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THE STORY OF THE HEAVENS
[Illustration: PLATE I.
THE PLANET SATURN, IN 1872.]
THE
STORY OF THE HEAVENS
SIR ROBERT STAWELL BALL, LL.D. D.Sc.
_Author of_ "_Star-Land_"
FELLOW OF THE ROYAL SOCIETY OF LONDON, HONORARY FELLOW OF THE ROYAL
SOCIETY OF EDINBURGH, FELLOW OF THE ROYAL ASTRONOMICAL SOCIETY,
SCIENTIFIC ADVISER TO THE COMMISSIONERS OF IRISH LIGHTS, LOWNDEAN
PROFESSOR OF ASTRONOMY AND GEOMETRY IN THE UNIVERSITY OF CAMBRIDGE, AND
FORMERLY ROYAL ASTRONOMER OF IRELAND
_WITH TWENTY-FOUR COLOURED PLATES AND NUMEROUS
ILLUSTRATIONS_
NEW AND REVISED EDITION
CASSELL AND COMPANY, LIMITED
_LONDON, PARIS, NEW YORK & MELBOURNE_
1900
ALL RIGHTS RESERVED
[Illustration: LA.BELLE SAUVAGE]
PREFACE TO ORIGINAL EDITION.
I have to acknowledge the kind aid which I have received in the
preparation of this book.
Mr. Nasmyth has permitted me to use some of the beautiful drawings of
the Moon, which have appeared in the well-known work published by him in
conjunction with Mr. Carpenter. To this source I am indebted for Plates
VII., VIII., IX., X., and Figs. 28, 29, 30.
Professor Pickering has allowed me to copy some of the drawings made at
Harvard College Observatory by Mr. Trouvelot, and I have availed myself
of his kindness for Plates I., IV., XII., XV.
I am indebted to Professor Langley for Plate II., to Mr. De la Rue for
Plates III. and XIV., to Mr. T.E. Key for Plate XVII., to Professor
Schiaparelli for Plate XVIII., to the late Professor C. Piazzi Smyth for
Fig. 100, to Mr. Chambers for Fig. 7, which has been borrowed from his
"Handbook of Descriptive Astronomy," to Dr. Stoney for Fig. 78, and to
Dr. Copeland and Dr. Dreyer for Fig. 72. I have to acknowledge the
valuable assistance derived from Professor Newcomb's "Popular
Astronomy," and Professor Young's "Sun." In revising the volume I have
had the kind aid of the Rev. Maxwell Close.
I have also to thank Dr. Copeland and Mr. Steele for their kindness in
reading through the entire proofs; while I have also occasionally
availed myself of the help of Mr. Cathcart.
ROBERT S. BALL.
OBSERVATORY, DUNSINK, CO. DUBLIN.
_12th May, 1886._
NOTE TO THIS EDITION.
I have taken the opportunity in the present edition to revise the work
in accordance with the recent progress of astronomy. I am indebted to
the Royal Astronomical Society for the permission to reproduce some
photographs from their published series, and to Mr. Henry F. Griffiths,
for beautiful drawings of Jupiter, from which Plate XI. was prepared.
ROBERT S. BALL.
CAMBRIDGE,
_1st May, 1900_.
CONTENTS.
PAGE
INTRODUCTION 1
CHAPTER
I. THE ASTRONOMICAL OBSERVATORY 9
II. THE SUN 29
III. THE MOON 70
IV. THE SOLAR SYSTEM 107
V. THE LAW OF GRAVITATION 122
VI. THE PLANET OF ROMANCE 150
VII. MERCURY 155
VIII. VENUS 167
IX. THE EARTH 192
X. MARS 208
XI. THE MINOR PLANETS 229
XII. JUPITER 245
XIII. SATURN 268
XIV. URANUS 298
XV. NEPTUNE 315
XVI. COMETS 336
XVII. SHOOTING STARS 372
XVIII. THE STARRY HEAVENS 409
XIX. THE DISTANT SUNS 425
XX. DOUBLE STARS 434
XXI. THE DISTANCES OF THE STARS 441
XXII. STAR CLUSTERS AND NEBULAE 461
XXIII. THE PHYSICAL NATURE OF THE STARS 477
XXIV. THE PRECESSION AND NUTATION OF THE EARTH'S AXIS 492
XXV. THE ABERRATION OF LIGHT 503
XXVI. THE ASTRONOMICAL SIGNIFICANCE OF HEAT 513
XXVII. THE TIDES 531
APPENDIX 558
LIST OF PLATES.
PLATE
I. The Planet Saturn _Frontispiece_
II. A Typical Sun-spot _To face page_ 9
A. The Sun " " 44
III. Spots and Faculae on the Sun " " 37
IV. Solar Prominences or Flames " " 57
V. The Solar Corona " " 62
VI. Chart of the Moon's Surface " " 81
B. Portion of the Moon " " 88
VII. The Lunar Crater Triesnecker " " 93
VIII. A Normal Lunar Crater " " 97
IX. The Lunar Crater Plato " " 102
X. The Lunar Crater Tycho " " 106
XI. The Planet Jupiter " " 254
XII. Coggia's Comet " " 340
C. Comet A., 1892, 1 Swift " " 358
XIII. Spectra of the Sun and of three Stars " " 47
D. The Milky Way, near Messier II. " " 462
XIV. The Great Nebula in Orion " " 466
XV. The Great Nebula in Andromeda " " 468
E. Nebulae in the Pleiades " " 472
F. o Centauri " " 474
XVI. Nebulae observed with Lord Rosse's Telescope " " 476
XVII. The Comet of 1882 " " 357
XVIII. Schiaparelli's Map of Mars " " 221
LIST OF ILLUSTRATIONS.
FIG. PAGE
1. Principle of the Refracting Telescope 11
2. Dome of the South Equatorial at Dunsink Observatory, Co. Dublin 12
3. Section of the Dome of Dunsink Observatory 13
4. The Telescope at Yerkes Observatory, Chicago 15
5. Principle of Herschel's Reflecting Telescope 16
6. South Front of the Yerkes Observatory, Chicago 17
7. Lord Rosse's Telescope 18
8. Meridian Circle 20
9. The Great Bear 27
10. Comparative Sizes of the Earth and the Sun 30
11. The Sun, photographed September 22, 1870 33
12. Photograph of the Solar Surface 35
13. An ordinary Sun-spot 36
14. Scheiner's Observations on Sun-spots 38
15. Zones on the Sun's Surface in which Spots appear 39
16. Texture of the Sun and a small Spot 43
17. The Prism 45
18. Dispersion of Light by the Prism 46
19. Prominences seen in Total Eclipses 53
20. View of the Corona in a Total Eclipse 62
21. View of Corona during Eclipse of January 22, 1898 63
22. The Zodiacal Light in 1874 69
23. Comparative Sizes of the Earth and the Moon 73
24. The Moon's Path around the Sun 76
25. The Phases of the Moon 76
26. The Earth's Shadow and Penumbra 78
27. Key to Chart of the Moon (Plate VI.) 81
28. Lunar Volcano in Activity: Nasmyth's Theory 97
29. Lunar Volcano: Subsequent Feeble Activity 97
30. " " Formation of the Level Floor by Lava 98
31. Orbits of the Four Interior Planets 115
32. The Earth's Movement 116
33. Orbits of the Four Giant Planets 117
34. Apparent Size of the Sun from various Planets 118
35. Comparative Sizes of the Planets 119
36. Illustration of the Moon's Motion 130
37. Drawing an Ellipse 137
38. Varying Velocity of Elliptic Motion 140
39. Equal Areas in Equal Times 141
40. Transit of the Planet of Romance 153
41. Variations in Phase and apparent Size of Mercury 160
42. Mercury as a Crescent 161
43. Venus, May 29, 1889 170
44. Different Aspects of Venus in the Telescope 171
45. Venus on the Sun at the Transit of 1874 177
46. Paths of Venus across the Sun in the Transits of 1874 and 1882 179
47. A Transit of Venus, as seen from Two Localities 183
48. Orbits of the Earth and of Mars 210
49. Apparent Movements of Mars in 1877 212
50. Relative Sizes of Mars and the Earth 216
51, 52. Drawings of Mars 217
53. Elevations and Depressions on the Terminator of Mars 217
54. The Southern Polar Cap on Mars 217
55. The Zone of Minor Planets between Mars and Jupiter 234
56. Relative Dimensions of Jupiter and the Earth 246
57-60. The Occultation of Jupiter 255
61. Jupiter and his Four Satellites 258
62. Disappearances of Jupiter's Satellites 259
63. Mode of Measuring the Velocity of Light 264
64. Saturn 270
65. Relative Sizes of Saturn and the Earth 273
66. Method of Measuring the Rotation of Saturn's Rings 288
67. Method of Measuring the Rotation of Saturn's Rings 289
68. Transit of Titan and its Shadow 295
69. Parabolic Path of a Comet 339
70. Orbit of Encke's Comet 346
71. Tail of a Comet directed from the Sun 363
72. Bredichin's Theory of Comets' Tails 366
73. Tails of the Comet of 1858 367
74. The Comet of 1744 368
75. The Path of the Fireball of November 6, 1869 375
76. The Orbit of a Shoal of Meteors 378
77. Radiant Point of Shooting Stars 381
78. The History of the Leonids 385
79. Section of the Chaco Meteorite 398
80. The Great Bear and Pole Star 410
81. The Great Bear and Cassiopeia 411
82. The Great Square of Pegasus 413
83. Perseus and its Neighbouring Stars 415
84. The Pleiades 416
85. Orion, Sirius, and Neighbouring Stars 417
86. Castor and Pollux 418
87. The Great Bear and the Lion 419
88. Booetes and the Crown 420
89. Virgo and Neighbouring Constellations 421
90. The Constellation of Lyra 422
91. Vega, the Swan, and the Eagle 423
92. The Orbit of Sirius 426
93. The Parallactic Ellipse 444
94. 61 Cygni and the Comparison Stars 447
95. Parallax in Declination of 61 Cygni 450
96. Globular Cluster in Hercules 463
97. Position of the Great Nebula in Orion 466
98. The Multiple Star th Orionis 467
99. The Nebula N.G.C. 1499 471
100. Star-Map, showing Precessional Movement 493
101. Illustration of the Motion of Precession 495
THE
STORY OF THE HEAVENS.
"The Story of the Heavens" is the title of our book. We have indeed a
wondrous story to narrate; and could we tell it adequately it would
prove of boundless interest and of exquisite beauty. It leads to the
contemplation of grand phenomena in nature and great achievements of
human genius.
Let us enumerate a few of the questions which will be naturally asked by
one who seeks to learn something of those glorious bodies which adorn
our skies: What is the Sun--how hot, how big, and how distant? Whence
comes its heat? What is the Moon? What are its landscapes like? How does
our satellite move? How is it related to the earth? Are the planets
globes like that on which we live? How large are they, and how far off?
What do we know of the satellites of Jupiter and of the rings of Saturn?
How was Uranus discovered? What was the intellectual triumph which
brought the planet Neptune to light? Then, as to the other bodies of our
system, what are we to say of those mysterious objects, the comets? Can
we discover the laws of their seemingly capricious movements? Do we know
anything of their nature and of the marvellous tails with which they are
often decorated? What can be told about the shooting-stars which so
often dash into our atmosphere and perish in a streak of splendour? What
is the nature of those constellations of bright stars which have been
recognised from all antiquity, and of the host of smaller stars which
our telescopes disclose? Can it be true that these countless orbs are
really majestic suns, sunk to an appalling depth in the abyss of
unfathomable space? What have we to tell of the different varieties of
stars--of coloured stars, of variable stars, of double stars, of
multiple stars, of stars that seem to move, and of stars that seem at
rest? What of those glorious objects, the great star clusters? What of
the Milky Way? And, lastly, what can we learn of the marvellous nebulae
which our telescopes disclose, poised at an immeasurable distance? Such
are a few of the questions which occur when we ponder on the mysteries
of the heavens.
The history of Astronomy is, in one respect, only too like many other
histories. The earliest part of it is completely and hopelessly lost.
The stars had been studied, and some great astronomical discoveries had
been made, untold ages before those to which our earliest historical
records extend. For example, the observation of the apparent movement of
the sun, and the discrimination between the planets and the fixed stars,
are both to be classed among the discoveries of prehistoric ages. Nor is
it to be said that these achievements related to matters of an obvious
character. Ancient astronomy may seem very elementary to those of the
present day who have been familiar from childhood with the great truths
of nature, but, in the infancy of science, the men who made such
discoveries as we have mentioned must have been sagacious philosophers.
Of all the phenomena of astronomy the first and the most obvious is that
of the rising and the setting of the sun. We may assume that in the dawn
of human intelligence these daily occurrences would form one of the
first problems to engage the attention of those whose thoughts rose
above the animal anxieties of everyday existence. A sun sets and
disappears in the west. The following morning a sun rises in the east,
moves across the heavens, and it too disappears in the west; the same
appearances recur every day. To us it is obvious that the sun, which
appears each day, is the same sun; but this would not seem reasonable to
one who thought his senses showed him that the earth was a flat plain of
indefinite extent, and that around the inhabited regions on all sides
extended, to vast distances, either desert wastes or trackless oceans.
How could that same sun, which plunged into the ocean at a fabulous
distance in the west, reappear the next morning at an equally great
distance in the east? The old mythology asserted that after the sun had
dipped in the western ocean at sunset (the Iberians, and other ancient
nations, actually imagined that they could hear the hissing of the
waters when the glowing globe was plunged therein), it was seized by
Vulcan and placed in a golden goblet. This strange craft with its
astonishing cargo navigated the ocean by a northerly course, so as to
reach the east again in time for sunrise the following morning. Among
the earlier physicists of old it was believed that in some manner the
sun was conveyed by night across the northern regions, and that darkness
was due to lofty mountains, which screened off the sunbeams during the
voyage.
In the course of time it was thought more rational to suppose that the
sun actually pursued his course below the solid earth during the course
of the night. The early astronomers had, moreover, learned to recognise
the fixed stars. It was noticed that, like the sun, many of these stars
rose and set in consequence of the diurnal movement, while the moon
obviously followed a similar law. Philosophers thus taught that the
various heavenly bodies were in the habit of actually passing beneath
the solid earth.
By the acknowledgment that the whole contents of the heavens performed
these movements, an important step in comprehending the constitution of
the universe had been decidedly taken. It was clear that the earth could
not be a plane extending to an indefinitely great distance. It was also
obvious that there must be a finite depth to the earth below our feet.
Nay, more, it became certain that whatever the shape of the earth might
be, it was at all events something detached from all other bodies, and
poised without visible support in space. When this discovery was first
announced it must have appeared a very startling truth. It was so
difficult to realise that the solid earth on which we stand reposed on
nothing! What was to keep it from falling? How could it be sustained
without tangible support, like the legendary coffin of Mahomet? But
difficult as it may have been to receive this doctrine, yet its
necessary truth in due time commanded assent, and the science of
Astronomy began to exist. The changes of the seasons and the recurrence
of seed-time and harvest must, from the earliest times, have been
associated with certain changes in the position of the sun. In the
summer at mid-day the sun rises high in the heavens, in the winter it is
always low. Our luminary, therefore, performs an annual movement up and
down in the heavens, as well as a diurnal movement of rising and
setting. But there is a third species of change in the sun's position,
which is not quite so obvious, though it is still capable of being
detected by a few careful observations, if combined with a philosophical
habit of reflection. The very earliest observers of the stars can hardly
have failed to notice that the constellations visible at night varied
with the season of the year. For instance, the brilliant figure of
Orion, though so well seen on winter nights, is absent from the summer
skies, and the place it occupied is then taken by quite different groups
of stars. The same may be said of other constellations. Each season of
the year can thus be characterised by the sidereal objects that are
conspicuous by night. Indeed, in ancient days, the time for commencing
the cycle of agricultural occupations was sometimes indicated by the
position of the constellations in the evening.
By reflecting on these facts the early astronomers were enabled to
demonstrate the apparent annual movement of the sun. There could be no
rational explanation of the changes in the constellations with the
seasons, except by supposing that the place of the sun was altering, so
as to make a complete circuit of the heavens in the course of the year.
This movement of the sun is otherwise confirmed by looking at the west
after sunset, and watching the stars. As the season progresses, it may
be noticed each evening that the constellations seem to sink lower and
lower towards the west, until at length they become invisible from the
brightness of the sky. The disappearance is explained by the supposition
that the sun appears to be continually ascending from the west to meet
the stars. This motion is, of course, not to be confounded with the
ordinary diurnal rising and setting, in which all the heavenly bodies
participate. It is to be understood that besides being affected by the
common motion our luminary has a slow independent movement in the
opposite direction; so that though the sun and a star may set at the
same time to-day, yet since by to-morrow the sun will have moved a
little towards the east, it follows that the star must then set a few
minutes before the sun.[1]
The patient observations of the early astronomers enabled the sun's
track through the heavens to be ascertained, and it was found that in
its circuit amid the stars and constellations our luminary invariably
followed the same path. This is called the _ecliptic_, and the
constellations through which it passes form a belt around the heavens
known as the _zodiac_. It was anciently divided into twelve equal
portions or "signs," so that the stages on the sun's great journey could
be conveniently indicated. The duration of the year, or the period
required by the sun to run its course around the heavens, seems to have
been first ascertained by astronomers whose names are unknown. The skill
of the early Oriental geometers was further evidenced by their
determination of the position of the ecliptic with regard to the
celestial equator, and by their success in the measurement of the angle
between these two important circles on the heavens.
The principal features of the motion of the moon have also been noticed
with intelligence at an antiquity more remote than history. The
attentive observer perceives the important truth that the moon does not
occupy a fixed position in the heavens. During the course of a single
night the fact that the moon has moved from west to east across the
heavens can be perceived by noting its position relatively to adjacent
stars. It is indeed probable that the motion of the moon was a discovery
prior to that of the annual motion of the sun, inasmuch as it is the
immediate consequence of a simple observation, and involves but little
exercise of any intellectual power. In prehistoric times also, the time
of revolution of the moon had been ascertained, and the phases of our
satellite had been correctly attributed to the varying aspect under
which the sun-illuminated side is turned towards the earth.
But we are far from having exhausted the list of great discoveries which
have come down from unknown antiquity. Correct explanations had been
given of the striking phenomenon of a lunar eclipse, in which the
brilliant surface is plunged temporarily into darkness, and also of the
still more imposing spectacle of a solar eclipse, in which the sun
itself undergoes a partial or even a total obscuration. Then, too, the
acuteness of the early astronomers had detected the five wandering stars
or planets: they had traced the movements of Mercury and Venus, Mars,
Jupiter, and Saturn. They had observed with awe the various
configurations of these planets: and just as the sun, and in a lesser
degree the moon, were intimately associated with the affairs of daily
life, so in the imagination of these early investigators the movements
of the planets were thought to be pregnant with human weal or human woe.
At length a certain order was perceived to govern the apparently
capricious movements of the planets. It was found that they obeyed
certain laws. The cultivation of the science of geometry went hand in
hand with the study of astronomy: and as we emerge from the dim
prehistoric ages into the historical period, we find that the theory of
the phenomena of the heavens possessed already some degree of coherence.
Ptolemy, following Pythagoras, Plato, and Aristotle, acknowledged that
the earth's figure was globular, and he demonstrated it by the same
arguments that we employ at the present day. He also discerned how this
mighty globe was isolated in space. He admitted that the diurnal
movement of the heavens could be accounted for by the revolution of the
earth upon its axis, but unfortunately he assigned reasons for the
deliberate rejection of this view. The earth, according to him, was a
fixed body; it possessed neither rotation round an axis nor translation
through space, but remained constantly at rest in what he supposed to be
the centre of the universe. According to Ptolemy's theory the sun and
the moon moved in circular orbits around the earth in the centre. The
explanation of the movements of the planets he found to be more
complicated, because it was necessary to account for the fact that a
planet sometimes advanced and that it sometimes retrograded. The ancient
geometers refused to believe that any movement, except revolution in a
circle, was possible for a celestial body: accordingly a contrivance was
devised by which each planet was supposed to revolve in a circle, of
which the centre described another circle around the earth.
Although the Ptolemaic doctrine is now known to be framed on quite an
extravagant estimate of the importance of the earth in the scheme of the
heavens, yet it must be admitted that the apparent movements of the
celestial bodies can be thus accounted for with considerable accuracy.
This theory is described in the great work known as the "Almagest,"
which was written in the second century of our era, and was regarded for
fourteen centuries as the final authority on all questions of astronomy.
Such was the system of Astronomy which prevailed during the Middle Ages,
and was only discredited at an epoch nearly simultaneous with that of
the discovery of the New World by Columbus. The true arrangement of the
solar system was then expounded by Copernicus in the great work to which
he devoted his life. The first principle established by these labours
showed the diurnal movement of the heavens to be due to the rotation of
the earth on its axis. Copernicus pointed out the fundamental difference
between real motions and apparent motions; he proved that the
appearances presented in the daily rising and setting of the sun and the
stars could be accounted for by the supposition that the earth rotated,
just as satisfactorily as by the more cumbrous supposition of Ptolemy.
He showed, moreover, that the latter supposition must attribute an
almost infinite velocity to the stars, so that the rotation of the
entire universe around the earth was clearly a preposterous supposition.
The second great principle, which has conferred immortal glory on
Copernicus, assigned to the earth its true position in the universe.
Copernicus transferred the centre, about which all the planets revolve,
from the earth to the sun; and he established the somewhat humiliating
truth, that our earth is merely a planet pursuing a track between the
paths of Venus and of Mars, and subordinated like all the other planets
to the supreme sway of the Sun.
This great revolution swept from astronomy those distorted views of the
earth's importance which arose, perhaps not unnaturally, from the fact
that we happen to be domiciled on that particular planet. The
achievements of Copernicus were soon to be followed by the invention of
the telescope, that wonderful instrument by which the modern science of
astronomy has been created. To the consideration of this important
subject we shall devote the first chapter of our book.
[Illustration: PLATE II.
A TYPICAL SUN-SPOT.
(AFTER LANGLEY.)]
CHAPTER I.
THE ASTRONOMICAL OBSERVATORY.
Early Astronomical Observations--The Observatory of Tycho
Brahe--The Pupil of the Eye--Vision of Faint Objects--The
Telescope--The Object-Glass--Advantages of Large Telescopes--The
Equatorial--The Observatory--The Power of a Telescope--Reflecting
Telescopes--Lord Rosse's Great Reflector at Parsonstown--How the
mighty Telescope is used--Instruments of Precision--The Meridian
Circle--The Spider Lines--Delicacy of pointing a
Telescope--Precautions necessary in making Observations--The Ideal
Instrument and the Practical One--The Elimination of
Error--Greenwich Observatory--The ordinary Opera-Glass as an
Astronomical Instrument--The Great Bear--Counting the Stars in the
Constellation--How to become an Observer.
The earliest rudiments of the Astronomical Observatory are as little
known as the earliest discoveries in astronomy itself. Probably the
first application of instrumental observation to the heavenly bodies
consisted in the simple operation of measuring the shadow of a post cast
by the sun at noonday. The variations in the length of this shadow
enabled the primitive astronomers to investigate the apparent movements
of the sun. But even in very early times special astronomical
instruments were employed which possessed sufficient accuracy to add to
the amount of astronomical knowledge, and displayed considerable
ingenuity on the part of the designers.
Professor Newcomb[2] thus writes: "The leader was Tycho Brahe, who was
born in 1546, three years after the death of Copernicus. His attention
was first directed to the study of astronomy by an eclipse of the sun on
August 21st, 1560, which was total in some parts of Europe. Astonished
that such a phenomenon could be predicted, he devoted himself to a study
of the methods of observation and calculation by which the prediction
was made. In 1576 the King of Denmark founded the celebrated observatory
of Uraniborg, at which Tycho spent twenty years assiduously engaged in
observations of the positions of the heavenly bodies with the best
instruments that could then be made. This was just before the invention
of the telescope, so that the astronomer could not avail himself of that
powerful instrument. Consequently, his observations were superseded by
the improved ones of the centuries following, and their celebrity and
importance are principally due to their having afforded Kepler the means
of discovering his celebrated laws of planetary motion."
The direction of the telescope to the skies by Galileo gave a wonderful
impulse to the study of the heavenly bodies. This extraordinary man is
prominent in the history of astronomy, not alone for his connection with
this supreme invention, but also for his achievements in the more
abstract parts of astronomy. He was born at Pisa in 1564, and in 1609
the first telescope used for astronomical observation was constructed.
Galileo died in 1642, the year in which Newton was born. It was Galileo
who laid with solidity the foundations of that science of Dynamics, of
which astronomy is the most splendid illustration; and it was he who, by
promulgating the doctrines taught by Copernicus, incurred the wrath of
the Inquisition.
The structure of the human eye in so far as the exquisite adaptation of
the pupil is concerned presents us with an apt illustration of the
principle of the telescope. To see an object, it is necessary that the
light from it should enter the eye. The portal through which the light
is admitted is the pupil. In daytime, when the light is brilliant, the
iris decreases the size of the pupil, and thus prevents too much light
from entering. At night, or whenever the light is scarce, the eye often
requires to grasp all it can. The pupil then expands; more and more
light is admitted according as the pupil grows larger. The illumination
of the image on the retina is thus effectively controlled in accordance
with the requirements of vision.
A star transmits to us its feeble rays of light, and from those rays
the image is formed. Even with the most widely-opened pupil, it may,
however, happen that the image is not bright enough to excite the
sensation of vision. Here the telescope comes to our aid: it catches all
the rays in a beam whose original dimensions were far too great to allow
of its admission through the pupil. The action of the lenses
concentrates those rays into a stream slender enough to pass through the
small opening. We thus have the brightness of the image on the retina
intensified. It is illuminated with nearly as much light as would be
collected from the same object through a pupil as large as the great
lenses of the telescope.
[Illustration: Fig. 1.--Principle of the Refracting Telescope.]
In astronomical observatories we employ telescopes of two entirely
different classes. The more familiar forms are those known as
_refractors_, in which the operation of condensing the rays of light is
conducted by refraction. The character of the refractor is shown in Fig.
1. The rays from the star fall upon the object-glass at the end of the
telescope, and on passing through they become refracted into a
converging beam, so that all intersect at the focus. Diverging from
thence, the rays encounter the eye-piece, which has the effect of
restoring them to parallelism. The large cylindrical beam which poured
down on the object-glass has been thus condensed into a small one, which
can enter the pupil. It should, however, be added that the composite
nature of light requires a more complex form of object-glass than the
simple lens here shown. In a refracting telescope we have to employ what
is known as the achromatic combination, consisting of one lens of flint
glass and one of crown glass, adjusted to suit each other with extreme
care.
[Illustration: Fig. 2.--The Dome of the South Equatorial at Dunsink
Observatory Co Dublin.]
[Illustration: Fig. 3.--Section of the Dome of Dunsink Observatory.]
The appearance of an astronomical observatory, designed to accommodate
an instrument of moderate dimensions, is shown in the adjoining figures.
The first (Fig. 2) represents the dome erected at Dunsink Observatory
for the equatorial telescope, the object-glass of which was presented to
the Board of Trinity College, Dublin, by the late Sir James South. The
main part of the building is a cylindrical wall, on the top of which
reposes a hemispherical roof. In this roof is a shutter, which can be
opened so as to allow the telescope in the interior to obtain a view of
the heavens. The dome is capable of revolving so that the opening may be
turned towards that part of the sky where the object happens to be
situated. The next view (Fig. 3) exhibits a section through the dome,
showing the machinery by which the attendant causes it to revolve, as
well as the telescope itself. The eye of the observer is placed at the
eye-piece, and he is represented in the act of turning a handle, which
has the power of slowly moving the telescope, in order to adjust the
instrument accurately on the celestial body which it is desired to
observe. The two lenses which together form the object-glass of this
instrument are twelve inches in diameter, and the quality of the
telescope mainly depends on the accuracy with which these lenses have
been wrought. The eye-piece is a comparatively simple matter. It
consists merely of one or two small lenses; and various eye-pieces can
be employed, according to the magnifying power which may be desired. It
is to be observed that for many purposes of astronomy high magnifying
powers are not desirable. There is a limit, too, beyond which the
magnification cannot be carried with advantage. The object-glass can
only collect a certain quantity of light from the star; and if the
magnifying power be too great, this limited amount of light will be
thinly dispersed over too large a surface, and the result will be found
unsatisfactory. The unsteadiness of the atmosphere still further limits
the extent to which the image may be advantageously magnified, for every
increase of power increases in the same degree the atmospheric
disturbance.
A telescope mounted in the manner here shown is called an _equatorial_.
The convenience of this peculiar style of supporting the instrument
consists in the ease with which the telescope can be moved so as to
follow a star in its apparent journey across the sky. The necessary
movements of the tube are given by clockwork driven by a weight, so
that, once the instrument has been correctly pointed, the star will
remain in the observer's field of view, and the effect of the apparent
diurnal movement will be neutralised. The last refinement in this
direction is the application of an electrical arrangement by which the
driving of the instrument is controlled from the standard clock of the
observatory.
[Illustration: Fig. 4.--The Telescope at Yerkes Observatory, Chicago.
(_From the Astrophysical Journal, Vol. vi., No. 1._)]
The power of a refracting telescope--so far as the expression has any
definite meaning--is to be measured by the diameter of its object-glass.
There has, indeed, been some honourable rivalry between the various
civilised nations as to which should possess the greatest refracting
telescope. Among the notable instruments that have been successfully
completed is that erected in 1881 by Sir Howard Grubb, of Dublin, at the
splendid observatory at Vienna. Its dimensions may be estimated from the
fact that the object-glass is two feet and three inches in diameter.
Many ingenious contrivances help to lessen the inconvenience incident to
the use of an instrument possessing such vast proportions. Among them we
may here notice the method by which the graduated circles attached to
the telescope are brought within view of the observer. These circles are
necessarily situated at parts of the instrument which lie remote from
the eye-piece where the observer is stationed. The delicate marks and
figures are, however, easily read from a distance by a small auxiliary
telescope, which, by suitable reflectors, conducts the rays of light
from the circles to the eye of the observer.
[Illustration: Fig. 5.--Principle of Herschel's Refracting Telescope.]
Numerous refracting telescopes of exquisite perfection have been
produced by Messrs. Alvan Clark, of Cambridgeport, Boston, Mass. One of
their most famous telescopes is the great Lick Refractor now in use on
Mount Hamilton in California. The diameter of this object-glass is
thirty-six inches, and its focal length is fifty-six feet two inches. A
still greater effort has recently been made by the same firm in the
refractor of forty inches aperture for the Yerkes Observatory of the
University of Chicago. The telescope, which is seventy-five feet in
length, is mounted under a revolving dome ninety feet in diameter, and
in order to enable the observer to reach the eye-piece without using
very large step-ladders, the floor of the room can be raised and lowered
through a range of twenty-two feet by electric motors. This is shown in
Fig. 4, while the south front of the Yerkes Observatory is represented
in Fig. 6.
[Illustration: Fig. 6.--South Front of the Yerkes Observatory, Chicago.
(_From the Astrophysical Journal, Vol. vi., No. 1._)]
[Illustration: Fig. 7.--Lord Rosse's Telescope.]
Within the last few years two fine telescopes have been added to the
instrumental equipment of the Royal Observatory, Greenwich, both by Sir
H. Grubb. One of these, containing a 28-inch object-glass, has been
erected on a mounting originally constructed for a smaller instrument by
Sir G. Airy. The other, presented by Sir Henry Thompson, is of 26 inches
aperture, and is adapted for photographic work.
There is a limit to the size of the refractor depending upon the
material of the object-glass. Glass manufacturers seem to experience
unusual difficulties in their attempts to form large discs of optical
glass pure enough and uniform enough to be suitable for telescopes.
These difficulties are enhanced with every increase in the size of the
discs, so that the cost has a tendency to increase at a very much
greater rate. It may be mentioned in illustration that the price paid
for the object-glass of the Lick telescope exceeded ten thousand pounds.
There is, however, an alternative method of constructing a telescope, in
which the difficulty we have just mentioned does not arise. The
principle of the simplest form of _reflector_ is shown in Fig. 5, which
represents what is called the Herschelian instrument. The rays of light
from the star under observation fall on a mirror which is both carefully
shaped and highly polished. After reflection, the rays proceed to a
focus, and diverging from thence, fall on the eye-piece, by which they
are restored to parallelism, and thus become adapted for reception in
the eye. It was essentially on this principle (though with a secondary
flat mirror at the upper end of the tube reflecting the rays at a right
angle to the side of the tube, where the eye-piece is placed) that Sir
Isaac Newton constructed the little reflecting telescope which is now
treasured by the Royal Society. A famous instrument of the Newtonian
type was built, half a century ago, by the late Earl of Rosse, at
Parsonstown. It is represented in Fig. 7. The colossal aperture of this
instrument has never been surpassed; it has, indeed, never been
rivalled. The mirror or speculum, as it is often called, is a thick
metallic disc, composed of a mixture of two parts of copper with one of
tin. This alloy is so hard and brittle as to make the necessary
mechanical operations difficult to manage. The material admits, however,
of a brilliant polish, and of receiving and retaining an accurate
figure. The Rosse speculum--six feet in diameter and three tons in
weight--reposes at the lower end of a telescope fifty-five feet long.
The tube is suspended between two massive castellated walls, which form
an imposing feature on the lawn at Birr Castle. This instrument cannot
be turned about towards every part of the sky, like the equatorials we
have recently been considering. The great tube is only capable of
elevation in altitude along the meridian, and of a small lateral
movement east and west of the meridian. Every star or nebula visible in
the latitude of Parsonstown (except those very near the pole) can,
however, be observed in the great telescope, if looked for at the right
time.
[Illustration: Fig. 8.--Meridian Circle.]
Before the object reaches the meridian, the telescope must be adjusted
at the right elevation. The necessary power is transmitted by a chain
from a winch at the northern end of the walls to a point near the upper
end of the tube. By this contrivance the telescope can be raised or
lowered, and an ingenious system of counterpoises renders the movement
equally easy at all altitudes. The observer then takes his station in
one of the galleries which give access to the eye-piece; and when the
right moment has arrived, the star enters the field of view. Powerful
mechanism drives the great instrument, so as to counteract the diurnal
movement, and thus the observer can retain the object in view until he
has made his measurements or finished his drawing.
Of late years reflecting telescopes have been generally made with
mirrors of glass covered with a thin film of silver, which is capable of
reflecting much more light than the surface of a metallic mirror. Among
great reflectors of this kind we may mention two, of three and five feet
aperture respectively, with which Dr. Common has done valuable work.
We must not, however, assume that for the general work in an observatory
a colossal instrument is the most suitable. The mighty reflector, or
refractor, is chiefly of use where unusually faint objects are being
examined. For work in which accurate measurements are made of objects
not particularly difficult to see, telescopes of smaller dimensions are
more suitable. The fundamental facts about the heavenly bodies have been
chiefly learned from observations obtained with instruments of moderate
optical power, specially furnished so as to enable precise measures of
position to be secured. Indeed, in the early stages of astronomy,
important determinations of position were effected by contrivances
which showed the direction of the object without any telescopic aid.
Perhaps the most valuable measurements obtained in our modern
observatories are yielded by that instrument of precision known as the
_meridian circle_. It is impossible, in any adequate account of the
Story of the Heavens, to avoid some reference to this indispensable aid
to astronomical research, and therefore we shall give a brief account of
one of its simpler forms, choosing for this purpose a great instrument
in the Paris Observatory, which is represented in Fig. 8.
The telescope is attached at its centre to an axis at right angles to
its length. Pivots at each extremity of this axis rotate upon fixed
bearings, so that the movements of the telescope are completely
restricted to the plane of the meridian. Inside the eye-piece of the
telescope extremely fine vertical fibres are stretched. The observer
watches the moon, or star, or planet enter the field of view; and he
notes by the clock the exact time, to the fraction of a second, at which
the object passes over each of the lines. A silver band on the circle
attached to the axis is divided into degrees and subdivisions of a
degree, and as this circle moves with the telescope, the elevation at
which the instrument is pointed will be indicated. For reading the
delicately engraved marks and figures on the silver, microscopes are
necessary. These are shown in the sketch, each one being fixed into an
aperture in the wall which supports one end of the instrument. At the
opposite side is a lamp, the light from which passes through the
perforated axis of the pivot, and is thence ingeniously deflected by
mirrors so as to provide the requisite illumination for the lines at the
focus.
The fibres which the observer sees stretched over the field of view of
the telescope demand a few words of explanation. We require for this
purpose a material which shall be very fine and fairly durable, as well
as somewhat elastic, and of no appreciable weight. These conditions
cannot be completely fulfilled by any metallic wire, but they are
exquisitely realised in the beautiful thread which is spun by the
spider. The delicate fibres are stretched with nice skill across the
field of view of the telescope, and cemented in their proper places.
With instruments so beautifully appointed we can understand the
precision attained in modern observations. The telescope is directed
towards a star, and the image of the star is a minute point of light.
When that point coincides with the intersection of the two central
spider lines the telescope is properly sighted. We use the word sighted
designedly, because we wish to suggest a comparison between the sighting
of a rifle at the target and the sighting of a telescope at a star.
Instead of the ordinary large bull's-eye, suppose that the target only
consisted of a watch-dial, which, of course, the rifleman could not see
at the distance of any ordinary range. But with the telescope of the
meridian circle the watch-dial would be visible even at the distance of
a mile. The meridian circle is indeed capable of such precision as a
sighting instrument that it could be pointed separately to each of two
stars which subtend at the eye an angle no greater than that subtended
by an adjoining pair of the sixty minute dots around the circumference
of a watch-dial a mile distant from the observer.
This power of directing the instrument so accurately would be of but
little avail unless it were combined with arrangements by which, when
once the telescope has been pointed correctly, the position of the star
can be ascertained and recorded. One element in the determination of the
position is secured by the astronomical clock, which gives the moment
when the object crosses the central vertical wire; the other element is
given by the graduated circle which reads the angular distance of the
star from the zenith or point directly overhead.
Superb meridian instruments adorn our great observatories, and are
nightly devoted to those measurements upon which the great truths of
astronomy are mainly based. These instruments have been constructed with
refined skill; but it is the duty of the painstaking astronomer to
distrust the accuracy of his instrument in every conceivable way. The
great tube may be as rigid a structure as mechanical engineers can
produce; the graduations on the circle may have been engraved by the
most perfect of dividing machines; but the conscientious astronomer will
not be content with mere mechanical precision. That meridian circle
which, to the uninitiated, seems a marvellous piece of workmanship,
possessing almost illimitable accuracy, is viewed in a very different
light by the astronomer who makes use of it. No one can appreciate more
fully than he the skill of the artist who has made that meridian circle,
and the beautiful contrivances for illumination and reading off which
give to the instrument its perfection; but while the astronomer
recognises the beauty of the actual machine he is using, he has always
before his mind's eye an ideal instrument of absolute perfection, to
which the actual meridian circle only makes an approximation.
Contrasted with the ideal instrument, the finest meridian circle is
little more than a mass of imperfections. The ideal tube is perfectly
rigid, the actual tube is flexible; the ideal divisions of the circle
are perfectly uniform, the actual divisions are not uniform. The ideal
instrument is a geometrical embodiment of perfect circles, perfect
straight lines, and perfect right angles; the actual instrument can only
show approximate circles, approximate straight lines, and approximate
right angles. Perhaps the spider's part of the work is on the whole the
best; the stretched web gives us the nearest mechanical approach to a
perfectly straight line; but we mar the spider's work by not being able
to insert those beautiful threads with perfect uniformity, while our
attempts to adjust two of them across the field of view at right angles
do not succeed in producing an angle of exactly ninety degrees.
Nor are the difficulties encountered by the meridian observer due solely
to his instrument. He has to contend against his own imperfections; he
has often to allow for personal peculiarities of an unexpected nature;
the troubles that the atmosphere can give are notorious; while the
levelling of his instrument warns him that he cannot even rely on the
solid earth itself. We learn that the earthquakes, by which the solid
ground is sometimes disturbed, are merely the more conspicuous
instances of incessant small movements in the earth which every night in
the year derange the delicate adjustment of the instrument.
When the existence of these errors has been recognised, the first great
step has been taken. By an alliance between the astronomer and the
mathematician it is possible to measure the discrepancies between the
actual meridian circle and the instrument that is ideally perfect. Once
this has been done, we can estimate the effect which the irregularities
produce on the observations, and finally, we succeed in purging the
observations from the grosser errors by which they are contaminated. We
thus obtain results which are not indeed mathematically accurate, but
are nevertheless close approximations to those which would be obtained
by a perfect observer using an ideal instrument of geometrical accuracy,
standing on an earth of absolute rigidity, and viewing the heavens
without the intervention of the atmosphere.
In addition to instruments like those already indicated, astronomers
have other means of following the motions of the heavenly bodies. Within
the last fifteen years photography has commenced to play an important
part in practical astronomy. This beautiful art can be utilised for
representing many objects in the heavens by more faithful pictures than
the pencil of even the most skilful draughtsman can produce. Photography
is also applicable for making charts of any region in the sky which it
is desired to examine. When repeated pictures of the same region are
made from time to time, their comparison gives the means of ascertaining
whether any star has moved during the interval. The amount and direction
of this motion may be ascertained by a delicate measuring apparatus
under which the photographic plate is placed.
If a refracting telescope is to be used for taking celestial
photographs, the lenses of the object-glass must be specially designed
for this purpose. The rays of light which imprint an image on the
prepared plate are not exactly the same as those which are chiefly
concerned in the production of the image on the retina of the human eye.
A reflecting mirror, however, brings all the rays, both those which are
chemically active and those which are solely visual, to one and the
same focus. The same reflecting instrument may therefore be used either
for looking at the heavens or for taking pictures on a photographic
plate which has been substituted for the observer's eye.
A simple portrait camera has been advantageously employed for obtaining
striking photographs of larger areas of the sky than can be grasped in a
long telescope; but for purposes of accurate measurement those taken
with the latter are incomparably better.
It is needless to say that the photographic apparatus, whatever it may
be, must be driven by delicately-adjusted clockwork to counteract the
apparent daily motion of the stars caused by the rotation of the earth.
The picture would otherwise be spoiled, just as a portrait is ruined if
the sitter does not remain quiet during the exposure.
Among the observatories in the United Kingdom the Royal Observatory at
Greenwich is of course the most famous. It is specially remarkable among
all the similar institutions in the world for the continuity of its
labours for several generations. Greenwich Observatory was founded in
1675 for the promotion of astronomy and navigation, and the observations
have from the first been specially arranged with the object of
determining with the greatest accuracy the positions of the principal
fixed stars, the sun, the moon, and the planets. In recent years,
however, great developments of the work of the Observatory have been
witnessed, and the most modern branches of the science are now
assiduously pursued there.
The largest equatorial at Greenwich is a refractor of twenty-eight
inches aperture and twenty-eight feet long, constructed by Sir Howard
Grubb. A remarkable composite instrument from the same celebrated
workshop has also been recently added to our national institution. It
consists of a great refractor specially constructed for photography, of
twenty-six inches aperture (presented by Sir Henry Thompson) and a
reflector of thirty inches diameter, which is the product of Dr.
Common's skill. The huge volume published annually bears witness to the
assiduity with which the Astronomer Royal and his numerous staff of
assistant astronomers make use of the splendid means at their disposal.
The southern part of the heavens, most of which cannot be seen in this
country, is watched from various observatories in the southern
hemisphere. Foremost among them is the Royal Observatory at the Cape of
Good Hope, which is furnished with first-class instruments. We may
mention a great photographic telescope, the gift of Mr. M'Clean.
Astronomy has been greatly enriched by the many researches made by Dr.
Gill, the director of the Cape Observatory.
[Illustration: Fig. 9.--The Great Bear.]
It is not, however, necessary to use such great instruments to obtain
some idea of the aid the telescope will afford. The most suitable
instrument for commencing astronomical studies is within ordinary reach.
It is the well-known binocular that a captain uses on board ship; or if
that cannot be had, then the common opera-glass will answer nearly as
well. This is, no doubt, not so powerful as a telescope, but it has some
compensating advantages. The opera-glass will enable us to survey a
large region of the sky at one glance, while a telescope, generally
speaking, presents a much smaller field of view.
Let us suppose that the observer is provided with an opera-glass and is
about to commence his astronomical studies. The first step is to become
acquainted with the conspicuous group of seven stars represented in Fig.
9. This group is often called the Plough, or Charles's Wain, but
astronomers prefer to regard it as a portion of the constellation of the
Great Bear (Ursa Major). There are many features of interest in this
constellation, and the beginner should learn as soon as possible to
identify the seven stars which compose it. Of these the two marked
a and b, at the head of the Bear, are generally called the
"pointers." They are of special use, because they serve to guide the eye
to that most important star in the whole sky, known as the "pole star."
Fix the attention on that region in the Great Bear, which forms a sort
of rectangle, of which the stars a b g d are the corners. The next fine
night try to count how many stars are visible within that rectangle. On
a very fine night, without a moon, perhaps a dozen might be perceived,
or even more, according to the keenness of the eyesight. But when the
opera-glass is directed to the same part of the constellation an
astonishing sight is witnessed. A hundred stars can now be seen with the
greatest ease.
But the opera-glass will not show nearly all the stars in this region.
Any good telescope will reveal many hundreds too faint for the feebler
instrument. The greater the telescope the more numerous the stars: so
that seen through one of the colossal instruments the number would have
to be reckoned in thousands.
We have chosen the Great Bear because it is more generally known than
any other constellation. But the Great Bear is not exceptionally rich in
stars. To tell the number of the stars is a task which no man has
accomplished; but various estimates have been made. Our great telescopes
can probably show at least 50,000,000 stars.
The student who uses a good refracting telescope, having an object-glass
not less than three inches in diameter, will find occupation for many a
fine evening. It will greatly increase the interest of his work if he
have the charming handbook of the heavens known as Webb's "Celestial
Objects for Common Telescopes."