. In
fact Pickering estimates the total number visible,
down to and including the sixth magnitude, to be
only 2,509 for the Northern Hemisphere, and on
that basis it may safely be assumed that 2,000 would
be the extreme limit for the average eye.
PLATE XXVII.
Region of the Milky Way in Sagittarius. Photographed by Professor E. E.
Barnard.
But this somewhat disappointing result is more
than atoned for when the telescope is called in and
the true richness of the heavenly host begins to
appear. Let us take for illustration a familiar group
of stars—the Pleiades. The number of stars visible
to an ordinary eye in this little group is six; keen-sighted
people see eleven, or even fourteen. A
small telescope converts the Pleiades into a brilliant
array of luminous points to be counted not by units
but by scores, while the plates taken with a modern
photographic telescope of 13 inches aperture show
2,326 stars. The Pleiades, of course, are a somewhat
notable group; but those who have seen any of the
beautiful photographs of the heavens, now so common,
will know that in many parts of the sky even this
great increase in number is considerably exceeded;
and that for every star the eye sees in such regions
a moderate telescope will show 1,000, and a great
instrument perhaps 10,000. It is extremely probable
that the number of stars visible with the largest
telescopes at present in use would not be overstated
at 100,000,000 (Plate XXVII.).
It is evident, on the most casual glance at the
sky, that in the words of Scripture, 'One star
differeth from another star in glory.' There are
[pg 234]
stars of every degree of brilliancy, from the sparkling
white lustre of Sirius or Vega, down to the
dim glimmer of those stars which are just on the
edge of visibility, and are blotted out by the faintest
wisp of haze. Accordingly, the stars have been
divided into 'magnitudes' in terms of scales which,
though arbitrary, are yet found to be of general
convenience. Stars of the first six magnitudes come
under the title of 'lucid' stars; below the sixth we
come to the telescopic stars, none of which are
visible to the naked eye, and which range down to
the very last degree of faintness. Of stars of the
first magnitude there are recognised about twenty,
more or less. By far the brightest star visible to us
in the Northern Hemisphere, though it is really
below the Equator, is Sirius, whose brightness
exceeds by no fewer than fourteen and a half times
that of Regulus, the twentieth star on the list. The
next brightest stars, Canopus and Alpha Centauri,
are also Southern stars, and are not visible to us in
middle latitudes. The three brightest of our truly
Northern stars, Vega, Capella, and Arcturus, come
immediately after Alpha Centauri, and opinions are
much divided as to their relative brightness, their
diversity in colour and in situation rendering a comparison
somewhat difficult. The other conspicuous
stars of the first magnitude visible in our latitudes
are, in order of brightness, Rigel, Procyon, Altair,
Betelgeux, Aldebaran, Pollux, Spica Virginis,
Antares, Fomalhaut, Arided (Alpha Cygni), and
Regulus, the well-known double star Castor following
[pg 235]
not far behind Regulus. The second magnitude
embraces, according to Argelander, 65 stars; the
third, 190; fourth, 425; fifth, 1,100; sixth, 3,200;
while for the ninth magnitude the number leaps up
to 142,000. It is thus seen that the number of stars
increases with enormous rapidity as the smaller
magnitudes come into question, and, according to
Newcomb, there is no evidence of any falling off in
the ratio of increase up to the tenth magnitude. In
the smaller magnitudes, however, the ratio of increase
does not maintain itself. The number of the stars,
though very great, is not infinite.
A further fact which quickly becomes apparent to
the naked eye is that the stars are not all of the
same colour. Sirius, for example, is of a brilliant
white, with a steely glitter; Betelgeux, comparatively
near to it in the sky, is of a beautiful topaz tint,
perhaps on the whole the most exquisite single star
in the sky, so far as regards colour; Aldebaran is
orange-yellow, while Vega is white with a bluish
cast, as is also Rigel. These diversities become
much more apparent when the telescope is employed.
At the same time the observer may be warned
against expecting too much in the way of colour,
for, as a matter of fact, the colours of the stars, while
perfectly manifest, are yet of great delicacy, and it
is difficult to describe them in ordinary terms without
some suspicion of exaggeration. Stars of a reddish
tone, which ranges from the merest shade of orange-yellow
up to a fairly deep orange, are not uncommon;
several first-magnitude stars, as already noted, have
[pg 236]
distinct orange tones. For anything approaching to
real blues and greens, we must go to the smaller
stars, and the finest examples of blue or green stars
are found in the smaller members of some of the
double systems. Thus in the case of the double
Beta Cygni (Albireo), one of the most beautiful and
easy telescopic objects in the northern sky, the
larger star is orange-yellow, and the smaller blue;
in that of Gamma Andromedæ the larger is yellow,
and the smaller bluish-green; while Gamma Leonis
has a large yellow star, and a small greenish-yellow
one in connection. The student who desires to
pursue the subject of star colours should possess
himself of the catalogue published in the Memoirs
of the British Astronomical Association, which gives
the colours of the lucid stars determined from the
mean of a very large number of observations made
by different observers.
In this connection it may be noticed that there is
some suspicion that the colours of certain stars have
changed within historic times, or at least that they
have not the same colour now which they are said to
have had in former days. The evidence is not in any
instance strong enough to warrant the assertion that
actual change has taken place; but it is perfectly
natural to suppose that it does, and indeed must
gradually progress. As the stars are intensely hot
bodies, there must have been periods when their
heat was gradually rising to its maximum, and there
must be periods when they will gradually cool off to
extinction, and these stages must be represented by
[pg 237]
changes in the colour of the particular star in question.
In all probability, then, the colour of a star gives
some indication of the stage to which it has advanced
in its life-history; and as a matter of fact, this proves
to be so, the colour of a star being found to be
generally a fair indication of what its constitution,
as revealed by the spectroscope, will be.
Another feature of the stars which cannot fail to
be noticed is the fact that they are not evenly distributed
over the heavens, but are grouped into a
variety of configurations or constellations. In the
very dawn of human history these configurations
woke the imaginations of the earliest star-gazers,
and fanciful shapes and titles were attached to the
star-groups, which have been handed down to the
present time, and are still in use. It must be
confessed that in some cases it takes a very lively
imagination to find any resemblance between the
constellation and the figure which has been associated
with it. The anatomy of Pegasus, for example,
would scarcely commend itself to a horse-breeder,
while the student will look in vain for any resemblance
to a human figure, heroic or unheroic, in the
straggling group of stars which bears the name of
Hercules. At the same time a few of the constellations
do more or less resemble the objects from
which their titles are derived. Thus the figure of a
man may without any great difficulty be traced
among the brilliant stars which form the beautiful
constellation Orion; while Delphinus presents at
least an approximation to a fish-like form, and
[pg 238]
Corona Borealis gives the half of a diadem of
sparkling jewels.
A knowledge of the constellations, and, if possible,
of the curious old myths and legends attaching to
them, should form part of the equipment of every
educated person; yet very few people can tell one
group from another, much less say what constellations
are visible at a given hour at any particular season
of the year. People who are content merely to gape
at the heavens in 'a wonderful clear night of stars'
little know how much interest they are losing.
When the constellations and the chief stars are
learned and kept in memory, the face of the sky
becomes instinct with interest, and each successive
season brings with it the return of some familiar
group which is hailed as one hails an old friend.
Nor is the task of becoming familiar with the constellations
one of any difficulty. Indeed, there are
few pleasanter tasks than to trace out the figures of
the old heroes and heroines of mythology by the
help of a simple star-map, and once learned, they
need never be forgotten. In this branch of the
subject there are many easily accessible helps. For
a simple guide, Peck's 'Constellations and how to
Find Them' is both cheap and useful, while Newcomb's
'Astronomy for Everybody' and Maunder's
'Astronomy without a Telescope' also give careful and
simple directions. Maunder's volume is particularly
useful for a beginner, combining, as it does, most
careful instructions as to the tracing of the constellations
with a set of clear and simple star-charts, and
[pg 239]
a most interesting discussion of the origin of these
ancient star-groups. A list of the northern constellations
with a few of the most notable objects of
interest in each will be found in Appendix II.
Winding among the constellations, and forming
a gigantic belt round the whole star-sphere, lies that
most wonderful feature of the heavens familiar to
all under the name of the Milky Way. This great
luminous girdle of the sky may be seen in some
portion of its extent, and at some hour of the night,
at all seasons of the year, though in May it is
somewhat inconveniently placed for observation.
Roughly speaking, it presents the appearance of a
broad arch or pathway of misty light, 'whose
groundwork is of stars'; but the slightest attention
will reveal the fact that in reality its structure is of
great complexity. It throws out streamers on either
side and at all angles, condenses at various points
into cloudy masses of much greater brilliancy than
the average, strangely pierced sometimes by dark
gaps through which we seem to look into infinite
and almost tenantless space (Plate XXVII.), while
in other quarters it spreads away in considerable
width, and to such a degree of faintness that the eye
can scarcely tell where it ends. At a point in the
constellation Cygnus, well seen during autumn and
the early months of winter, it splits up into two
great branches which run separate to the Southern
horizon with a well-marked dark gap dividing them.
When examined with any telescopic power, the
Milky Way reveals itself as a wonderful collection
[pg 240]
of stars and star-clusters; and it will also be found
that there is a very remarkable tendency among the
stars to gather in the neighbourhood of this great
starry belt. So much is this case that, in the words
of Professor Newcomb, 'Were the cloud-forms
which make up the Milky Way invisible to us, we
should still be able to mark out its course by the
crowding of the lucid stars towards it.' Not less
remarkable is the fact that the distribution of the
nebulæ with regard to the Galaxy is precisely the
opposite of that of the stars. There are, of course,
many nebulæ in the Galaxy; but, at the same time,
they are comparatively less numerous along its
course, and grow more and more numerous in proportion
as we depart from it. It seems impossible
to avoid the conclusion that these twin facts are
intimately related to one another, though the explanation
of them is not yet forthcoming.
In the year 1665 the famous astronomer Hooke
wrote concerning the small star Gamma Arietis:
'I took notice that it consisted of two small stars
very near together; a like instance of which I have
not else met with in all the heavens.' This is the
first English record of the observation of a double
star, though Riccioli detected the duplicity of Zeta
Ursæ Majoris (Mizar), in 1650, and Huygens saw
three stars in Theta Orionis in 1656. These were
the earliest beginnings of double-star observation,
which has since grown to such proportions that
double stars are now numbered in the heavens by
thousands. Of course, certain stars appear to be
[pg 241]
double even when viewed with the unaided eye.
Thus Mizar, a bright star in the handle of the
Plough, referred to above, has not far from it a
fainter companion known as Alcor, which the Arabs
used to consider a test of vision. Either it has
brightened in modern times, or else the Arabs have
received too much credit for keenness of sight, for
Mizar and Alcor now make a pair that is quite easy
to very ordinary sight even in our turbid atmosphere.
Alpha Capricorni, and Zeta Ceti, with Iota
Orionis are also instances of naked-eye doubles,
while exceptionally keen sight will detect that the
star Epsilon Lyræ, which forms a little triangle
with the brilliant Vega and Zeta Lyræ, is double, or
at least that it is not single, but slightly elongated in
form. Astronomers, however, would not call such
objects as these 'double stars' at all; they reserve
that title for stars which are very much closer
together than the components of a naked-eye double
can ever be. The last-mentioned star, Epsilon
Lyræ, affords a very good example of the distinction.
To the naked eye it is, generally speaking,
not to be distinguished from a single star. Keen
sight elongates it; exceptionally keen sight divides
it into two stars extremely close to one another.
But on using even a very moderate telescope, say
a 2½-inch with a power of 100 or upwards, the two
stars which the keenest sight could barely separate
are seen widely apart in the field, while each of
them has in its turn split up into two little dots of
light. Thus, to the telescope, Epsilon Lyræ is
[pg 242]
really a quadruple star, while in addition there is
a faint star forming a triangle with the two pairs,
and a large instrument will reveal two very faint
stars, the 'debilissima,' one on either side of the line
joining the larger stars. These I have seen with
3⅞-inch.
What the telescope does with Epsilon Lyræ,
it does with a great multitude of other stars.
There are thousands of doubles of all degrees of
easiness and difficulty—doubles wide apart, and
doubles so close that only the finest telescopes in the
world can separate them; doubles of every degree
of likeness or of disparity in their components, from
Alpha Geminorum (Castor), with its two beautiful
stars of almost equal lustre, to Sirius, where the
chief star is the brightest in all the heavens, and the
companion so small, or rather so faint, that it takes
a very fine glass to pick it out in the glare of
its great primary. The student will find in these
double stars an extremely good series of tests for
the quality of his telescope. They are, further,
generally objects of great beauty, being often characterized,
as already mentioned, by diversity of
colour in the two components. Thus, in addition to
the examples given above, Eta Cassiopeiæ presents
the beautiful picture of a yellow star in conjunction
with a red one, while Epsilon Boötis has been
described as 'most beautiful yellow and superb blue,'
and Alpha Herculis consists of an orange star close
to one which is emerald green. It has been suggested
that the colours in such instances are merely
[pg 243]
complementary, the impression of orange or yellow
in the one star producing a purely subjective impression
of blue or green when the other is viewed; but
it has been conclusively proved that the colours of
very many of the smaller stars in such cases are
actual and inherent.
Not only are there thousands of double stars in
the heavens, but there are also many multiple stars,
where the telescope splits an apparently single star
up into three, four, or sometimes six or seven
separate stars. Of these multiples, one of the best
known is Theta Orionis. It is the middle star of
the sword which hangs from the belt of Orion, and
is, of course, notable from its connection with the
Great Nebula; but it is also a very beautiful
multiple star. A 2½-inch telescope will show that
it consists of four stars in the form of a trapezium;
large instruments show two excessively faint stars
in addition. Again, in the same constellation lies
Sigma Orionis, immediately below the lowermost
star of the giant's belt. In a 3-inch telescope
this star splits up into a beautiful multiple of six
components, their differences in size and tint making
the little group a charming object.
Looking at the multitude of double and multiple
stars, the question can scarcely fail to suggest itself:
Is there any real connection between the stars
which thus appear so close to one another? It can
be readily understood that the mere fact of their
appearing close together in the field of the telescope
does not necessarily imply real closeness. Two
[pg 244]
gas-lamps, for instance, may appear quite close
together to an observer who is at some distance
from them, when in reality they may be widely
separated one from the other—the apparent closeness
being due to the fact that they are almost in
the same line of sight. No doubt many of the
stars which appear double in the telescope are of
this class—'optical doubles,' as they are called, and
are in reality separated by vast distances from one
another. But the great majority have not only an
apparent, but also a real closeness; and in a number
of cases this is proved by the fact that observation
shows the stars in question to be physically
connected, and to revolve around a common centre
of gravity. Double stars which are thus physically
connected are known as 'binaries.' The discovery
of the existence of this real connection between
some double stars is due, like so many of the most
interesting astronomical discoveries, to Sir William
Herschel. At present the number of stars known
to be binary is somewhat under one thousand; but
in the case of most of these, the revolution round
a common centre which proves their physical connection
is extremely slow, and consequently the
majority of binary stars have as yet been followed
only through a small portion of their orbits, and the
change of position sufficient to enable a satisfactory
orbit to be computed has occurred in only a small
proportion of the total number. The first binary
star to have its orbit computed was Xi Ursæ
Majoris, whose revolution of about sixty years has
[pg 245]
been twice completed since, in 1780, Sir William
Herschel discovered it to be double.
The star which has the shortest period at present
known is the fourth magnitude Delta Equulei,
which has a fifth magnitude companion. The pair
complete their revolution, according to Hussey, in
5·7 years. Kappa Pegasi comes next in speed of
revolution, with a period of eleven and a half years,
while the star 85 of the same constellation takes
rather more than twice as long to complete its
orbit. From such swiftly circling pairs as these,
the periods range up to hundreds of years. Thus,
for example, the well-known double star Castor,
probably the most beautiful double in the northern
heavens, and certainly the best object of its class
for a small telescope, is held to have a period of
347 years, which, though long enough, is a considerable
reduction upon the 1,000 once attributed to it.
But the number of binary stars known is not
confined to those which have been discovered and
measured by means of the telescope and micrometer.
One of the most wonderful results of modern astronomical
research has been the discovery of the fact
that many stars have revolving round them invisible
companions, which are either dark bodies, or else
are so close to their primaries as for ever to defy
the separating powers of our telescopes. The discovery
of these dark, or at least invisible, companions
is one of the most remarkable triumphs of
the spectroscope. It was in 1888 that Vogel first
applied the spectroscopic method to the well-known
[pg 246]
variable star, Beta Persei—known as Algol,
'the Demon,' from its 'slowly-winking eye.' The
variation in the light of Algol is very large, from
second to fourth magnitude; Vogel therefore
reasoned that if this variation were caused by a
dark companion partially eclipsing the bright star, the
companion must be sufficiently large to cause motion
in Algol—that is, to cause both stars to revolve
round a common centre of gravity. Should this be
the case, then at one point of its orbit Algol must
be approaching, and at the opposite point receding
from the earth; and therefore the shift of the lines
of its spectrum towards the violet in the one instance
and towards the red in the other would settle the
question of whether it had or had not an invisible
companion. The spectroscopic evidence proved
quite conclusive. It was found that before its
eclipses, Algol was receding from the sun at the
rate of 26⅓ miles per second, while after eclipse
there was a similar motion of approach; and therefore
the hypothesis of an invisible companion was
proved to be fact. Vogel carried his researches
further, his inquiry into the questions of the size
and distance apart of the two bodies leading him to
the conclusion that the bright star is rather more,
and its companion rather less than 1,000,000 miles
in diameter; while the distance which divides them
is somewhat more than 3,000,000 miles. Though
larger, both bodies prove to be less massive than
our sun, Algol being estimated at four-ninths and
its companion at two-ninths of the solar mass.
[pg 247]
The class of double star disclosed in this manner
is known as the 'spectroscopic binary,' and has
various other types differing from the Algol type.
Thus the type of which Xi Ursæ Majoris was the
first detected instance has two component bodies
not differing greatly in brightness from one another.
In such a case the fact of the star being binary
is revealed through the consideration that in any
binary system the two components must necessarily
always be moving in opposite directions. Hence
the shift of the lines of their spectrum will be in
opposite directions also, and when one of the stars
(A) is moving towards us, and the other (B) away
from us, all the lines of the spectrum which are
common to the two will appear double, those of A
being displaced towards the violet and those of B
towards the red. After a quarter of a revolution,
when the stars are momentarily in a straight line
with us, the lines will all appear single; but after
half a revolution they will again be displaced, those
of A this time towards the red and those of B
towards the violet.
There has thus been opened up an entirely new
field of research, and the idea, long cherished, that
the stars might prove to have dark, or, at all events,
invisible, companions attendant on them, somewhat
as our own sun has its planets, has been proved to
be perfectly sound. So far, in the case of dark
companions, only bodies of such vast size have
been detected as to render any comparison with the
planets of our system difficult; but the principle is
[pg 248]
established, and the probability of great numbers of
the stars having real planetary systems attendant on
them is so great as to become practically a certainty.
'We naturally infer,' says Professor Newcomb,
'that ... innumerable stars may have satellites,
planets, or companion stars so close or so faint as
to elude our powers of observation.'
From the consideration of spectroscopic binaries
we naturally turn to that of variable stars, the two
classes being, to some extent at least, coincident, as
is evidenced by the case of Algol. While the discovery
of spectroscopic binaries is one of the latest
results of research, that of variability among stars
dates from comparatively far back in the history of
astronomy. As early as the year 1596 David
Fabricius noted the star now known as Omicron
Ceti, or Mira, 'the Wonderful,' as being of the
third magnitude, while in the following year he
found that it had vanished. A succession of appearances
and disappearances was witnessed in the
middle of the next century by Holwarda, and from
that time the star has been kept under careful
observation, and its variations have been determined
with some exactness, though there are anomalies as
yet unexplained. 'Once in eleven months,' writes
Miss Clerke, 'the star mounts up in about 125 days
from below the ninth to near the third, or even to
the second magnitude; then, after a pause of two
or three weeks, drops again to its former low level
in once and a half times, on an average, the duration
of its rise.' This most extraordinary fluctuation
[pg 249]
means that at a bright maximum Mira emits
1,500 times as much light as at a low minimum.
The star thus subject to such remarkable outbursts
is, like most variables, of a reddish colour, and at
maximum its spectrum shows the presence of glowing
hydrogen. Its average period is about 331 days;
but this period is subject to various irregularities,
and the maximum has sometimes been as much as
two months away from the predicted time. Mira
Ceti may be taken as the type of the numerous class
of stars known as 'long-period variables.'
Not less interesting are those stars whose variations
cover only short periods, extending from less
than thirty days down to a few hours. Of these,
perhaps the most easily observed, as it is also one
of the most remarkable, is Beta Lyræ. This star is
one of the two bright stars of nearly equal magnitude
which form an obtuse-angled triangle with the
brilliant first-magnitude star Vega. The other star
of the pair is Gamma Lyræ, and between them lies
the famous Ring Nebula, to be referred to later.
Ordinarily Beta Lyræ is of magnitude 3·4, but from
this it passes, in a period of rather less than thirteen
days, through two minima, in one of which it
descends to magnitude 3·9 and in the other to 4·5.
This fluctuation seems trifling. It really means,
however, that at maximum the star is two and
three-quarter times brighter than when it sinks to
magnitude 4·5; and the variation can be easily
recognised by the naked eye, owing to the fact of
the nearness of so convenient a comparison star
[pg 250]
as Gamma Lyræ. Beta Lyræ is a member of the
class of spectroscopic binaries, and belongs to that
type of the class in which the mutually eclipsing
bodies are both bright. In such cases the variation
in brilliancy is caused by the fact that when the
two bodies are, so to speak, side by side, light is
received from both of them, and a maximum is
observed; while, when they are end on, both in
line with ourselves, one cuts off more or less of the
other's light from us, thus causing a minimum.
A third class, distinct from either of the preceding,
is that of the Algol Variables, so-called from the
bright star Beta Persei, which has already been
mentioned as a spectroscopic binary. Than this
star there is no more notable variable in the
heavens, and its situation fortunately renders it
peculiarly easy of observation to northern students.
Algol shines for about fifty-nine hours as a star of
small second magnitude, then suddenly begins to
lose light, and in four and a half hours has fallen to
magnitude three and a half, losing in so short a space
two-thirds of its normal brilliancy. It remains in this
degraded condition for only fifteen minutes, and then
begins to recover, reaching its normal lustre in about
five hours more. These remarkable changes, due,
as before mentioned, to the presence of an invisible
eclipsing companion, are gone through with the
utmost regularity, so much so that, as Gore says,
the minima of Algol 'can be predicted with as much
certainty as an eclipse of the sun.' The features of
the type-star are more or less closely reproduced in
[pg 251]
the other Algol Variables—a comparatively long
period of steady light emission, followed by a rapid
fall to one or more minima, and a rapid recovery of
light. The class as yet is a small one, but new
members are gradually being added to it, the
majority of them white, like the type-star.
The study of variable stars is one which should
seem to be specially reserved for the amateur
observer. In general, it requires but little instrumental
equipment. Many variables can be seen at
maximum, some even at minimum, with the unaided
eye; in other cases a good opera or field glass is all
that is required, and a 2½ or 3-inch telescope will
enable the observer to command quite an extensive
field of work. Here, again, the beginner may be
referred to the Memoirs of the British Astronomical
Association for help and guidance, and may be
advised to connect himself with the Variable Star
Section.
With the exception of such variations in the
lustre of certain stars as have been described, the
aspect of the heavens is, in general, fixed and unchanging.
There are, as we shall see, real changes
of the vastest importance continually going on; but
the distances separating us from the fixed stars are
so enormous that these changes shrink into nothingness,
and the astronomers of forty centuries before
our era would find comparatively little change today
in the aspect of the constellations with which
they were familiar. But occasionally a very remarkable
change does take place, in the apparition
[pg 252]
of a new or temporary star. The accounts of the
appearance of such objects are not very numerous,
but are of great interest. We pass over those
recorded, in more or less casual fashion, by the
ancients, for the reason that the descriptions given
are in general more picturesque than illuminative.
It does not add much to one's knowledge, though it
may excite wonder, to find the Chinese annals
recording the appearance, in A.D. 173, of a new star
'resembling a large bamboo mat!'
The first Nova, of which we have a really
scientific record, was the star which suddenly blazed
out, in November, 1572, in the familiar W of
Cassiopeia. It was carefully observed by the great
astronomer, Tycho Brahé, and, according to him,
was brighter than Sirius, Alpha Lyræ, or Jupiter.
Tycho followed it till March, 1574, by which time
it had sunk to the limit of unaided vision, and
further observation became impossible. There is at
present a star of the eleventh magnitude close to the
place fixed for the Nova from Tycho's observations.
In 1604 and 1670, new stars were observed, the
first by Kepler and his assistants, the second by the
monk Anthelme; but from 1670 there was a long
break in the list of discoveries, which was ended by
Hind's observation of a new star in Ophiuchus
(April, 1848). This was never a very conspicuous
object, rising only to somewhat less than fourth
magnitude, and soon fading to tenth or eleventh.
We can only mention the 'Blaze Star' of Corona
Borealis, discovered by Birmingham in 1866, the
[pg 253]
Nova discovered in 1876 by Schmidt of Athens,
near Rho Cygni—an object which seems to have
faded out into a planetary nebula, a fate apparently
characteristic of this class of star—and the star
which appeared in 1885, close to the nucleus of the
Great Nebula in Andromeda.
In 1892, Dr. Anderson of Edinburgh discovered
in the constellation Auriga a star which he estimated
as of fifth magnitude. The discovery was made on
January 31, and the new star was found to have
been photographed at Harvard on plates taken from
December 16, 1891, to January 31, 1892. Apparently
this Nova differed from other temporary
stars in the fact that it attained its full brightness
only gradually. By February 3 it rose to magnitude
3·5, then faded by April 1 to fifteenth, but in August
brightened up again to about ninth magnitude. It
is now visible as a small star. The great development
of spectroscopic resources brought this object,
otherwise not a very conspicuous one, under the
closest scrutiny. Its spectrum showed many bright
lines, which were accompanied by dark ones on the
side next the blue. The idea was thus suggested
that the outburst of brilliancy was due to a collision
between two bodies, one of which, that causing the
dark lines, was approaching the earth, while the
other was receding from it. Lockyer considered
the conflagration to be due to a collision between
two swarms of meteorites, Huggins that it was
caused by the near approach to one another of two
gaseous bodies, while others suggested that the rush
[pg 254]
of a star or of a swarm of meteorites through a
nebula would explain the facts observed. Subsequent
observations of the spectrum of Nova
Aurigæ have revealed the fact that it has obeyed
the destiny which seems to wait on temporary stars,
having become a planetary nebula.
Dr. Anderson followed up his first achievement
by the discovery of a brilliant Nova in the constellation
Perseus. The discovery was made on the
night of February 21-22, 1901, the star being then
of magnitude 2·7. Within two days it became
about the third brightest star in the sky, being a
little more brilliant than Capella; but before the
middle of April it had sunk to fifth magnitude. The
rapidity of its rise must have been phenomenal!
A plate exposed at Harvard on February 19, and
showing stars to the eleventh magnitude, bore no
trace of the Nova. 'It must therefore,' says Newcomb,
'have risen from some magnitude below the
eleventh to the first within about three days. This
difference corresponds to an increase of the light
ten thousandfold!' Such a statement leaves the
mind simply appalled before the spectacle of a
cataclysm so infinitely transcending the very wildest
dreams of fancy. Subsequent observations have
shown the usual tendency towards development
into a nebula, and in August, 1901, photographs
were actually obtained of a nebulosity round the star,
showing remarkable condensations. These photographs,
taken at Yerkes Observatory, when compared
with others taken at Mount Hamilton in
November, revealed the startling fact that the condensations
[pg 255]
of the nebula were apparently in extraordinarily
rapid motion. Now the Nova shows no
appreciable parallax, or in other words is so distant
that its distance cannot be measured; on what
scale, therefore, must these motions have been to be
recorded plainly across a gulf measurable perhaps in
hundreds of light years!
Nova Geminorum, discovered by Professor Turner,
at Oxford, in March, 1903, had not the striking
features which lent so much interest to Nova Persei.
It showed a crimson colour, and its spectrum indicated
the presence in its blaze of hydrogen and
helium; but it faded so rapidly as to show that the
disturbance affected a comparatively small body, and
it has exhibited the familiar new star change into a
nebula.
One point with regard to the Novæ in Auriga and
Perseus deserves notice. These discoveries, so
remarkable in themselves, and so fruitful in the
extension of our knowledge, were made by an
amateur observer with no greater equipment than a
small pocket telescope and a Klein's Star-Atlas.
The thorough knowledge of the face of the heavens
which enabled Dr. Anderson to pick out the faint
glimmer of Nova Aurigæ and to be certain that the
star was a new one is not in the least unattainable
by anyone who cares to give time and patience to
its acquisition; and even should the study never be
rewarded by a capture so dramatic as that of Nova
Persei, the familiarity gained in its course with the
beauty and wonder of the star-sphere will in itself
be an ample reward.
[pg 256]
CHAPTER XV
CLUSTERS AND NEBULÆ
Even the most casual observer of the heavens
cannot have failed to notice that in certain instances
the stars are grouped so closely together as to form
well-marked clusters. The most familiar example
is the well-known group of the Pleiades, in the
constellation Taurus, while quite close is the more
scattered group of the Hyades. Another somewhat
coarsely scattered group is that known as Coma
Berenices, the Hair of Berenice, which lies beneath
the handle of the Plough; and a fainter group is the
cluster Præsepe, which lies in the inconspicuous
constellation Cancer, between Gemini and Leo,
appearing to the naked eye like a fairly bright, hazy
patch, which the smallest telescope resolves into a
cloud of faint stars.
PLATE XXVIII.
1.
2.
Irregular Star Clusters. Photographed by E. E. Barnard.
1. Messier 35 in Gemini. 2. Double Cluster in Perseus.
The Pleiades form undoubtedly the most remarkable
naked-eye group in the heavens. The six
stars which are visible to average eyesight are
Alcyone, 3rd magnitude; Maia, Electra, and Atlas,
of the 4th; Merope, 4⅓; and Taygeta, 4½. While
Celæno, 5⅓; Pleione, 5½; and Asterope, 6, hang
[pg 257]
on the verge of visibility. With an opera-glass
about thirty more may be counted, while photographs
show between 2,000 and 3,000. It is
probable that the fainter stars have no real connection
with the cluster itself, which is merely
seen upon a background of more distant star-dust.
Modern photographs have shown that this cluster
is involved in a great nebula, which stretches in
curious wisps and straight lines from star to star,
and surrounds the whole group. The Pleiades make
a brilliant object for a small telescope with a low
magnifying power, but are too scattered for an
instrument of any size to be effective upon them.
The finest of all irregular star-clusters is that known
as the Sword-handle of Perseus. Midway between
Perseus and the W of Cassiopeia, and directly in
the line of the Galaxy, the eye discerns a small,
hazy patch of light, of which even a 2 or 3 inch glass
will make a beautiful object, while with a large
aperture its splendour is extraordinary. It consists
of two groups of stars which are both in the same
field with a small instrument and low powers.
Towards the edge of the field the stars are comparatively
sparsely scattered; but towards the two
centres of condensation the thickness of grouping
steadily increases. Altogether there is no more
impressive stellar object than this magnificent
double cluster (Plate XXVIII., 2). Another very
fine example of the irregular type of grouping is
seen in M. 35, situated in the constellation Gemini,
and forming an obtuse-angled triangle with the stars
[pg 258]
Mu and Eta Geminorum (Plate XXVIII., 1). There
are many other similar groups fairly well within
the reach of comparatively small instruments, and
some of these are mentioned in the list of objects
(Appendix II.).
Still more remarkable than the irregular clusters
are those which condense into a more or less
globular form. There are not very many objects
of this class in the northern sky visible with a small
telescope, but the beauty of those which are visible
is very notable. The most splendid of all is the
famous cluster M. 13 Herculis. (The M. in these
cases refers to the catalogue of such objects drawn
up by Messier, the French 'comet ferret,' to guide
him in his labours.) M. 13 is situated almost on
the line between Zeta and Eta Herculis, and at
about two-thirds of the distance from Zeta towards
Eta. It is faintly visible to the unaided eye when
its place is known, and, when viewed with sufficient
telescopic power, is a very fine object. Nichol's
remark that 'perhaps no one ever saw it for the
first time through a large telescope without uttering
a shout of wonder' seems to be based on a somewhat
extravagant estimate of the enthusiasm and
demonstrativeness of the average star-gazer; but
the cluster is a very noble object all the same, consisting,
according to a count made on a negative
taken in 1899, of no fewer than 5,482 stars, which
condense towards the centre into a mass of great
brilliancy. It takes a large aperture to resolve the
centre of the cluster into stars, but even a 3-inch
[pg 259]
will show a number of twinkling points of light in
the outlying streamers (Plate XXIX.). In the
same constellation will also be found the cluster
M. 92, similar to, but somewhat fainter than M. 13;
and other globular clusters are noted in the
Appendix. Most of these objects, however, can
only be seen after a fashion with small instruments.
Of the true nature and condition of these wonderful
aggregations we are so far profoundly ignorant.
The question of whether they are composed of
small stars, situated at no very great distance from
the earth, or of large bodies, which are rendered
faint to our vision by immense distance, has been
frequently discussed. Gore concludes that they are
'composed of stars of average size and mass, and
that the faintness of the component stars is simply
due to their immense distance from the earth.' If
so, the true proportions of some of these clusters
must be indeed phenomenal! A very remarkable
feature to be noticed in connection with some of
them is the high proportion of variable stars which
they contain. Professor Bailey has found that in
such clusters as M. 3 and M. 5 the proportion of
variables is one in seven and one in eleven
respectively, while several other groups show proportions
ranging from one in eighteen up to one in
sixty. As the general proportion of variables is
somewhere about one in a hundred, these ratios are
remarkable. They only characterize a certain number
of clusters, however, and are absent in cases which
seem strictly parallel to others where they exist.
PLATE XXIX.
Cluster M. 13 Herculis. Photographed by Mr. W. E. Wilson.
[pg 260]
We now pass from the star-clusters to the nebulæ
properly so called. Till after the middle of last
century it was an open question whether there was
any real distinction between the two classes of
bodies. Herschel had suggested the existence of
a 'shining fluid,' distributed through space, whose
condensations gave rise to those objects known as
nebulæ; but it was freely maintained by many that
the objects which could not be resolved into stars
were irresolvable only because of their vast distance,
and that the increase of telescopic power would
result in the disclosure of their stellar nature. This
view seemed to be confirmed when it was confidently
announced that the great Rosse telescope
had effected the resolution of the Orion Nebula,
which was looked upon as being in some sort a test
case. But the supposed proof of the stellar character
of nebulæ did not hold its ground for long, for in
1864 Sir William Huggins, on applying the spectroscope
to the planetary nebula in Draco, found that
its spectrum consisted merely of bright lines, one
of which—the most conspicuous—was close to the
position of a nitrogen line, but has proved to be
distinct from it; while of the other two, one was
unmistakably the F line of hydrogen and the other
remains still unidentified. Thus it became immediately
manifest that the nebula in Draco did
not consist of distant stars, but was of gaseous
constitution; and Sir William Herschel's idea of
the existence of non-stellar matter in the universe
was abundantly justified. Subsequent research has
[pg 261]
proved that multitudes of nebulæ yield a bright-line
spectrum, and are therefore gaseous. Of these, by
far the most remarkable and interesting is the Great
Nebula of Orion. The observer will readily distinguish
even with the unaided eye that the middle
star of the three that form the sword which hangs
down from Orion's belt has a somewhat hazy
appearance. A small telescope reveals the fact that
the haziness is due to the presence of a great misty
cloud of light, in shape something like a fish-mouth,
and of a greenish colour. At the junction of the
jaws lies the multiple star Theta Orionis, which
with a 2- or 3-inch glass appears to consist of four
stars—'the trapezium'—large instruments showing
in addition two very faint stars.
With greater telescopic power additional features
begin to reveal themselves; the mist immediately
above the trapezium assumes a roughly triangular
shape, and is evidently much denser than the rest
of the nebula, presenting a curdled appearance
similar to that of the stretches of small cloud in a
'mackerel' sky; while from the upper jaw of the
fish-mouth a great shadowy horn rises and stretches
upward, until it gradually loses itself in the darkness
of the background. This wonderful nebula appears
to have been discovered in 1618, but was first really
described and sketched by Huygens in 1656, since
when it has been kept under the closest scrutiny,
innumerable drawings of it having been made and
compared from time to time with the view of detecting
any traces of change. The finest drawings
[pg 262]
extant are those of Sir John Herschel and Mr.
Lassell, and the elaborate one made with the help of
the Rosse 6-foot mirror.
Drawing, however, at no time a satisfactory
method of representing the shadowy and elusive
forms of nebulæ, has now been entirely superseded
by the work of the sensitive plate. Common,
Roberts, Pickering, and others have succeeded
admirably in photographing the Great Nebula with
exposures ranging from half an hour up to six hours.
The extension of nebulous matter revealed by these
photographs is enormous (Plate XXX.), so much
so that many of the central features of the nebula
with which the eye is familiar are quite masked
and overpowered in the photographic print. The
spectrum of the Orion Nebula exhibits indications
of the presence of hydrogen and helium, as well as
the characteristic green ray which marks the unknown
substance named 'nebulium.'
The appearance of this 'tumultuous cloud, instinct
with fire and nitre,' is always amazing. Sir Robert
Ball considers it one of the three most remarkable
objects visible in the northern heavens, the other
two being Saturn and the Great Cluster in Hercules.
But, beautiful and wonderful as both of these may
be, the Orion Nebula conveys to the mind a sense
of mystery which the others, in spite of their
extraordinary features, never suggest. Absolutely
staggering is the thought of the stupendous
dimensions of the nebula. Professor Pickering
considers its parallax to be so small as to indicate
[pg 263]
a distance of not less than 1,000 years light journey
from our earth! It is almost impossible to realize
the meaning of such a statement. When we look
at this shining mist, we are seeing it, not as it is
now, but as it was more than a hundred years
before the Norman Conquest; were it blotted out
of existence now, it would still shine to us and our
descendants for another ten centuries in virtue of
the rays of light which are already speeding across
the vast gulf that separates our world from its
curdled clouds of fire-mist, and the astronomers of
A.D. 2906 might still be speculating on the nature
and destiny of a thing which for ages had been non-existent!
That an object should be visible at all
at such a distance demands dimensions which are
really incomprehensible; but the Orion Nebula is
not only visible, it is conspicuous!
PLATE XXX.
Photograph of the Orion Nebula (W. H. Pickering).
The rival of this famous nebula in point of
visibility is the well-known spiral in the girdle of
Andromeda. On a clear night it can easily be seen
with the naked eye near the star Nu Andromedæ,
and may readily be, as it has often been, mistaken
for a comet. Its discovery must, therefore, have
been practically coincident with the beginnings of
human observation of the heavens; but special
mention of it does not occur before the tenth century
of our era. A small telescope will show it
fairly well, but it must be admitted that the first
view is apt to produce a feeling of disappointment.
The observer need not look for anything like the
whirling streams of light which are revealed on
[pg 264]
modern long exposure photographs (Plate XXXI., 1).
He will see what Simon Marius so aptly described
under the simile of 'the light seen from a great
distance through half-transparent horn plates'—a
lens-shaped misty light, brightening very rapidly
towards a nucleus which seems always on the point
of coming to definition but is never defined, and
again fading away without traceable boundary into
obscurity on every side. The first step towards an
explanation of the structure of this curious object
was made by Bond in the middle of last century.
With the 15-inch refractor of the Cambridge
(U.S.A.) Observatory, he detected two dark rifts
running lengthwise through the bright matter of
the nebula; but it was not till 1887 and 1888 that
its true form was revealed by Roberts's photographs.
It was then seen to be a gigantic spiral
or whirlpool, the rifts noticed by Bond being the
lines of separation between the huge whorls of the
spiral. Of course, small instruments are powerless
to reveal anything of this wonderful structure; still
there is an interest in being able to see, however
imperfectly, an object which seems to present to our
eyes the embodiment of that process by which some
assume that our own system may have been shaped.
So far as the powers of the best telescopes go, the
Andromeda Nebula presents no appearance of stellar
constitution. Its spectrum, according to Scheiner, is
continuous, which would imply that in spite of
appearances it is in reality composed of stars; but
Sir William Huggins has seen also bright lines in
[pg 265]
it. Possibly it may represent a stage intermediate
between the stellar and the gaseous.
PLATE XXXI.
North.
1.
North.
2.
Photographs of Spiral Nebulæ. By Dr. Max Wolf.
1. Great Nebula in Andromeda. 2. Spiral in Triangulum (M. 33).
Another remarkable example of a spiral nebula
will be found in M. 51. It is situated in the constellation
Canes Venatici, and may be easily picked
up, being not far from the end star of the Plough-handle
Eta Ursæ Majoris. This strange object,
'gyre on gyre' of fire-mist, was one of the first
spirals to have its true character demonstrated by
the Rosse telescope. It is visible with moderate
optical powers, but displays to them none of that
marvellous structure which the great 6-foot mirror
revealed for the first time, and which has been
amply confirmed by subsequent photographic
evidence (Plate XXXII.).
PLATE XXXII.
Photograph of Whirlpool Nebula (M. 51). Taken by Mr. W. E. Wilson,
March 6, 1897.
Among other classes of nebulæ we can only
mention the ring and the planetary. Of each of
these, one good example can be seen, though, it
must be admitted, not much more than seen, with
very modest instrumental equipment. Midway
between the two stars Beta and Gamma Lyræ,
already referred to in connection with the variability
of the former, the observer by a little fishing will
find the famous Ring Nebula of Lyra. With low
powers it appears simply as a hazy oval spot; but
it bears magnifying moderately well, and its annular
shape comes out fairly with a power of eighty on a
2½ inch, though it can scarcely be called a brilliant
object with that aperture, or indeed with anything
much under 8 inches. None the less, it is of great
interest, the curious symmetry of this gaseous ring
[pg 266]
making it an almost unique object. It resembles
nothing so much as those vortex rings which an
expert smoker will sometimes send quivering
through the air. Photographs show clearly a star
within the ring, and this star has a very curious
history, having been frequently visible in comparatively
small telescopes, and again, within a year or
two, invisible in much larger ones. Photography
seems to have succeeded in persuading it to forgo
these caprices, though it presents peculiarities of
light which are still unexplained. The actinic plate
reveals also very clearly that deficiency of light at
the ends of the longer diameter of the ring which
can be detected, though with more difficulty, by the
eye. The class of annular nebulæ is not a large
one, and none of its other members come within the
effective range of small instruments.
Planetary nebulæ are so called because with
ordinary powers they present somewhat of the
appearance of a planet seen very dimly and
considerably out of focus. The appearance of
uniformity in their boundaries vanishes under higher
telescopic power, and they appear to be generally
decidedly elliptical; they yield a gaseous spectrum
with strong evidence of the presence of 'nebulium,'
the unknown substance which gives evidence of its
presence in the spectrum of every true nebula, and
has, so far (with one doubtful exception) been found
nowhere else. The chief example of the class is
that body in Draco which first yielded to Huggins
the secret of the gaseous nature of the nebulæ. It
[pg 267]
lies nearly half-way between Polaris and Gamma
Draconis, and is described by Webb as a 'very
luminous disc, much like a considerable star out of
focus.' It is by no means a striking object, but has
its own interest as the first witness to the true
nature of that great class of heavenly bodies to
which it belongs.
The multitude of nebulous bodies scattered over
the heavens may be judged from the fact that
Professor Keeler, after partial surveys carried out
by means of photography with the Crossley reflector,
came to the conclusion that the number within the
reach of that instrument (36-inch aperture) might
be put down at not less than 120,000. It is a
curious fact that the grouping of this great multitude
seems to be fundamentally different from that of the
stars. Where stars are densely scattered, nebulæ
are comparatively scarce; where nebulæ abound,
the stars are less thickly sown. So much is this
the case, that, when Herschel in his historic 'sweeps'
of the heavens came across a notably starless region,
he used to call out to his assistant to 'prepare for
nebulæ.' The idea of a physical connection between
the two classes of bodies is thus underlined in a
manner which, as Herbert Spencer saw so early as
1854, is quite unmistakable.
There remain one or two questions of which the
very shortest notice must suffice—not because they
are unimportant, but because their importance is
such that any attempt at adequate discussion of
them is impossible in our limited space. One of
[pg 268]
these inevitably rises to the mind in presence of the
myriads of the heavenly host—the familiar question
which was so pleasingly suggested to our growing
minds by the nursery rhyme of our childhood. To
the question, What is a star? it has now become
possible to give an answer which is satisfactory so
far as it goes, though it is in a very rudimentary
stage as regards details.
The spectroscope has taught us that the stars
consist of incandescent solid bodies, or of masses of
incandescent gas so large and dense as not to be
transparent; and further, that they are surrounded
by atmospheres consisting of gases cooler than
themselves. The nature of the substances incandescent
in the individual bodies has also to some
extent been learned. The result has been to show
that, while there is considerable variety in the
chemical constitution and condition of the stars, at
least five different types being recognised, each
capable of more minute subdivision, the stars are,
in the main, composed of elements similar to those
existing in the sun; and, in Professor Newcomb's
words, 'as the sun contains most of the elements
found on the earth and few or no others, we may say
that earth and stars seem to be all made out of like
matter.' It is, of course, impossible to say what
unknown elements may exist in the stars; but at
least it is certain that many substances quite familiar
to us, such as iron, magnesium, calcium, hydrogen,
oxygen, and carbon, are present in their constitution.
Indeed, our own sun, in spite of its overwhelming
[pg 269]
importance to ourselves is to be regarded, relatively
to the stellar multitudes, as merely one star among
many; nor, so far as can be judged, can it be considered
by any means a star of the first class. There
can be no doubt that, if removed to the average
distance of first magnitude stars—thirty-three years
light journey—our sun would be merely a common-place
member of the heavenly host, far outshone by
many of its fellow-suns. In all probability it would
shine as about a fifth magnitude star, with suspicions
of variability in its light.
There remains to be noted the fact that the sun
is not to be regarded as a fixed centre, its fixity
being only relative to the members of its own
system. With all its planets and comets it is
sweeping continually through space with a velocity of
more than 1,000,000 miles in the twenty-four hours.
This remarkable fact was first suspected by Sir
William Herschel, who also, with that insight which
was characteristic of his wonderful genius, saw, and
was able roughly to apply, the method which would
either confirm or disprove the suspicion.
The principle which lies at the bottom of the
determination is in itself simple enough, though its
application is complicated in such a manner as to
render the investigation a very difficult one. A
wayfarer passing up the centre of a street lighted
on both sides by lamps will see that the lamps in
front of him appear to open out and separate from
one another as he advances, while those that he is
leaving behind him have an opposite motion,
[pg 270]
appearing to close in upon one another. Now,
with regard to the solar motion, if the case were
absolutely simple, the same effect would be produced
upon the stars among which we are moving; that
is to say, were the stars absolutely fixed, and our
system alone in motion among them, there would
appear to be a general thinning out or retreating of
the stars from the point towards which the sun is
moving, and a corresponding crowding together of
them towards the point, directly opposite in the
heavens, from which it is receding. In actual fact
the case is not by any means so simple, for the stars
are not fixed; they have motions of their own, some
of them enormously greater than the motion of the
sun. Thus the apparent motion caused by the
advance of our system is masked to a great extent
by the real motion of the stars. It is plain, however,
that the perspective effect of the sun's motion
must really be contained in the total motion of each
star, or, in other words, that each star, along with its
own real motion, must have an apparent motion
which is common to all, and results from our movement
through space. If this common element can
be disentangled from the individual element, the
proper motion of each star, then the materials for
the solution of the problem will be secured. It has
been found possible to effect this disentanglement,
and the results of all those who have attempted the
problem are, all things considered, in remarkably
close agreement.
Herschel's application of his principle led him to
[pg 271]
the conclusion that there was a tendency among the
stars to widen out from the constellation Hercules,
and to crowd together towards the opposite constellation
of Argo Navis in the southern hemisphere,
and the point which he fixed upon as the apex of
the sun's path was near the star Lambda Herculis.
Subsequent discussions of the problem have confirmed,
to a great extent, his rough estimate, which
was derived from a comparatively small number of
stars. So far as general direction was concerned,
he was entirely right; the conclusion which he
reached as to the exact point towards which the
motion is directed has, however, been slightly
modified by the discussion of a much larger number
of stars, and it is now considered that the apex of
the solar journey 'is in the general direction of the
constellation Lyra, and perhaps near the star Vega,
the brightest of that constellation' (Newcomb, 'The
Stars,' p. 91). There are but few stars more
beautiful and interesting than Vega; to its own
intrinsic interest must now be added that arising
from the fact that each successive night we look
upon it we have swept more than 1,000,000 miles
nearer to its brilliant globe, and that with every
year we have lessened, by some 400,000,000 miles,
the distance that divides us from it. There can
surely be no thought more amazing than this! It
seems to gather up and bring to a focus all the other
impressions of the vastness of celestial distances and
periods. So swift and ceaseless a motion, and yet
the gulfs that sever us from our neighbours in space
[pg 272]
are so huge that a millennium of such inconceivable
travelling makes no perceptible change upon the
face of the heavens! There rise other thoughts to
the mind. Towards what goal may our world and
its companions be voyaging under the sway of the
mighty ruler of the system, and at the irresistible
summons of those far-off orbs which distance reduces
to the mere twinkling points of light that in man's
earliest childlike thought were but lamps hung out
by the Creator to brighten the midnight sky for his
favourite children? What strange chances may be
awaiting sun and planet alike in those depths of
space towards which we are rushing with such
frightful speed? Such questions remain unanswered
and unanswerable. We are as ignorant of the end
of our journey, and of the haps that may attend it,
as we are helpless in the grasp of the forces that
compel and control it.
[pg 273]
APPENDIX I
The following is a list of the Lunar Formations numbered as on
the Key-map, Plate XIX.:
1. Newton.
2. Short.
3. Simpelius.
4. Manzinus.
5. Moretus.
6. Gruemberger.
7. Casatus.
8. Klaproth.
9. Wilson.
10. Kircher.
11. Bettinus.
12. Blancanus.
13. Clavius.
14. Scheiner.
15. Zuchius.
16. Segner.
17. Bacon.
18. Nearchus.
19. Vlacq.
20. Hommel.
21. Licetus.
22. Maginus.
23. Longomontanus.
24. Schiller.
25. Phocylides.
26. Wargentin.
27. Inghirami.
28. Schickard.
29. Wilhelm I.
30. Tycho.
31. Saussure.
32. Stöfler.
33. Maurolycus.
34. Barocius.
35. Fabricius.
36. Metius.
37. Fernelius.
|
38. Heinsius.
39. Hainzel.
40. Bouvard.
41. Piazzi.
42. Ramsden.
43. Capuanus.
44. Cichus.
45. Wurzelbauer.
46. Gauricus.
47. Hell.
48. Walter.
49. Nonius.
50. Riccius.
51. Rheita.
52. Furnerius.
53. Stevinus.
54. Hase.
55. Snellius.
56. Borda.
57. Neander.
58. Piccolomini.
59. Pontanus.
60. Poisson.
61. Aliacensis.
62. Werner.
63. Pitatus.
64. Hesiodus.
65. Mercator.
66. Vitello.
67. Fourier.
68. Lagrange.
69. Vieta.
70. Doppelmayer.
71. Campanus.
72. Kies.
73. Purbach.
74. La Caille.
|
75. Playfair.
76. Azophi.
77. Sacrobosco.
78. Fracastorius.
79. Santbech.
80. Petavius.
81. Wilhelm Humboldt.
82. Polybius.
83. Geber.
84. Arzachel.
85. Thebit.
86. Bullialdus.
87. Hippalus.
88. Cavendish.
89. Mersenius.
90. Gassendi.
91. Lubiniezky.
92. Alpetragius.
93. Airy.
94. Almanon.
95. Catherina.
96. Cyrillus.
97. Theophilus.
98. Colombo.
99. Vendelinus.
100. Langrenus.
101. Goclenius.
102. Guttemberg.
103. Isidorus.
104. Capella.
105. Kant.
106. Descartes.
107. Abulfeda.
108. Parrot.
109. Albategnius.
110. Alphonsus.
|
111. Ptolemæus.[pg 274]
112. Herschel.
113. Davy.
114. Gueriké.
115. Parry.
116. Bonpland.
117. Lalande.
118. Réaumur.
119. Hipparchus.
120. Letronne.
121. Billy.
122. Fontana.
123. Hansteen.
124. Damoiseau.
125. Grimaldi.
126. Flamsteed.
127. Landsberg.
128. Mösting.
129. Delambre.
130. Taylor.
131. Messier.
132. Maskelyne.
133. Sabine.
134. Ritter.
135. Godin.
136. Sömmering.
137. Schröter.
138. Gambart.
139. Reinhold.
140. Encke.
141. Hevelius.
142. Riccioli.
143. Lohrmann.
144. Cavalerius.
145. Reiner.
146. Kepler.
147. Copernicus.
148. Stadius.
149. Pallas.
150. Triesnecker.
|
151. Agrippa.
152. Arago.
153. Taruntius.
154. Apollonius.
155. Schubert.
156. Firmicus.
157. Silberschlag.
158. Hyginus.
159. Ukert.
160. Boscovich.
161. Ross.
162. Proclus.
163. Picard.
164. Condorcet.
165. Plinius.
166. Menelaus.
167. Manilius.
168. Eratosthenes.
169. Gay Lussac.
170. Tobias Mayer.
171. Marius.
172. Olbers.
173. Vasco de Gama.
174. Seleucus.
175. Herodotus.
176. Aristarchus.
177. La Hire.
178. Pytheas.
179. Bessel.
180. Vitruvius.
181. Maraldi.
182. Macrobius.
183. Cleomedes.
184. Römer.
185. Littrow.
186. Posidonius.
187. Geminus.
188. Linné.
189. Autolycus.
190. Aristillus.
|
191. Archimedes.
192. Timocharis.
193. Lambert.
194. Diophantus.
195. Delisle.
196. Briggs.
197. Lichtenberg.
198. Theætetus.
199. Calippus.
200. Cassini.
201. Gauss.
202. Messala.
203. Struve.
204. Mason.
205. Plana.
206. Burg.
207. Baily.
208. Eudoxus.
209. Aristoteles.
210. Plato.
211. Pico.
212. Helicon.
213. Maupertuis.
214. Condamine.
215. Bianchini.
216. Sharp.
217. Mairan.
218. Gérard.
219. Repsold.
220. Pythagoras.
221. Fontenelle.
222. Timæus.
223. Epigenes.
224. Gärtner.
225. Thales.
226. Strabo.
227. Endymion.
228. Atlas.
229. Hercules.
|
In the accompanying brief notes on a few important formations, the
diameter of each is given in miles, and the height of the highest
peak on wall in feet. The day of each lunation on which it may
be well seen is also added.
NO.
22. Maginus.—Great walled plain; 100 miles; 14,000 feet. Central
mountain 2,000 feet. Difficult in full, owing to rays from
Tycho. Plate XIV. Eighth and ninth days.
23. Longomontanus.—Walled plain; 90 miles; 13,314 feet.
Crossed by rays from Tycho. Plate XV. Ninth day.
[pg 275]
26. Wargentin; 28. Schickard.—Close together. 26. Curious
ring plain; 54 miles. Seemingly filled with lava. 'Resembles
a large thin cheese.' 28. Great walled plain; 134 miles;
9,000 feet. Floor 13,000 square miles area, very varied in
colour. Walls would be invisible to spectator in centre of
enclosure. Plate XII. Thirteenth and fourteenth days.
30. Tycho.—Splendid ring plain; 54 miles; 17,000 feet. Central
mountain 5,000 feet. Great system of streaks from neighbourhood.
Plates XII., XIII., XV. Ninth and tenth days.
32. Stöfler.—Walled plain. Peak on N.E. wall 12,000 feet. Floor
very level. Beautiful steel-grey colour. Plate XVI. Seventh day.
33. Maurolycus.—Walled plain; 150 miles; 14,000 feet. In area
equal to about half of Ireland. Floor in full covered with
bright streaks. Plate XVI. Seventh day.
58. Piccolomini.—Ring plain; 57 miles; 15,000 feet on E. Fine
central mountain. Very rugged neighbourhood. Plate XI.
Fifth and sixth days.
63. Pitatus.—58 miles. Wall massive on S., but breached on
N. side, facing Mare Nubium. Two clefts in interior shown
Plate XV. Ninth day.
78. Fracastorius.—Another partially destroyed formation;
60 miles. Wall breached on N., facing Mare Nectaris.
Under low sun traces of wall can be seen. Plate XI. Fifth
and sixth days.
80. Petavius.—Fine object; 100 miles; 11,000 feet. Fine central
peak 6,000 feet. Great cleft from central mountain to S.E.
wall can be seen with 2-inch. Third and fourth days, but best
seen on waning moon a day or two after full.
90. Gassendi.—Walled plain; 55 miles. Wall on N. broken by
intrusive ring-plain of Gassendi A. Fine central mountain
4,000 feet. Between thirty and forty clefts in floor, more or less
difficult. Plates XII., XIII. Eleventh and twelfth days.
95. Catherina; 96. Cyrillus; 97. Theophilus.—Fine group
of three great walled plains. 95. Very irregular; 70 miles;
16,000 feet. Connected by rough valley with 96. 96 has outline
approaching a square; walls much terraced, overlapped
by 97, and partially ruined on N.E. side. 97 is one of the
finest objects on moon; 64 miles; terraced wall, 18,000 feet.
Fine central mountain 6,000 feet. Plates XI., XVI. Sixth
day.
84. Arzachel; 110. Alphonsus; 111. Ptolemæus.—Another
fine group. 84 is southernmost; 66 miles; 13,000 feet. Fine
central mountain. 110. Walled plain; 83 miles; abutting on
111. Wall rises to 7,000 feet. Bright central peak. Three
peculiar dark patches on floor, best seen towards full. 111 is
largest of three; 115 miles. Many large saucer-shaped hollows
[pg 276]
on floor under low sun. Area 9,000 square miles. Plate XIII.
Eighth and ninth days.
125. Grimaldi.—Darkest walled plain on moon; 148 miles by 129;
area 14,000 square miles; 9,000 feet. Plate XII. Thirteenth
and fourteenth days.
131. Messier and Messier A.—Two bright craters; 9 miles.
Change suspected in relative sizes. From Messier A two
straight light rays like comet's tail extend across Mare Fœcunditatis.
Fourth and fifth days.
147. Copernicus.—Grand object; 56 miles; 10,000 to 12,000 feet.
Central mountain 2,400 feet. Centre of system of bright rays.
On W. a remarkable crater row; good test for definition.
Plates XII., XIII. Ninth and tenth days.
150. Triesnecker.—Small ring plain; 14 miles. Terraced wall
5,000 feet. Remarkable cleft-system on W. Rather delicate
for small telescopes. Plate XIII. Seventh and eighth days.
158. Hyginus.—Crater-pit 3·7 miles. Remarkable cleft runs through
it; visible with 2-inch: connected with Ariadæus rill to W.,
which also an object for a 2-inch. Dark spot to N.W. on
Mare Vaporum named Hyginus N. Has been suspected to be
new formation. Plate XII. Seventh day.
168. Eratosthenes.—Fine ring plain at end of Apennines;
38 miles. Terraced wall 16,000 feet above interior, which is
8,000 feet below Mare Imbrium. Fine central mountain.
Plate XIII. Remarkable contrast to 148 Stadius, which has
wall only 200 feet, with numbers of craters on floor. Ninth and
tenth days.
175. Herodotus; 176. Aristarchus.—Interesting pair. 175 is
23 miles; 4,000 feet. Floor very dusky. Great serpentine
valley; most interesting object. Easy with 2-inch. 176 is
most brilliant crater on moon; 28 miles; 6,000 feet. Central
peak very bright. Readily seen on dark part of moon by
earth-shine. Plates XII., XIII. Twelfth day.
188. Linné.—Small crater on M. Serenitatis near N.W. end of
Apennines. Suspected of change, but varies much in appearance
under different lights. Visible on Plate XVII. as whitish
oval patch to left of end of Apennines. Seventh day.
191. Archimedes.—Fifty miles; 7,000 feet. Floor very flat; crossed
by alternate bright and dark zones. Makes with 189 and 190
fine group well shown Plate XVII. Eighth day.
208. Eudoxus; 209. Aristoteles.—Beautiful pair of ring plains.
208 is 40 miles. Walls much terraced; 10,000 to 11,000 feet;
209 is 60 miles; 11,000 feet. Plate XVII. Sixth and seventh
days.
210. Plato.—Great walled plain; 60 miles; 7,400 feet. Dark grey
floor, which exhibits curious changes of colour under different
[pg 277]
lights, also spots and streaks too difficult for small telescope.
Landslip on E. side. Shadows very fine at sunrise. Plates
XII., XIII. Ninth day.
211. Pico.—Isolated mountain; 7,000 to 8,000 feet. S. of 210. Casts
fine shadow when near terminator. Ninth and tenth days.
228. Atlas; 229. Hercules.—Beautiful pair. 228 is 55 miles;
11,000 feet. Small but distinct central mountain. 229 is
46 miles. Wall reaches same height as 228, and is finely
terraced. Landslip on N. wall. Conspicuous crater on floor.
Plate XI. Fifth day.
[pg 278]
APPENDIX II
The following list includes a number of double and multiple stars,
clusters, and nebulæ, which may be fairly well seen with instruments
up to 3 inches in aperture. A few objects have been added on account
of their intrinsic interest, which may prove pretty severe tests. The
places given are for 1900, and the position-angles and distances are
mainly derived from Mr. Lewis's revision of Struve's 'Mensuræ
Micrometricæ,' Royal Astronomical Society's Memoirs, vol. lvi., 1906.
For finding the various objects, Proctor's larger Star Atlas, though
constructed for 1880, is still, perhaps, the most generally useful.
Cottam's 'Charts of the Constellations' (Epoch 1890) are capital, but
somewhat expensive. A smaller set of charts will be found in Ball's
'Popular Guide to the Heavens,' while Peck has also published
various useful charts. The student who wishes fuller information than
that contained in the brief notes given below should turn to Gore's
exceedingly handy volume, 'The Stellar Heavens.'
The brighter stars are generally known by the letters of the Greek
alphabet, prefixed to them by Bayer. When these are used up,
recourse is had either to the numbers in Flamsteed's Catalogue, or to
those in Struve's 'Mensuræ Micrometricæ.' The Struve numbers are
preceded by the Greek Σ. A few of the more notable variable and
red stars are included; these are generally marked by capital letters,
as V. AQUILÆ. The order of the notes is as follows. First is given
the star's designation, then its place in hours and minutes of right
ascension and degrees and minutes of declination, N. and S. being
marked respectively by + and −; then follow the magnitudes; the
position-angles, which are measured in degrees from the north, or
bottom point of the field, round by east, south, and west to north
again; the distances of the components from one another in seconds
of arc; and, finally, short notes as to colour, etc. According to
Dawes, one inch aperture should separate the components of a 4·56″
double star, two inches those of a 2·28″, three those of a 1·52″, and so
on. If the observer's glass can do this on good nights there is little
fault to find with it. Double stars may be difficult for other reasons
than the closeness of the components; thus, a faint companion to a
bright star is more difficult to detect than a companion which is not
far below its primary in brightness. Clusters and nebulæ, with a few
exceptions, are apt to prove more or less disappointing in small
instruments. The letters of the Greek alphabet are as follows:
[pg 279]
α Alpha. |
η Eta. |
ν Nu. |
τ Tau. |
β Beta. |
θ Theta. |
ξ Xi. |
υ Upsilon. |
γ Gamma. |
ι Iota. |
ο Omicron. |
φ Phi. |
δ Delta. |
κ Kappa. |
π Pi. |
χ Chi. |
ε Epsilon. |
λ Lambda. |
ρ Rho. |
ψ Psi. |
ζ Zeta. |
μ Mu. |
σ Sigma. |
ω Omega. |
Andromeda.
M. 31: 0 h. 37 m. + 40° 43′. Great Spiral Nebula. Visible to naked
eye near ν Andromedæ. Rather disappointing in small glass.
Σ 205 or γ : 1 h. 58 m. + 41° 51′ : 3-5 : 62′5° : 10·2″. Yellow, bluish-green.
5 is also double, a binary, but a very difficult object at
present.
Aquarius.
M. 2 : 21 h. 28 m. − 1° 16′. Globular cluster; forms flat triangle
with α and β.
Σ 2909 or ζ : 22 h. 24 m. − 0° 32′ : 4-4·1 : 319·1° : 3·29″. Yellow, pale
yellow. Binary.
Aquila.
M. 11 : 18 h. 46 m. − 6° 23′. Fine fan-shaped cluster. Just visible to
naked eye.
V : 18 h. 59 m. − 5° 50′. Red star, variable from 6·5 to 8·0.
Argo Navis.
M. 46 : 7 h. 37 m. − 14° 35′. Cluster of small stars, about ½° in
diameter.
Aries.
Σ 180 or γ : 1 h. 48 m. + 18° 49′ : 4·2-4·4 : 359·4° : 8·02″. Both white.
Easy and pretty.
λ 1 h. 52 m. + 23° 7′ : 4·7-6·7 : 47° : 36·5″. Yellow, pointed to by γ
and β.
Auriga.
(Capella) α : 5 h. 9 m. + 45° 54′. Spectroscopic binary; period 104
days.
M. 37 : 5 h. 46 m. + 32° 31′. Fine cluster. M. 36 and M. 38 also fine.
All easily found close to straight line drawn from κ to φ Aurigæ.
β : 5 h. 52 m. + 44° 57′. Spectroscopic binary, period 3·98 days.
41: 6 h. 4 m. + 48° 44′ : 5·2-6·4 : 353·7 : 7·90″. Yellowish-white, bluish-white.
Boötes.
Σ 1864 or π : 14 h. 36 m. + 16° 51′ : 4·9-6 : 103·3° : 5·83″. Both white.
Σ 1877 or ε : 14 h. 40 m. + 27° 30′ : 3-6·3 : 326·4° : 2·86″. Yellow, blue.
Fine object and good test.
Σ 1888 or ξ : 14 h. 47 m. + 19° 31′ : 4·5-6·5 : 180·4° : 2·70″. Yellow,
purple, binary.
Σ 1909 or 44 : 15 h. 0 m. + 48° 2′ : 5·2-6·1 : 242° : 4·32″.
Camelopardus.
V. : 3 h. 33 m. + 62° 19′. Variable, 7·3 to 8·8. Fiery red.
Cancer.
Σ 1196 or ζ : 8 h. 6 m. + 17° 57′ : 5-5·7-6·5 : 349·1°, 109·6° : 1·14″, 5·51″.
Triple ; 5 and 5·7 binary, period 60 years; 6·5 revolves round
centre of gravity of all in opposite direction.
Σ 1268 or ι : 8 h. 41 m. + 29° 7′ : 4·4-6·5 : 307° : 30·59″. Yellow, blue.
Præsepe: Cluster, too widely scattered for anything but lowest powers.
[pg 280]
Canes Venatici.
Σ 1622 or 2 : 12 h. 11 m. + 41° 13′ : 5-7·8 : 258° : 11·4″. Gold, blue.
Σ 1645 : 12 h. 23 m. + 45° 21′ : 7-7·5 : 160·5° : 10·42″. White. Pretty,
though faint.
Σ 1692, 12, or α : 12 h. 51 m. + 38° 52′ : 3·1-5·7 : 227° : 19·69″. Cor
Caroli. White, violet.
M. 51 : 13 h. 26 m. + 47° 43′. Great spiral. 3° S.W. of η Ursæ
Majoris.
M. 3 : 13 h. 38 m. + 28° 53′. Fine globular cluster; on line between
Cor Caroli and Arcturus, rather nearer the latter.
Canis Major.
M. 41 : 6 h. 43 m. − 20° 38′. Fine cluster, visible to naked eye, 4°
below Sirius.
Canis Minor.
(Procyon) α : 7 h. 34 m. + 5° 30′ : 0·5-14 : 5° 4·46″. Binary, companion
discovered, Lick, 1896, only visible in great instruments.
Capricornus.
α : 20 h. 12 m. − 12° 50′ : 3·2-4·2. Naked eye double, both yellow.
M. 30 : 21 h. 35 m. − 23° 38′. Fairly bright cluster.
Cassiopeia.
Σ 60 or η : 0 h. 43 m. + 57° 18′ : 4-7 : 227·8° : 5·64″. Binary; period
about 200 years.
Σ 262 or ι : 2 h. 21 m. + 66° 58′ : 4·2-7·1-7·5 : 250°, 112·6° : 1·93″, 7·48″.
Triple.
H. vi. 30 : 23 h. 52 m. + 56° 9′. Large cloud of small stars.
Σ 3049 or σ : 23 h. 54 m. + 55° 12′ : 5-7·5 : 325·9° : 3·05″. Pretty
double, white, blue.
Cepheus.
κ : 20 h. 12 m. + 77° 25′ : 4-8 : 123° : 7·37″. Yellowish-green.
Σ 2806 or β : 21 h. 27 m. + 70° 7′ : 3-8 : 250·6° : 13·44″. White, blue.
S : 21 h. 36 m. + 78° 10′. Variable, 7·4 to 12·3. Very deep red.
Σ 2863 or ξ : 22 h. 1 m. + 64° 8′ : 4·7-6·5 : 283·3°: 6·87″. Yellow, blue.
δ : 22 h. 25 m. + 57° 54′ : variable-5·3 : 192° : 40″. Yellow, blue.
Primary varies from 3·7 to 4·9. Period, 5·3 days. Spectroscopic
binary.
Σ 3001 or ο : 23 h. 14 m. + 67° 34′ : 5·2-7·8 : 197·3° : 2·97″. Yellow,
yellowish-green.
Cetus.
(Mira) ο : 2 h. 14 m. − 3° 26′. Variable. Period about 331 days.
Maxima, 1·7 to 5; minima, 8 to 9. Colour, deep yellow to
deep orange.
Σ 281 or ν : 2 h. 31 m. + 5° 10′ : 5-9·4 : 83·1°: 7·74″. Yellow, ashy.
Σ 299 or γ : 2 h. 38 m. + 2° 49′ : 3-6·8 : 291° : 3·11″. Yellow, blue,
slow binary.
Coma Berenices.
Σ 1657 or 24 : 12 h. 30 m. + 18° 56′ : 5·5-7 : 271·1° : 20·23″. Orange,
blue.
M. 53 : 13 h. 8 m. + 18° 42′. Cluster of faint stars.
Corona Borealis.
Σ 1965 or ζ : 15 h. 36 m. + 36° 58′ : 4·1-5 : 304·3° : 6·15″. White
greenish.
R : 15 h. 44 m. + 28° 28′. Irregularly variable, 5·5 to 10·1.
Σ 2032 or σ : 16 h. 11 m. + 34° 6′ : 5-6·1 : 216·3° : 4·80″. Yellow,
bluish. Binary, period about 400 years.
[pg 281]
Corvus.
δ : 12 h. 25 m. − 15° 57′ : 3-8·5 : 214° : 24·3″. Yellow, lilac.
Crater.
R. : 10 h. 56 m. − 17° 47′. Variable. About 8 magnitude. Almost
blood-colour.
Cygnus.
Σ 2486 : 19 h. 9 m. + 49° 39′ : 6-6·5 : 218·2° : 9·63″. 'Singular and
beautiful field' (Webb).
(Albireo) β : 19 h. 27 m. + 27° 45′ : 3-5·3 : 55° : 34·2″. Orange-yellow,
blue. Easy and beautiful.
Σ 2580 or χ : 19 h. 43 m. + 33° 30′ : 4·5-8·1 : 71·6° : 25·50″. 4·5 is
variable to 13·5. Period 406 days.
Z : 19 h. 58 m. + 49° 45′. Variable, 7·1 to 12. Deep red.
RS : 20 h. 10 m. + 38° 27′. Variable, 6 to 10. Deep red.
U : 20 h. 16 m. + 47° 35′. Variable, 7 to 11·6. Very red.
V : 20 h. 38 m. + 47° 47′. Variable, 6·8 to 13·5. Very red.
Σ 2758 or 61 : 21 h. 2 m. + 38° 13′ : 5·3-5·9 : 126·8° : 22·52″. First star
whose distance was measured.
RV : 21 h. 39 m. + 37° 33′. Variable, 7·1 to 9·3. Splendid red.
Σ 2822 or μ : 21 h. 40 m. + 28° 18′ : 4-5 : 122·2° : 2·29″. Fine double;
probably binary.
Delphinus.
Σ 2727 or γ : 20 h. 42 m. + 15° 46′ : 4-5 : 269·8° : 10·99″. Yellow,
bluish-green.
V : 20 h. 43 m. + 18° 58′. Variable, 7·3 to 17·3. Period 540 days.
Widest range of magnitude known.
Draco.
Σ 2078 or 17 : 16 h. 34 m. + 53° 8′ : 5-6 : 109·5° : 3·48″. White.
Σ 2130 or μ : 17 h. 3 m. + 54° 37′ : 5-5·2 : 144·2° : 2·17″. White.
H. iv. 37 : 17 h. 59 m. + 66° 38′. Planetary nebula, nearly half-way
between Polaris and γ Draconis. Gaseous; first nebula discovered
to be so.
Σ 2323 or 39: 18 h. 22 m. + 58° 45′ : 4·7-7·7-7·1 : 358·2°, 20·8° : 3·68″,
88·8″. Triple.
ε : 19 h. 48 m. + 70° 1′ : 4-7·6 : 7·5° : 2·84″. Yellow, blue.
Equuleus.
Σ 2737 or ε : 20 h. 54 m. + 3° 55′ : 5·7-6·2-7·1 : 285·9°, 73·8° : 0·53″,
10·43″. Triple with large instruments.
Eridanus.
Σ 518 or 40 or 0^2 : 4 h. 11 m. − 7° 47′ : 4-9-10·8 : 106·3°, 73·6° : 82·4″,
2·39″. Triple, close pair binary.
Gemini.
M. 35 : 6 h. 3 m. + 24° 21′. Magnificent cluster; forms obtuse
triangle with μ and η.
Σ 982 or 38 : 6 h. 49 m. + 13° 19′ : 5·4-7·7 : 159·7° : 6·63″. Yellow,
blue. Probably binary.
ζ : 6 h. 58 m. + 20° 43′. Variable, 3·8 to 4·3. Period 10·2 days.
Non-eclipsing binary.
Σ 1066 or δ : 7 h. 14 m. + 22° 10′ : 3·2-8·2 : 207·3° : 6·72″. Pale yellow,
reddish.
(Castor) α : 7 h. 28 m + 32° 7′ : 2-2·8 : 224·3° : 5·68″. White, yellowish-green.
Finest double in Northern Hemisphere.
[pg 282]
Hercules.
M. 13 : 16 h. 38 m. + 36° 37′. Great globular cluster, two-thirds of
way from ζ to η.
Σ 2140 or α : 17 h. 10 m. + 14° 30′ : 3-6·1 : 113·6° : 4·78″. Orange-yellow,
bluish-green. Fine object.
Σ 2161 or ρ : 17 h. 20 m. + 37° 14′ : 4-5·1 : 314·4° : 3·80″. 'Gem of a
beautiful coronet' (Webb).
M. 92 : 17 h. 14 m. + 43° 15′. Globular cluster; fainter than M. 13.
Σ 2264 or 95 : 17 h. 57 m. + 21° 36′ : 4·9-4·9 : 259·7° : 6·44″. 'Apple-green,
cherry-red' (Smyth), but now both pale yellow.
Σ 2280 or 100 : 18 h. 4 m. + 26° 5′ : 5·9-5·9 : 181·7° : 13·87″. Greenish-white.
Hydra.
Σ 1273 or ε : 8 h. 41 m. + 6° 48′ : 3·8-7·7 : 231·6° : 3·33″. The brighter
star is itself a close double.
V : 10 h. 47 m. − 20° 43′. Variable, 6·7 to 9·5. Copper-red.
W : 13 h. 44 m. − 27° 52′. Variable, 6·7 to 8·0. Deep red.
Lacerta.
Leo.
Σ 1424 or γ : 10 h. 14 m. + 20° 21′ : 2-3·5 : 116·5° : 3·70″. Fine double,
yellow, greenish-yellow.
Σ 1487 or 54 : 10 h. 50 m. + 25° 17′ : 5-7 : 107·5° : 6·38″. Greenish-white,
blue.
Σ 1536 or ι : 11 h. 19 m. + 11° 5′ : 3·9-7·1 : 55·0° : 2·36″. Yellow, blue.
Leo Minor.
Lepus.
R : 4 h. 55 m. − 14° 57′. Variable, 6·7 to 8·5. Intense crimson.
Libra.
M. 5 : 15 h. 13 m. + 2° 27′. Globular cluster, close to star 5 Serpentis.
Remarkable for high ratio of variables in it—1 in 11.
Lynx.
Σ 948 or 12 : 6 h. 37 m. + 59° 33′ : 5·2-6·1-7·4 : 116°, 305·8° : 1·41″,
8·23″. Triple, greenish, white, bluish.
Σ 1334 or 38 : 9 h. 13 m. + 37° 14′ : 4-6·7 : 235·6° : 2·88″. White blue.
Lyra.
T : 18 h. 29 m. + 36° 55′. Variable, 7·2 to 7·8. Crimson.
(Vega) α : 18 h. 34 m. + 38° 41′ : 1-10·5 : 160° : 50·77″. Very pale blue.
The faint companion is a good test for small telescopes. Vega is
near the apex of the solar way.
|
|
ε1 : 18 h. 41·1 m. + 39° 30′ : 4·6-6·3 : 12·4° : 2·85″.
Pale yellow, pale orange yellow |
ε |
|
|
ε2 : 4·9-5·2 : 127·3° : 2·15″. Both pale yellow. |
ζ : 18 h. 41 m. + 37° 30′ : 4·2-5·5 : 150° : 43·7″. Easy, both pale yellow.
β : 18 h. 46 m. + 33° 15′ : 3-6·7 : 149·8° : 45·3″. 3 variable, 12·91 days.
Spectroscopic binary.
M. 57 : 18 h. 50 m. + 32° 54′. Ring Nebula, between β and γ. Faint
in small telescope. Gaseous.
Monoceros.
Σ 919 or 11 : 6 h. 24 m. − 6° 57′ : A 5-B 5·5-C 6 : AB 131·6° : 7·27″ :
BC 105·7° : 2·65″. Fine triple.
Σ 950 or 15 : 6 h. 35 m. + 10°·0′ : 6-8·8-11·2 : 212·2°, 17·9° : 2·69″, 16·54″.
Triple, green, blue, orange.
[pg 283]
Ophiuchus.
ρ : 16 h. 19 m. − 23° 13′ : 6-6 : 355° : 3·4″.
39 : 17 h. 12 m. − 24° 11′ : 5·5-6 : 358° : 15″. Pale orange, blue.
Σ 2202 or 61 : 17 h. 40 m. + 2° 37′ : 5·5-5·8 : 93·4° : 20·68″. White.
Σ 2272 or 70 : 18 h. 1 m. + 2° 32′ : 4·5-6 : 178° : 2·10″. Yellow, purple.
Rather difficult.
Orion.
(Rigel) β : 5 h. 10 m. − 8° 19′ : 1-8 : 202·2° : 9·58″. Bluish-white,
dull bluish. Fair test for small glass.
δ : 5 h. 27 m. − 0° 23′ : 2-6·8 : 359° : 52·7″. White, very easy.
Σ 738 or λ : 5 h. 30 m. + 9° 52′ : 4-6 : 43° 1′ : 4·55″. Yellowish, purple.
Pretty double.
θ : 5 h. 30 m. − 5° 28′ : 6-7-7·5-8. The 'Trapezium' in the Great
Nebula.
M. 42 : 5 h. 30 m. − 5° 28′ : 6-7-7·5-8. Great Nebula of Orion.
Σ 752 or ι : 5 h. 30 m. − 5° 59′ : 3·2-7·3 : 141·7° : 11·50″. White, fine
field.
σ : 5 h. 34 m. − 2° 39′. Fine multiple, double triple in small glass.
ζ : 5 h. 36 m. − 2° 0′ : 2-6 : 156·3° : 2·43″. Yellowish-green, blue.
U : 5 h. 50 m. + 20° 10′. Variable, 5·8-12·3. Period 375 days.
Pegasus.
M. 15 : 21 h. 25 m. + 11° 43′. Fine globular cluster, 4° N.E. of
δ Equulei.
Perseus.
H. VI. 33·34 : 2 h. 13 m. + 56° 40′. Sword-handle of Perseus.
Splendid field.
M. 34 : 2 h. 36 m. + 42° 21′. Visible to naked eye. Fine low-power
field.
Σ 296 or θ : 2 h. 37 m. + 48° 48′ : 4·2-10-11 : 299°, 225° : 17·4″, 80″.
Triple.
Σ 307 or η : 2 h. 43 m. + 55° 29′ : 4-8·5 : 300° : 28″. Orange-yellow,
blue.
(Algol) β : 3 h. 2 m. + 40° 34′. Variable, 2·1 to 3·2. Period 2·8 days.
Spectroscopic eclipsing binary.
Σ 464 or ζ : 3 h. 48 m: + 31° 35′ : 2·7-9·3 : 206·7° : 12·65°. Greenish-white,
ashy. Three other companions more distant.
Σ 471 or ε : 3 h. 51 m. + 39° 43′ : 3·1-8·3 : 7·8° : 8·8″. White, bluish-white.
Pisces.
Σ 12 or 35 : 0 h. 10 m. + 8° 16′ : 6-8 : 150° : 12″. White, purplish.
Σ 88 or ψ : 1 h. 0·4 m. + 20° 56′ : 4·9-5 : 160° : 29·96″. White.
Σ 100 or ζ : 1 h. 8 m. + 7° 3′ : 4·2-5·3 : 64° : 23·68″. White, reddish-violet.
Σ 202 or α : 1 h. 57 m. + 2° 17′ : 2·8-3·9 : 318° : 2·47″. Reddish, white.
Sagitta.
Sagittarius.
M. 20 : 17 h. 56 m. − 23° 2′. The Trifid Nebula.
Scorpio.
β : 15 h. 59·6 m. − 19° 31′ : 2-5 : 25° : 13·6″. Orange, pale yellow.
(Antares) α : 16 h. 23 m. − 26° 13′ : 1-7 : 270° : 3″. Difficult with
small glass.
[pg 284]
Scutum Sobieskii.
M. 24 : 18 h. 12 m. − 18° 27′. Fine cluster of faint stars on Galaxy.
M. 17 : 18 h. 15 m. − 16° 14′. The Omega Nebula. Gaseous.
R : 18 h. 42 m. − 5° 49′. Irregular, variable, 4·8 to 7·8.
Serpens.
Σ 1954 or δ : 15 h. 30 m. + 10° 53′ : 3·2-4·1 : 189·3° : 3·94″. Yellow,
yellowish-green, binary.
Σ 2417 or θ : 18 h. 51 m. + 4° 4′ : 4-4·2 : 103° : 22″. Both pale yellow.
Sextans.
Taurus.
Σ 528 or χ : 4 h. 16 m. + 25° 23′ : 5·7-7·8 : 24·2° : 19·48″. White, lilac.
Σ 716 or 118 : 5 h. 23 m. + 25° 4′ : 5·8-6·6 : 201·8 : 4·86″. White,
bluish-white.
M. 1 : 5 h. 28 m. + 21° 57′. The Crab Nebula. Faint in small glass.
Triangulum.
Σ 227 or ι : 2 h. 7 m. + 29° 50′ : 5-6·4 : 74·6°: 3·79″. Yellow, blue,
beautiful.
Ursa Major.
Σ 1523 or ξ : 11 h. 13 m. + 32° 6′ : 4-4·9 : 137·2° : 2·62″. Yellowish,
binary. Period 60 years.
Σ 1543 or 57 : 11 h. 24 m. + 39° 54′ : 5·2-8·2 : 2·1° : 5·40″. White,
ashy.
(Mizar) ζ : 13 h. 20 m. + 55° 27′ : 2·1-4·2 : 149·9° : 14·53″. Fine pair,
yellow and yellowish-green. Alcor, 5 magnitude in same field
with low power, also 8 magnitude star.
Ursa Minor.
(Polaris) α : 1 h. 22 m. + 88° 46′ : 2-9 : 215·6° : 18·22″. Yellow, bluish,
test for 2-inch.
Virgo.
Σ 1670 or γ : 12 h. 37 m. − 0° 54′ : 3-3 : 328·3° : 5·94″. Both pale
yellow. Binary, 185 years.
Vulpecula.
M. 27 : 19 h. 55 m. + 22° 27′. The Dumb-bell Nebula. Just visible
with 1¼-inch. Gaseous.
[pg 285]
INDEX
A | B | C | D |
E | F | G | H |
I | J | K | L |
M | N | O | P |
R | S | T | U |
V | W | Y | Z
A
- Achromatic. See Telescope
- Adams, search for Neptune, 198-201
- Aerolites, 227
- Airy, search for Neptune, 197-201
- Albireo, colour of, 236
- Alcor, 241
- Alcyone, 256
- Aldebaran, 234;
- Algol, spectroscopic binary, 246;
- diameter and mass of components, 246;
- period of, 250;
- variables, 250
- Alps, lunar, 116;
- Altai Mountains, 117
- Altair, 234
- Altazimuth, 25-28
- Anderson discovers Nova Aurigæ, 253;
- discovers Nova Persei, 254
- Andromeda, great nebula of, 263, 264
- Andromedæ γ, colour of, 236
- Andromedes, 214, 215, 225, 226
- Annular eclipse, 69, 70
- Antares, 234
- Anthelme observes new star, 252
- Apennines, lunar, 116
- Archimedes, 117
- Arcturus, 234
- Argelander, number of stars, 235
- Ariadæus cleft, 119
- Arided, 234
- Arietis γ, observed by Hooke, 240
- Aristillus, 117
- Asteroids, number of, 150;
- methods of discovery, 150, 151
- Asterope, 256
- Astræa, discovery of, 150
- Atlas, 256
- Atmosphere, solar, 75
- Autolycus, 117
- Auzout, aerial telescopes, 4
B
- Bacon, Roger, 1
- Bailey, cluster variables, 259
- Ball, Sir R., 154, 262;
- Popular Guide to the Heavens,' 278
- Barnard, measures of Venus, 89;
- markings on Venus, 95;
- on Mars, 133;
- measures of asteroids, 152;
- discovers Jupiter's fifth satellite, 167;
- measures of Saturn, 172;
- drawing of Saturn, 172;
- rotation of Saturn, 174;
- on Saturnian markings, 184-185;
- observation of Comet 1882 (iii.), 218
- Bayer, lettering of stars, 278
- Beer. See Mädler
- Bélopolsky, rotation of Venus, 96
- Bessel, search for Neptune, 197
- Betelgeux, 234;
- Biela's comet, 213, 214, 215, 224, 225
- Birmingham observes Nova Coronæ, 252
- Bode's law, 148, 149
- Bond, G. P., discovers rifts in Andromeda nebula, 264
- Bond, W. C., discovers Crape Ring, 178;
- discovers Saturn's eighth satellite, 187;
- verifies discovery of Neptune's satellite, 201
- Boötis ε, double star, 242
- Bouvard, tables of Uranus, 197
- Bradley uses aerial telescope, 4
- Bremiker's star-charts, 200
- Brooks' comet, 210;
- observation of comet 1882 (iii.), 218
- Brorsen's comet, 213
C
- Calcium in chromosphere, 73
- Campbell, atmosphere of Mars, 140;
- bright projections on Mars, 141;
- spectroscopic investigation of Saturn's rings, 180
- Canals. See Mars
- Canes Venatici, great spiral nebula in, 265
- Canopus, 234
- Capella, 234
- Capricorni α, naked-eye double, 241[pg 286]
- Carpathians, 117
- Carrington, solar rotation, 59
- Cassegrain. See Telescope, forms of
- Cassini uses aerial telescope, 4;
- discovers four satellites of Saturn and division of ring, 4;
- observations on Jupiter, 160;
- discovers division in Saturn's ring, 177;
- four satellites of Saturn, 184, 186, 187
- Cassiopeiæ η, double star, 242;
- Castor, 234;
- Caucasus, lunar, 116
- Cauchoix constructs 12-inch O.G., 6
- Celaeno, 256
- Celestial cycle, 18
- Centauri α, 231, 234
- Ceres, discovery of, 149;
- diameter of, 152;
- reflective power, 152
- Ceti ζ, naked-eye double, 241;
- Mira (ο) variable star, 248;
- period, 249
- Challis, search for Neptune, 199
- Chambers, G. F., on comets, 208-209;
- Chromosphere, 71, 73, 76;
- depth of, 73;
- constitution of, 73
- Clark, Alvan, constructs 18-½-inch, 8;
- 26-inch, 8;
- 30-inch Pulkowa telescope and 36-inch Lick, 8;
- 40-inch Yerkes, 9
- Clavius, lunar crater, 113, 114, 120
- Clerke, Miss Agnes, 60, 73;
- climate of Mercury, 85;
- on Mars, 139;
- albedo of asteroids, 152;
- Jupiter's red spot, 161;
- on comet 1882 (iii.), 218;
- on Mira Ceti, 248
- Clerk-Maxwell, constitution of Saturn's rings, 179
- Cluster variables, 259
- Clusters, irregular, 256;
- Coggia's comet, 211
- Coma Berenices, 256
- Comas Solà, rotation of Saturn, 174
- Comet of 1811, 206;
- of 1843, 206, 215, 216;
- of Encke, 207;
- of Halley, 207, 213;
- Brooks, 210;
- Donati, 205, 210;
- Tempel, 211;
- 1866 (i.), 214, 224;
- Winnecke, 211;
- Coggia, 211;
- Holmes, 211;
- Biela, 213;
- and Andromeda meteors, 214, 215, 224, 225;
- great southern (1901), 211;
- Wells, 213;
- of 1882, 213, 216-219;
- De Vico, 213;
- Brorsen, 213;
- of Swift 1862 (iii.), and Perseid meteors, 214, 224;
- great southern (1880), 216;
- of 1881, 216;
- of 1807, 216
- Comets, 203 et seq.;
- structure of, 205;
- classes of, 206-208;
- number of, 209;
- spectra of, 211-213, 218;
- constitution of, 212, 218;
- connection with meteors, 214, 215, 224;
- families of, 215-218;
- observation of, 219-222
- Common 5-foot reflector, 12;
- photographs Orion nebula, 262
- Constellations, formation of, 237, 238
- Contraction of sun, 79
- Cooke, T., and Sons, 25-inch Newall telescope, 8;
- mounting of 6-inch refractor, 31
- Copernicus, prediction of phases of Venus, 92;
- Corona, 71, 72, 76;
- tenuity of, 71;
- variations in structure, 71;
- minimum type of, 71, 72;
- maximum type of, 72;
- constitution of, 72
- Corona Borealis, 238;
- Coronal streamers, analogy with Aurora, 71
- Coronium, 72, 73
- Cottam, charts of the constellations, 278
- Crape ring of Saturn, 178
- Craters, lunar, 109, 112;
- ruined and 'ghost,' 111;
- number and size, 112;
- classification of, 112
- Cygni, 61, 231;
- alpha], 234;
- β, colour of, 236
D
- Darwin, G. H., evolution of Saturnian system, 186
- Dawes discovers crape ring, 178;
- Deimos, satellite of Mars, 143
- Delphinus, 237
- Denning, absence of colour in reflector, 22;
- measuring sun-spots, 51, 53;
- on naked-eye views of Mercury, 82;
- abnormal features on Venus, 94;
- on canals of Mars, 136;
- observations of cloud on Mars, 139, 140;
- changes on Jupiter, 159, 160;
- rotation of Saturn, 174;
- visibility of Cassini's division, 182;
- number of meteor radiants, 225;
- classification of sporadic meteors, 227;
- meteoric observation, 227, 228;
- stationary radiants, 229
- Deslandres, calcium photographs of sun, 60;
- on form of corona, 72;
- photographs chromosphere and prominences, 74[pg 287]
- De Vico's comet, 213
- Dew-cap, 39
- Digges, supposed use of telescopes, 1
- Dollond, John, invention of achromatic, 5;
- Donati, comet of 1858, 205, 210;
- spectrum of comet Tempel, 211
- Doppler's principle, 180
- Dorpat refractor, 6, 7, 31
- Douglass, markings on Venus, 95
- Draco, planetary nebula in, 266
- Dunér, rotation of sun, 59
E
- Earth-light on moon, 105
- Eclipse, Indian, 1898, 70;
- 1878, July 29, 72;
- 1870, December 22, 74
- Eclipses, solar, 68-70;
- Electra, 256
- Electrical influence of sun on earth, 63
- Elger on lunar Maria, 111;
- lunar clefts, 119;
- lunar chart, 125
- Elkin observes transit of comet 1882 (iii.), 212
- Encke discovers division in ring of Saturn, 177;
- Equatorial mountings, 29-31, 36
- Equulei δ, short-period binary, 245
- Erck, Dr. Wentworth, satellites of Mars, 144
- Eros, discovery of, distance of, 151;
F
- Fabricius observes Mira Ceti, 248
- Faculæ, 59;
- Faculides, 60
- Finder. See Telescope
- Finlay, transit of comet 1882 (iii.), 212
- Flamsteed, catalogue of stars, 278
- Fomalhaut, 234
- Fowler, 'Telescopic Astronomy,' 17
- Fracastorius, 111
G
- Galaxy. See Milky Way
- Galilean telescope. See Telescope, forms of
- Galileo Galilei, invention of telescope, 2;
- loss of sight, 47;
- discovery of phases of Venus, 92;
- on lunar craters, 112;
- discovers four satellites of Jupiter, 166;
- observations of Saturn, 175, 176
- Galle discovers Neptune, 200
- Gassendi observes transit of Mercury, 87;
- Geminorum α. See Castor
- George III. pensions Herschel, 193
- Georgium Sidus, 194
- Gore, period of Algol, 250;
- globular clusters, 259;
- 'The Stellar Heavens,' 278
- Gregorian. See Telescope, forms of
- Grubb, 27-inch Vienna telescope, 8;
- Gruithuisen, changes on moon, 126
H
- Hale, calcium photographs of sun, 60
- Hall, Asaph, discovers satellites of Mars, 8, 143;
- Hall, Chester Moor, discovers principle of achromatic, 5
- Halley's comet, 207, 213
- Harding discovers Juno, 149
- Hebe, discovery of, 150
- Hegel proves that there are only seven planets, 149
- Helium in chromosphere, 73
- Helmholtz, speed of sensation, 48;
- Hencke discovers Astræa and Hebe, 150
- Henry, 30-inch Nice telescope, 8
- Heraclides promontory, 117
- Hercules, 237
- Herculis α, double star, 242
- Herodotus, valley of, 118, 119, 126
- Herschel, Sir John, drawing of Orion nebula, 262
- Herschel, Sir William, 4-foot telescope, 13;
- impairs sight, 47;
- misses satellites of Mars, 143, 144;
- rotation of Saturn, 173;
- discovers Saturn's sixth and seventh satellites, 186, 187;
- early history, 190, 191;
- discovers Uranus, 191;
- discovers two satellites of Uranus, 196;
- binary stars, 244;
- gaseous constitution of nebulæ, 260;
- distribution of nebulæ, 267;
- translation of solar system, 269
- Herschelian. See Telescope, forms of
- Hevelius, description of Saturn, 176
- Hind discovers Nova Ophiuchi, 252
- Hirst, colouring of Jupiter, 159
- Hirst, Miss, colouring of Jupiter, 159
- Holden on solar rotation, 59, 60[pg 288]
- Holmes, Edwin, telescope-house, 38;
- Holmes, Oliver Wendell, 'Poet at the Breakfast-table,' 13
- Holwarda observes ο Ceti, 248
- Hooke, observation of Gamma Arietis, 240
- Howlett, criticism of Wilsonian theory of sun-spots, 61
- Huggins, atmosphere of Mars, 140;
- gaseous nature of nebulæ, 210;
- spectrum of Winnecke's comet, 211;
- discovers nebula in Draco to be gaseous, 260;
- spectrum of Andromeda nebula, 264
- Humboldtianum, Mare, 111
- Humboldt observes meteor-shower of 1799, 224
- Hussey, search for Neptune, 197
- Hussey, W. J., period of δ Equulei, 245
- Huygens, improvement on telescopes, 3;
- aerial telescopes, 4;
- discovers nature of Saturn's ring and first satellite of Saturn, 177, 186;
- observation of θ Orionis, 240;
- of great nebula in Orion, 261
- Huygens, Mount, 116
- Hydrogen in chromosphere, 73
- Hyginus cleft, 119
I
- Imbrium, Mare, 116
- Iron in chromosphere, 73
J
- Jansen, Zachariah, claim to invention of telescope, 1
- Janssen, photographs of sun, 57
- Journal of British Astronomical Association, 23, 38
- Juno, discovery of, 149;
- Jupiter, brilliancy compared with Venus, 90;
- period of, 155;
- distance of, 155;
- diameter of, 155;
- compression, volume, density, 155;
- brilliancy, 156;
- apparent diameter of, 156;
- belts of, 157 et seq.;
- colouring, 158, 159;
- changes on surface of, 159, 160;
- great red spot, 160-164;
- rotation period, 163-165;
- resemblance to sun, 164-166;
- satellites of, 166-169;
- observation of, 169-171;
- visibility of satellites, 166;
- diameters of, 167;
- occultations of, eclipses of, transits of, 167
K
- Kaiser sea, Mars, 145
- Keeler, report on Yerkes telescope, 9;
- rotation of Saturn, 174;
- constitution of Saturn's rings, 180;
- photographic survey of nebulæ, 267
- Kelvin, solar combustion, 78, 79
- Kepler, suggestion for improved refractor, 3;
- predicts transit of Mercury, 87;
- lunar crater, ray-system of, 120, 121;
- observes new star, 252
- Kirchhoff, production of Fraunhofer lines, 75
- Kirkwood, theory of asteroid formation, 153;
- Kitchiner, visibility of Saturn's satellites, 188
- Klein's Star Atlas, 255
L
- Lampland, photographs of Mars, 137
- Langley, heat of umbra of sun-spot, 50;
- Lassell, 4-foot reflector, 37;
- discovers Saturn's eighth satellite, 187;
- discovers satellite of Uranus, 196;
- search for Neptune, 200;
- discovers satellite of Neptune, 201;
- drawing of Orion nebula, 262
- Leibnitz, mountains, 117
- Lemonnier, observations of Uranus, 193
- Leonid, meteors, 214, 224, 225, 226
- Leonis γ, colour of, 236
- Leverrier, search for Neptune, 199-201
- Lewis, revision of Struve's 'Mensuræ Micrometricæ,' 278
- Lick, 36-inch telescope, 8
- Light, speed of, 231
- Light-year, 230
- Lippershey, claim to invention of telescope, 1
- Lohrmann, lunar chart of, 122
- Lowell, rotation of Mercury, 85;
- surface of Mercury, 86;
- surface of Venus, 95;
- rotation of Venus, 96;
- 'oases' of Mars, 137, 138;
- projections on Mars, 141
- Lunar observation, 123-125
- Lyræ ε, double double, 241, 242;
- β, variable star, 249;
- spectroscopic binary, 250
- Lyra, ring nebula in, 265;[pg 289]
- Lyrid, meteors, 214, 224, 226
M
- M. 35, cluster, 257;
- MacEwen, drawing of Venus, 94, 95
- Mädler, heights of lunar mountains, 118;
- Maginus, 120
- Magnesium in chromosphere, 73
- Maia, 256
- Maintenance of solar light and heat, 78, 79
- Marius, Simon, description of Andromeda nebula, 264
- Markwick, Colonel, 117
- Mars, distance, diameter, rotation, year of, phase of, 130-132;
- oppositions of, 130, 131;
- polar caps, 132;
- canals, 135-137;
- dark areas, 133;
- 'oases,' 137, 138;
- atmosphere of, 139, 140;
- projections on terminator, 141;
- satellites of, 142-144;
- visibility of details of, 144
- Maunder, Mrs., photographs of coronal streamers, 70
- Maunder, E. W., adjustment of equatorial, 22, 23;
- electrical influence of sun on earth, 63;
- 'Astronomy without a Telescope,' 238
- Mee, Arthur, on amateur observation, 17;
- visibility of Cassini's division, 183
- Melbourne 4-foot reflector, 12
- Mellor, lunar chart, 124
- Mendenhall, illustration of sun's distance, 48
- Mercury, elongations of, 81;
- diameter of, 82;
- orbit, 83;
- bulk, weight, density, reflective power, 83;
- phases, 84;
- surface, 84;
- rotation period, 85;
- transits, 87, 88;
- anomalous appearances in, 87
- Merope, 256
- Merz, Cambridge (U.S.A.), and Pulkowa refractors, 6
- Messier, lunar crater, 126;
- 'the comet ferret,' 219;
- catalogue of nebulæ, 258
- Meteors, 222 et seq.;
- shower of 1833, 223;
- of 1866, 224;
- Perseid, 214, 224, 225;
- Leonid, 214, 224, 225;
- Lyrid, 214, 224, 226;
- Andromedes, 214, 215, 224, 225;
- radiant point, 223, 224;
- sporadic, 226;
- observation of, 227-229
- Metius's claim to invention of telescope, 1
- Milky Way, 239;
- clustering of stars towards, 240;
- nebulæ in, 240
- Mira, ο Ceti, 248;
- Mizar, 240, 241
- Montaigne, 219
- Month, lunar and sidereal, 103
- Moon, size, orbit, area, volume, density, mass, force of gravity, 100;
- lunar tides, 101, 102;
- phases, 102;
- synodic period, 103;
- reflective power, 104;
- 'old moon in young moon's arms,' 104;
- earth's light on, 105;
- lunar eclipses, 105, 106;
- 'black eclipses,' 105;
- Maria of, 109-111;
- craters of, 109, 112-114;
- mountain ranges, 109, 116-118;
- clefts or rills, 109, 118, 119;
- ray systems, 109, 120, 121;
- atmosphere of, 126;
- evidence of change, 127, 128
- Mountings. See Telescope
N
- Nasmyth, willow-leaf structure of solar surface, 57;
- lunar clefts, 119;
- on lunar ray systems, 121;
- and Carpenter, lunar chart, 125;
- on powers for lunar observation, 127
- Nebula of Orion, 261-263;
- drawings of, 262;
- photographs, 262;
- distance of, 263;
- of Andromeda, 263, 264;
- photographs of, 264;
- spectrum, 264
- Nebulæ, few in neighbourhood of Galaxy, 240;
- Neison on lunar walled plains, 115, 120;
- Neptune, 148, 196 et seq.;
- diameter, distance, period, spectrum, satellite of, 201
- Newall, 25-inch refractor, 8
- Newcomb on scale of solar operations, 77, 78;
- on markings of Venus, 93;
- phosphorescence of dark side of Venus, 97;
- ratio of stellar increase, 235;
- 'Astronomy for Everybody,' 238;
- stars in galaxy, 240;
- spectroscopic binaries, 248;
- on Nova Persei, 254;
- on constitution of stars, 268;[pg 290]
- apex of solar path, 271
- Newton, Sir Isaac, invents Newtonian reflector, 10
- Nice, 30-inch refractor, 8
- Nichol on M. 13, 258
- Nilosyrtis, 145
- Noble, method of observing sun, 67;
- visibility of Saturn's satellites, 188
- Nova Cassiopeiæ, 252;
- Coronæ, 252;
- Cygni, 253;
- Andromedæ, 253;
- Ophiuchi, 252;
- Aurigæ, 253;
- spectrum of, 253;
- changes into planetary nebula, 254;
- Persei, 254;
- photographs of, 254;
- nebulosity round, 254;
- Geminorum, 255;
- colour, spectrum of, 255
O
- Object-glass, treatment of, 19, 20;
- Observation, methods of solar, 65-67
- Olbers discovers Pallas and Vesta, 149;
- theory of asteroid formation, 150, 152
- Oppolzer, E. von, discovers variability of Eros, 152
- Opposition, 130 (note);
- Orion, 237;
- Orionis θ, observation of, 240;
- ι, naked-eye double, 241;
- θ, multiple star, 243;
- σ, multiple star, 243
P
- Palisa discovers asteroids, 151
- Pallas, discovery of, 149;
- Peck, 'Constellations and How to Find Them,' 238;
- Pegasi κ, short-period binary, 245
- Pegasus, 237
- Perihelion of planets, 131 (note)
- Period, synodic, of moon, 103
- Perrine discovers Jupiter's sixth and seventh satellites, 167
- Perseid, meteors, 214, 224, 225
- Perseus, sword-handle of, 257
- Petavius cleft, 119
- Peters discovers asteroids, 151
- Phillips, Rev. T. E. R., polar cap of Mars, 134;
- canals of Mars, 137;
- clouds on Mars, 140
- Phobos satellite of Mars, 143
- Phosphorescence of dark side of Venus, 97
- Photosphere, 75
- Piazzi discovers Ceres, 149
- Pickering, E. C., number of lucid stars in northern hemisphere, 233;
- parallax of Orion nebula, 262
- Pickering, W. H., on lunar ray systems, 120, 121;
- changes on moon, 126;
- on polar cap of Mars, 134, 135;
- discovers Saturn's ninth and tenth satellites, 187;
- photographs Orion nebula, 262
- Planetary nebulæ, 266;
- spectra of, 266;
- nebula in Draco, 266
- Plato, 117, 126
- Pleiades, number of stars in, 233, 256, 257;
- Pleione, 256
- Polarizing eye-piece, 66
- Pollux, 234
- Præsepe, 256
- Procellarum Oceanus, 111
- Proctor, 2;
- method of finding Mercury, 82;
- on state of Jupiter, 166
- Proctor on the Saturnian system, 181;
- visibility of Cassini's division, 182;
- on Challis's search for Neptune, 199;
- Star Atlas, 278
- Procyon, 234
- Projecting sun's image, 67
- Projections on terminator of Mars, 141
- Prominences, 73, 74
- Ptolemäus, 112
- Pulkowa, 30-inch refractor, 8, 9
R
- Radiant point of meteors, 223, 224;
- number of, 225;
- stationary, 229
- Ranyard Cowper on parallax measures, 231
- Regulus, 234
- Reversing layer seen by Young, 74;
- spectrum photographed by Shackleton, 75;
- depth of, 75
- Riccioli observes duplicity of ζ Ursæ Majoris, 240
- Rigel, 232, 234;
- Ritchey, 5-foot reflector Yerkes Observatory, 12
- Roche's limit, 186
- Rosse, Earl of, 6-foot reflector, 12;
- colouring of Jupiter, 158, 159;
- telescope, resolution of Orion nebula, 260;
- drawing of Orion nebula with, 262;
- spiral character of M. 51, 265
- Rotation period of Mercury, 85;
[pg 291]
S
- Satellite of Venus, question of, 97, 98;
- Saturn, orbit of, sun-heat received by, period of, diameter of, compression and density of, 172;
- features of globe, rotation period, 173;
- varying aspects of rings, 178;
- measures of rings, 178;
- constitution of rings, 179;
- satellites of, 186-189;
- satellites, transits of, 189
- Scheiner, construction of refractors, 2
- Scheiner, Julius, spectrum of Andromeda nebula, 264
- Schiaparelli, rotation of Mercury, 85;
- surface of Mercury, 86;
- rotation of Venus, 96;
- discovery of Martian canals, 135-137;
- connection of comets and meteors, 214, 224
- Schmidt, lunar map, 114;
- observation of comet 1882 (iii.), 217, 218;
- observes Nova Cygni, 253
- Schröter, observations of Venus, 94;
- lunar mountains, 118;
- rills, 118;
- lunar atmosphere, 126
- Schwabe, discovery of sun-spot period, 61, 62
- See, Dr., duration of sun's light and heat, 80
- Serenitatis, Mare, serpentine ridge on, 110, 111;
- crossed by ray from Tycho, 120
- Shackleton photographs spectrum of reversing layer, 75
- Sidereal month, 103
- Siderites and siderolites, 227
- Sinus Iridum, 117
- Sirius, companion of, discovered, 8;
- brightness, 234;
- colour, 235;
- brilliancy compared with Venus, 90;
- with Jupiter, 156
- Sirsalis cleft, 119
- Smyth, Admiral, on amateur observers, 18, 19, 45
- Sodium in chromosphere, 73
- Solar system, translation of, 269-272
- South, Sir James, 12-inch telescope, 6
- Spectroscope, 73, 76
- Spectroscopic observations of rotation of Venus, 96;
- of Martian atmosphere, 140;
- investigations of Saturn's rings, 180;
- of Uranus, 195
- Spectrum of reversing layer, 75;
- Spencer, Herbert, relation of stars and nebulæ, 267
- Spica Virginis, 234
- Stars, distance of, 231;
- number of, 232, 233;
- magnitudes, 234;
- numbers in different magnitudes, 235;
- colours, 235-237;
- change of colour in, 236, 237;
- constellations, 237, 238;
- double, 240;
- multiple, 243;
- binary, 244;
- spectroscopic binaries, 245-248;
- variable, 248-251;
- new or temporary, 251-255;
- constitution of, 268
- Struve, F. G. W., 'Mensuræ Micrometricæ,' 278
- Struve (Otto) discovers satellite of Uranus, 196;
- verifies discovery of Neptune's satellite, 201
- Sun, size, distance, 47, 48;
- rotation period of, 57-59;
- methods of observing, 65-67;
- atmosphere of, 75;
- light and heat of, 78
- Sun-spots, 49, 50;
- rapid changes in, 54, 55;
- period of, 62;
- zones and variation of latitude of, 62
- Synodic period, 103
- Syrtis Major, 145
- Swift, Dean, satellites of Mars, 142
- Swift's comet, 214, 224
T
- Taygeta, 256
- Telescope, invention of, 1, 2;
- refracting, 3;
- achromatic, 5;
- reflecting, 10, 11;
- forms of reflecting, Newtonian, Gregorian, Herschelian, Cassegrain, 10, 11;
- mirrors of reflecting, 11, 12;
- finders, 23, 24;
- mountings of, Altazimuth, 25-28;
- equatorial, 30, 31;
- house for, 37, 38;
- management of, 39, 40;
- powers of, 40, 41
- Tempel's comet, 211
- Terminator of moon, 107;
- Titius, discovery of Bode's law, 148
- Turner discovers Nova Geminorum, 255
- Tycho, 114;
- ray-system of, 108, 120, 121;
- Brahé observes Nova Cassiopeiæ, 252
U
- Uranus, 190;
- distance from sun, period, diameter, visibility, 194;
- spectrum and density, 195;
- satellites, 196
- Ursæ Majoris ζ, duplicity of, 240;
- ξ binary, 244;[pg 292]
- spectroscopic binary, 247
V
- Variable stars, 248-251
- Variation in sun-spot latitude, 62
- Vega, 234;
- colour of, 235;
- apex of solar path, 271
- Venus, diameter, 89;
- orbit and elongations, 89;
- visibility of, 89, 90;
- brilliancy, 90;
- reflective power, 90;
- phases, 92;
- as telescopic object, 93;
- atmosphere, 93;
- blunting of south horn, 94;
- rotation period, 96;
- 'phosphorescence' of dark side, 97;
- question of satellite of, 97, 98;
- transits, 98;
- opportunities for observation, 98, 99
- Vesta, discovery of 149;
- diameter of, 152;
- reflective power, 152
- Vienna, 27-inch refractor, 8
- Vogel, atmosphere of Mars, 140;
- discovery of spectroscopic binaries, 245, 246
W
- Washington, 26-inch refractor, 8
- Watson, asteroid discoveries, 151, 153
- Webb, Rev. J. W., remarks on telescope, 17;
- on amateurs, 18;
- on cleaning of eye pieces, 20;
- visibility of Saturn's rings, 181;
- lunar chart, 124;
- 'Celestial Objects,' 124;
- colouring of Jupiter, 158;
- description of planetary nebula in Draco, 267
- Williams, A. Stanley, seasonal variations in colour of Jupiter's belts, 159;
- periods of rotation (Jupiter), 163;
- rotation of Saturn, 174
- Wells's comet, 213
- Wilson, theory of sun-spots, 60, 61
- Winnecke's comet, 211
- Wolf, asteroid discoveries, 151
Y
- Yerkes observatory, 40-inch refractor, 8, 9;
- Young, illustrations from 'The Sun,' 48;
- electric influence of sun on earth, 63;
- observations of prominences, 74;
- of reversing layer, 74
Z
-
Zöllner, reflective power of Jupiter, 156
THE END
BILLING AND SONS, LTD., PRINTERS, GUILDFORD.
Transcriber's Note
° indicates hours (or degrees); ′ indicates minutes (prime = minutes = feet); ″ indicates seconds (double prime = seconds = inches).
Sundry missing or damaged punctuation has been repaired.
Illustrations (or Plates) which interrupted paragraphs have been moved to more convenient positions between paragraphs.
A few words appear in both hyphenated and unhyphenated versions.
A couple have been corrected, for consistency; the others have been
retained.
Page x: 'XI' corrected to 'IX'
"IX. THE ASTEROIDS 148"
Page 4: Corrected 'lengthwas' to 'length was'.
"... with a glass whose focal length was 212¼ feet."
Page 25: 'familar' corrected to 'familiar'.
"... or, to use more familiar terms,..."
Page 90: "... more especially if the
object casting the shadow have a sharply defined
edge,..."
'have' is correct, and has been retained (subjunctive after 'if').
Page 92: 'firstfruits' corrected to 'first-fruits'. (OED, and matches 2 other occurrences.)
"The actual proof of the
existence of these phases was one of the first-fruits
which Galileo gathered by means of his newly
invented telescope."
Page 109: 'eyeryone' corrected to 'everyone'.
"... —'the man in the moon'—with which everyone is familiar."
Page 118: 'of' added - missing at page-turn.
"They embrace some of the loftiest lunar peaks reaching...."
Page 128: 'lnnar' corrected to 'lunar'.
"The lunar night would be lit by our own earth,..."
Page 157: 'imch' corrected to 'inch'.
Jupiter, October 9, 1891, 9.30 p.m.; 3⅞-inch, power 120."
Page 158: 'eyepiece' corrected to 'eye-piece', to match all the rest.
"... and a single lens eye-piece giving a power of 36."
Page 194: The code for the astronomical symbol for Uranus is U+26E2 or ⛢ (& # 9954;), but it does not seem to work, except, perhaps, in the very latest browsers)
so an image has been used instead:
Page 205: removed extraneous 'of'.
"The nucleus is the only part of [of] a comet's structure "
Page 209: 'unconsidreed' corrected to 'unconsidered'.
"... that some unconsidered little patch of haze...."
Page 240: 'Ursae' corrected to 'Ursæ' to match entries in the Index, and for consistency.
"... though Riccioli detected the duplicity of Zeta Ursæ Majoris (Mizar), in 1650,..."
Page 248: 'in once and a half times,'. 'once' is as printed (and may have been intended).
As it is part of a quote, it has been retained.
"'Once in eleven months,' writes
Miss Clerke, 'the star mounts up in about 125 days
from below the ninth to near the third, or even to
the second magnitude; then, after a pause of two
or three weeks, drops again to its former low level
in once and a half times, on an average, the duration
of its rise.'"
Page 256: 'Celæno' appears here in the text; 'Celaeno, 256' is the Index entry. Both are as printed.
Page 281: 285·9″ corrected to 285·9°
"Equuleus.
Σ 2737 or ε : 20 h. 54 m. + 3° 55′ : 5·7-6·2-7·1 : 285·9°, 73·8° : 0·53″,
10·43″. Triple with large instruments."
This follows the pattern of preceding
Draco.
Σ 2323 or 39: 18 h. 22 m. + 58° 45′ : 4·7-7·7-7·1 : 358·2°, 20·8° : 3·68″,
88·8″. Triple.
Page 282: 3·80° corrected to 3·80″ to match pattern.
"Σ 2161 or ρ : 17 h. 20 m. + 37° 14′ : 4-5·1 : 314·4° : 3·80″. 'Gem of a
beautiful coronet' (Webb)."
Page 288: 'Lyrae' corrected to 'Lyræ'.
"Lyræ ε, double double, 241, 242;"
Page 291: 'obsering' corrected to 'observing'.
"methods of observing, 65-67;"
Page 292: 'elongagations' corrected to 'elongations'.
"orbit and elongations, 89;"
Page 292: 'GUIDFORD' corrected to 'GUILDFORD'.
"BILLING AND SONS, LTD., PRINTERS, GUILDFORD."
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