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ELEMENTARY LESSONS
ASTRONOMY.
ELEMENTARY LESSONS
m
ASTRONOMY.
J. NORMAN LOCKYER,
FELLOW OF THE ROVAL ASTRONOMICAL SOCIETY,
EDITOR OF "THE HEAVENS," ETC.
MACMILLAN AND CO.
1868.
[The right of translation and reproduction is reserved.]
LONDON :
R. CLAY, SON, AND TAYLOR, PRINTERS, BREAD STREET HILL.
PREFACE.
THESE "Elementary Lessons in Astronomy" are in- tended, in the main, to serve as a text-book for use in Schools, but I believe they will be found useful to " children of a larger growth," who wish to make them- selves acquainted with the basis and teachings of one of the most fascinating of the Sciences.
The arrangement adopted is new ; but it is the result of much thought. I have been especially anxious in the descriptive portion to show the Sun's real place in the Cosmos, and to separate the real from the apparent movements. I have therefore begun with the Stars, and have dealt with the apparent movements in a separate chapter.
It may be urged that this treatment is objectionable, as it reduces the mental gymnastic to a minimum ; it is right, therefore, that I should state that my aim throughout the book has been to give a connected view of the whole subject rather than to discuss any particular parts of it ; and to supply facts, and ideas founded on the facts, to serve as a basis for subsequent study and discussion. A companion volume to the present one — I allude to the altogether admirable " Popular Astronomy " from the pen of the Astronomer Royal — may, from this point of view, be looked upon as a sequel to two or three chapters in this book.
vi PREFACE.
It has been my especial endeavour to incorporate the most recent astronomical discoveries. Spectrum-analysis and its results are therefore fully dealt with ; and distances, masses, &c. are based upon the recent determination of the solar parallax.
The use of the Globes and of the Telescope have both been touched upon. Now that our best opticians are employed in producing " Educational Telescopes," more than powerful enough for school purposes, at a low price, it is to be hoped that this aid to knowledge will soon find its place in every school, side by side with the black- board and much questioning.
All the steel plates in the book, acknowledged chefs- d'ceuvres of astronomical drawing, have been placed at my disposal by my friend Mr. Warren De La Rue. I take this opportunity of expressing my thanks to him, and also to the Council of the Royal Astrono- mical Society, M. Guillemin, Mr. R. Bentley, the Rev. H. Godfray, Mr. Cooke, and Mr. Browning, who have kindly supplied me with many of the other illustrations.
I am also under obligations to other friends, espe- cially to Mr. Balfour Stewart and Mr. J. M. Wilson, for valuable advice and criticism, while the work has been passing through the press.
J. N. L.
CONTENTS.
INTRODUCTION.
LESSON PACK
General Notions. — The Uses of Astronomy i
CHAPTER I. THE STARS AND NEBULM.
I. — Magnitudes and Distances of the Stars. Shape of our
Universe , . . 9
II.— -The Constellations. Movements of the Stars. Movement of
our Sun 13
III.— Double and Multiple Stars. Variable Stars 18
IV.— Coloured Stars. Apparent Size. The Structure of the Stars.
Clusters of Stars 23
V. — Nebulae. Classification and Description 3*
VI.— Nebulae (concluded). Their Faintness. Variable Nebulae. Distribution in Space, Their Structure. Nebular Hypo- thesis 35
CHAPTER II.
THE SUN. V I L— Its relative Brightness, its Size, Distance, and Weight ... 38
VIII.— Telescopic Appearance of the Sun-spots. Penumbra, Umbra,
Nucleus. Faculae. Granules. Red Flames ...... 45
IX.— Explanation of the Appearances on the Sun's Surface. The Sun's Light and Heat. Sun-force. The Past and Future of the Sun ........ 4 /..., 48
viii CONTENTS.
CHAPTER III. THE SOLAR SYSTEM.
LESSON PAGE
X. — General Description. Distances of the Planets from the Sun. Sizes of the Planets. The Satellites. Volume, Mass, and Density of the Planets 54
XT.— The Earth. Its Shape. Poles. Equator. Latitude and
Longitude. Diameter 61
XII. — The Earth's Movements. Rotation. Movement round
the Sun. Succession of Day and Night 66
XIII.— The Seasons 7i
XIV. — Structure of the Earth. The Earth's Crust. Interior Heat of the Earth. Cause of its Polar Compression. The Earth once a Star f 77
XV. —The Earth (concluded}. The Atmosphere. Belts of Winds and Calms. The Action of Solar and Terrestrial Radiation. Clouds. Chemistry of the Earth. The Earth's Past and Future . . . . ' 83
XVI.— The Moon : its Size, Orbit, and Motions : its Physical
Constitution 88
XVIt. — Phases of the Moon. Eclipses: how caused. Eclipses
of the Moon 95
XVIII.— Eclipses (concluded}. Eclipses of the Sun. Total Eclipses
and their Phenomena Corona. Red Flames .... 100
XIX.— The other Planets compared with the Earth. Physical
Description of Mars 104
XX. — The other Planets compared with the Earth (continued}. Jupiter : his Belts and Moons. Saturn : general Sketch of his System m
XXL— The other Planets compared with the Earth (concluded). Dimensions of Saturn and his Rings. Probable Nature of the Rings. Effects produced by the Rings on the Planet Uranus. Neptune : its Discovery 116
CONTENTS. ix
LESSON PA<;E
XX 1 1.— The Asteroids, or Minor Planets. Bode's Law. Size of
the Minor Planets : their Orbits: how they are observed 120
XXIII.— Comets: their Orbits. Short-period Comets. Head, Tail, Coma, Nucleus, Jets, Envelopes. Their probable Number and Physical Constitution 123
XXIV. — Luminous Meteors. Shooting Stars. November Showers.
Radiant Points 130
XXV. —Luminous Meteors (concluded). Cause of the Phenomena of Meteors. Orbits of Shooting Stars. Detonating Meteors. Meteorites : their Classification. Falls. Chemical and Physical Constitution 136
CHAPTER IV. APPARENT MOVEMENTS OF THE HEAVENLY BODIES.
XXVI.— The Earth a moving Observatory. The Celestial Sphere. Effects of the Earth's Rotation upon the apparent Move- ments of the Stars. Definitions 142
XXVII.— Apparent Motions of the Heavens, as seen from different parts of the Earth. Parallel, Right, and Oblique Spheres. Circumpolar Stars. Equatorial Stars, and Stars invisible in the Latitude of London. Use of the Globes . ... 147
XXVIII.— Position of the Stars seen at Midnight. Depends upon the two Motions of the Earth. How to tell the Stars. Celestial Globe. Star Maps. The Equatorial Constel- lations. Method of Alignments 1 56
XXIX. — Apparent Motion of the Sun. Difference in Length be- tween the Sidereal and Solar Day. Celestial Latitude and Longitude. The Signs of the Zodiac. Sun's apparent Path. How the Times of Sunrise and Sunset, and the Length of the Day and Night, may be determined by means of the Celestial Globe 165
XXX. — Apparent Motions of the Moon and Planets. Extreme Meridian Heights of the Moon. Angle of her Path with the Horizon at different Times. Harvest Moon. Varying Distances, and varying apparent Size of the Planets. Conjunction and Opposition 170
CONTENTS.
LESSON PAGE
XXXI. — Apparent Motions of the Planets (concluded). Elongations and Stationary Points. Synodic Period, and Periodic Time 176
XXXII. — Apparent Motions of the Planets (concluded). In- clinations and Nodes of the Orbits. Apparent Paths among the Stars. Effects on Physical Observations. Mars. Saturn's Rings . , 182
CHAPTER V. THE MEASUREMENT OF TIME.
XXXIII. — Ancient Methods of Measurement. 'Clepsydrae. Sun Dials. Clocks and Watches. Mean Sun. Equation of Time 190
XXXIV.— Difference of Time. How determined on the Terrestrial Globe. Greenwich Mean Time. Length of the various Days. Sidereal Time. Conversion of Time .... 198
XXXV.— The Week. The Month. The Year. The Calendar.
Old Style. New Style 202
CHAPTER VI. LIGHT.— THE TELESCOPE AND SPECTROSCOPE.
XXXVI.— What Light is ; its Velocity ; how determined. Aberra- tion of Light. Reflection and Refraction. Index of Re- fraction. Dispersion. Lenses 207
XXXVII. — Achromatic Lenses. The Telescope. Illuminating Power.
Magnifying Power 214
XXXVIII.— The Telescope (concluded). Powers of Telescopes of different Apertures. Large Telescopes. Methods of Mounting the Equatorial Telescope 220
XXXI X. — The Solar Spectrum. The Spectroscope. Kirchhoff's
Discovery. Physical Constitution of the Sun .... 227
XL. — Importance of this Method of Research. Physical Con- stitution of the Stars, Nebulas, Moon, and Planets. Construction of the Spectroscope. Celestial Photography 233
CONTENTS. xi
CHAPTER VII.
DETERMINATION OF THE APPARENT PLACES OF THE HEAVENLY BODIES.
LESSON PAGE
XLI. — Geometrical Principles. Circle. Angles. Plane and Spherical Trigonometry. Sextant. Micrometer. The Altazimuth and its Adjustment 239
XLII. — The Transit Circle and its Adjustments. Principles of its Use. Methods of taking Transits. The Chronograph. The Equatorial 249
XLHI. — Corrections applied to Observed Places. Instrumental and Clock Errors. Corrections for Refraction and Aber- ration. Corrections for Parallax. Corrections for Luni- Solar Precession. Change of Equatorial into Ecliptic Co-ordinates 258
XLI V.— Summary of the Methods by which True Positions of the Heavenly Bodies are obtained. Use that is made of these Positions. Determination of Time : of Latitude : of Longitude „ 268
CHAPTER VIII.
DETERMINATION OF THE REAL DISTANCES AND DIMENSIONS OF THE HE A VENLY BODIES.
XLV. — Measurement of a Base Line. Ordnance Survey. Deter- mination of the Length of a Degree. Figure and Size of the Earth. Measurement of the Moon's Distance . . 273
XLVI. — Determination of the Distances of Venus and Mars : of
the Sun. Transit of Venus. The Transit of 1882 . . . 280
XLVI I.— Comparison of the Old and New Values of the Sun's Distance. Distance of the Stars. Determination of Real Sizes 289
xii CONTENTS.
CHAPTER IX. UNIVERSAL GRAVITATION.
LESSON PAGE
XLVIII. — Rest and Motion. Parallelogram of Forces. Law of Falling Bodies. Curvilinear Motion. Newton's Dis- covery. Fall of the Moon to the Earth. Kepler's Laws 293
X LI X.— Kepler's Second Law proved. Centrifugal Tendency. Centripetal Force. Kepler's Third Law proved. The Conic Sections. Movement in an Ellipse 302
L. — Attracting and Attracted Bodies considered separately. Centre of Gravity. Determination of the Weight of the Earth : of the Sun : of the Satellites 307
LI.— General Effect of Attraction. Precession of the Equinoxes : how caused. Nutation. Motions of the Earth's Axis. The Tides. Semi-diurnal, Spring, and Neap Tides. Cause of the Tides. Their probable Effect on the Earth's Rotation 315
APPENDIX , 327
INDEX 335
ILLUSTRATIONS.
PAGE
FRONTISPIECE : Spectra of the Sun, Stars, and Nebula (to face Title], PLATE I. Star Clusters ... 27
II. Nebulae 31
IIL The Sun . . to face 38
IV. Sun-spots 44
. V. The Solar System . 54-55 VI. The Lunar Crater, Co- pernicus . to face 94 VII. Eclipse of the Sun . . 102
PAGE
PLATE VIII. Mars in 1856, to face 104 ,, IX. Mars in 1862 . . 109 „ X. Jupiter and Saturn,
to face 112 „ XI. Radiant - point of
Shooting Stars . 133 „ XII. Equatorial Tele- scope .... 225 ,, XIII. Spectroscope . . . 229 „ XIV. Portable Altazimuth 245 „ XV. Transit Circle . . .251
FIG. x. Orbit of a Double Star . 18
2. The Double-double Star
e Lyrae 19
3. Position of the Sun's axis . 41
4. Part of a Sun-spot, as
seen in a powerful tele- scope 47
5. Section of the Plane of the
Ecliptic 56
6. Mode of constructing an
Ellipse 67
7. The Seasons 68
8. Explanation of the differ-
ent Altitudes of the Sun in Summer and Winter 72
9. 10, ii, 12. The Earth, as
seen from the Sun : — At the Summer Solstice 73 ,, Winter „ 74
,, Vernal Equinox 75 ,, Autumnal ,, . 76
13. Cause of the Earth's sphe
roidal form ... 82
14. Phases of the Moon . 96
FIG. 15. General Theory of
Eclipses 98
16. General View of Jupiter
and his Moons . . .113
17. General View of Saturn
and his Moons . . .114
1 8. Ecliptic Chart .... 122
19. Donati's Comet (general
view) 125
20. Ditto, showing Head and
Envelopes 127
21. Fire-ball, as seen in a
telescope 138
22. A Parallel Sphere . . .148
23. A Right Sphere . . . . tb.
24. An Oblique Sphere . . 149
25. Southern Circumpolar
Constellations. . . .152
26. Northern Circumpolar
Constellations . . .153
27. The Great Bear at inter-
vals of six hours . . . 155
28. Equatorial Constella-
tions:—Orion . . . 162
29. Bootes . . .163
XIV
ILLUSTRA TIONS.
PAGE
FIG. 30. Equatorial Constella- tions : — Perseus, Cas-
siopea . . 164
31. Sidereal and" Solar Days. 166
32. Harvest Moon .... 172
33. Retrogradations, Elonga-
tions, and Stationary Points of Planets . .178
34. Path of Venus among the
Stars in 1868 .... 183
35. Path of Saturn among
the Stars, 1862-5 . .184
36. Orbits of Mars and the
Earth 185
37. Varying Appearances of
Saturn's Rings . . .187
38. Saturn when the Plane
of the Ring passes through the Earth . .188
39. Ditto, when the North
Surface of the Rings is visible ib.
40. Construction of the Sun-
dial 192
Aberration of Light . . 208 Atmospheric Refraction
41. 42.
209 43t Action of a Prism on a
beam of light. . . . 210
44. Action of two Prisms
placed base to base . .212
45. Action of a Convex Lens 213
46. Ditto, showing how the
Image is inverted . . ib.
47. Concave Lens . . . .215
48. Theory of the Astrono-
mical Telescope . . .217 .49. Star Spectroscope . . . 235
50. Direct - vision Spectro-
,scope 237
51. triangles 241
52. Ditto, with equal bases . 242
53. Trigonometrical Ratios . 243
PAGE
FIG. 54. Effect of the Aberration of Light on a Star's ap- parent place .... 260
55. Mode of Correction for
Aberration 261
56. Parallax 262
57. Transformation of Equa-
torial and Ecliptic Co- ordinates 266
58. Measurement of the dis-
tance of the Moon . . 279
59. Ditto of Mars . . . .281
60. Transit of Venus . . . 283
61. Ditto, Sun's Disc, as seen
from the Earth . . . 285
62. Ditto, reversed .... ib.
63. Ditto, illuminated side of
the Earth at ingress . 286
64. Ditto, ditto, at egress . 287
65. Parallelogram of Forces . 294
66. Action of Gravity on the
Mjoon'spath .... 298
67. Kepler's Second Law . 300
68. Proof of ditto . . . .302
69. Circular Motion . . . 303
70. The Conic Sections . . 306
71. Orbital Velocities . . . 307
72. Centre of Gravity, in the
case of Equal Masses . 309
73. Ditto, in the case of
Unequal Masses . . . ib.
74. Fall of Planets to the Sun 310
75. The Cavendish Experi-
ment 312
76. Showing the effects of
Precession on the posi- tion of the Earth's axis 318
77. Nutation 319
78. Apparent motion of the
Pole of the Equator round the Pole of the Heavens (or Ecliptic) . 320
ELEMENTARY LESSONS
ASTRONOMY.
INTRODUCTION.
GENERAL NOTIONS.— THE USES OF ASTRONOMY.
1. AT night, if the sky be cloudless, we see it spangled with so many stars, that it seems impossible to count them ; and we see the same sight whether we are in England, or in any other part of the world. The earth on which we live is, in fact, surrounded by stars on all sides ; and this was so evident to even the first men who studied the stars that they pictured the earth standing in the centre of a hollow crystal sphere, in which the
r/ stars were fixed like golden nails.
2. In the daytime the scene is changed : in place of thousands of stars, our eyes behold a glorious orb whose rays light up and warm the earth, and this body we call the »un. So bright are his beams that, in his presence, all the " lesser lights," the stars, are extinguished. But if we doubt their being still there we have only to take a candle from a dark room into the sunshine to understand how their feeble light, like that of the candle, is " put out " by the greater light of the sun.
3. There are, however, other bodies which attract our attention. The moon shines at night now as a cres- cent and now as a full moon, sometimes rendering the stars invisible in the same way as the sun does,
2 ASTRONOMY.
though in a less degree, and showing us by its changes that there is some difference between it and the sun ; for while the sun always appears round, because we receive light from all parts of its surface turned towards us, the shape of the bright or lit-up portion of the moon varies from night to night, that part only being visible which is turned towards the sun.
4. Again, if we examine the heavens more closely still, we may see, after a few nights' watching, one, or perhaps two, of the brighter " stars " change their position with regard to the stars lying near them, or with regard to the sun if we watch that body closely at sunrise and sunset. These are the planets ; the ancients called them " wan- dering stars."
5. But the planets are not the only bodies which move across the face of the sky. Sometimes a comet may by its sudden appearance and strange form awaken our interest and make us acquainted with another class of objects unlike any of those which we have previously mentioned.
6. Such are the celestial bodies ordinarily visible to us. Far away, and comparatively so dim that the naked eye can make little out of them, lie the nebulae, so called because in the telescope they often put on strange cloud- like forms ; they differ as much from stars in their ap- pearance as comets do from planets.
7. There are other bodies, to which we shall refer by and by. But we will, in this place, content ourselves with stating generally what Astronomy teaches us concerning star and sun, moon and planet, comet and nebula.
8. To begin, then, with the stars. So far from being fixed, and being stuck as it were in a hollow glass globe, which state of things would cause all to be at pretty nearly equal distances from us, they are all in rapid mo- tion, and their distances vary enormously ; although
INTRODUCTION. 3
all of them are so very far away that they appear to us to be at rest, as a ship does when sailing along at a great distance from us. In spite, however, of their great and varying distances, science has been able to get a mental bird's-eye view of all the hosts of stars which the heavens reveal to our eyes, as they would appear to us if we could plant ourselves far on the other side of the most distant one. The telescope — an instrument which will be fully described further on— has, in fact, taught us that all the stars which we see, form, after all, but a cluster of islands as it were in an infinite ocean of space ; so that we may think of all the stars which we see, as forming our universe, and when we have got that thought well into our minds we may think of space being peopled with other universes, as there are other towns besides London in England.
9. Further, we know that our sun is one of the stars which compose this star-cluster, and that the reason that it appears so much bigger and brighter than the other stars is simply because it is the nearest star to us.
We all of us know how small a distant house looks or how feebly a distant gas-lamp or candle seems to shine ; but the distant house may be larger than the one we live in, or the distant light may really be brighter than the one which, being nearer to us, renders the other insig- nificant. It is precisely so with the stars. Not only would they appear to us as bright as the sun, if we were as near to them, but we know for a fact that some of them are larger and brighter.
10. Now, why do the stars and the sun shine? They shine, or give out light, because they are •white hot. At their surfaces masses of metals and other substances are burning together more fiercely than anything we can imagine. They are globes of the fiercest
B 2
4 ASTRONOMY.
fire, compared to which a mass of white-hot iron is as cold as ice,
11. What, then, are the planets? We may first state that they are comparatively small bodies travelling round our sun at various distances from him. Our earth is one of them. A moment's thought on what we have said about the sun and stars will, however, show us another important difference between the planets and the sun. We have seen that the sun is a white-hot body ; our earth, we know, is, on the contrary, a cold one : all the heat we get is from the sun. Because the earth is cold it cannot give out light any more than a cold poker can. Astronomers have learnt that all the other planets are like the earth in this respect. They are all dark bodies — having no light in themselves — and they all, like us, get their light and heat from the sun. When, therefore, we see a planet in the sky. we know that its light is sunshine second-hand ; that, as far as its light is concerned, it is but a looking-glass reflecting to us the light of the sun.
We have now got thus far: planets are dark, or obscure, or non-self-luminous bodies travel- ling round the sun, which is a bright body — bright because it is white hot; and the sun is a star, one of the stars which together form our universe ; the reason that it appears larger and brighter than the other stars being because we are nearer to it than we are to the others. It seems likely that the other stars have planets revolving round them, although astronomers have as yet no positive knowledge on the matter, as they are so very far away that the telescopes we possess at present are not powerful enough to show us their planets, if they have any.
12. We now come to the moon. What is it ? The moon goes round the earth in the same way as we have seen the planets revolving round the sun : it is in fact
INTRODUCTION. 5
a planet of the earth ; it is to the earth what the earth is to the sun. Like the earth and planets, it is a dark body, and this is the reason it does not always appear round as the sun does. We only see that part of it that is lit up by the sun. In the moon we have a speci- men of a third order of bodies called satellites, or com- panions, as they are the companions of the planets, accompanying them in their courses round the sun.
We have then to sum up again — (i) the sun, a star, like all the other stars in motion, (2) the planets revolving round the sun, and (3) satel- lites revolving round planets.
13. The nebulce and comets are quite distinct from stars and planets, for they are masses of gas. The nebulae lie far away from us, some of them perhaps out of our universe altogether ; the comets rush for the most part from distant regions to our sun, and having gone round him they go back again, and we only see them for a small part of their journey.
We saw in Art. 10 that the stars shone because they are white hot ; so also nebulae and comets shine because they are white hot.; but in the case of the stars we are dealing with solid or liquid matter, while in the case of the nebulae and comets we are dealing with burning gas.
14-. Such, then, are some of the bodies with which the science of ASTRONOMY has to deal ; but astronomers have not rested content with the appearances of these bodies : they have measured and weighed them in order to assign to them their true place. Thus they have found out that the sun is 1,260,000 times larger than the earth, and the earth is 50 times larger than the moon. On the other hand, as we have seen, they have discovered that, while we travel round the sun, the moon travels round us, and at a distance which is quite insignificant in comparison
6 ASTRONOMY.
In other words, the moon travels round us at a distance of 240,000 miles, while we travel round the sun at a distance of 91,000,000 miles.
15. We thus see how it is that the greater size of the sun is balanced, so to speak, by its greater distance ; the result being that the large distant sun looks about the same size as the small near moon.
16. We already see how enormous are the distances dealt with in astronomy, although they are measured in the same way as a land-surveyor measures the breadth of a river that he cannot cross. The numbers we obtain when we attempt to measure any distance beyond our own little planetary system convey no impression to the mind. Thus the nearest fixed star is more than 19,000,000,000,000 miles away, the more distant ones so far away that light, which travels at the rate of 186,000 miles in a second of time, requires 50,000 years to dart from the stars to our eyes !
17. In spite, however, of this immensity, the methods employed by astronomers are so sure that, in the case of the nearer bodies, their distances, sizes, weights, and motions are now well known. We can indeed predict the place that the moon — the most difficult one to deal with — will occupy ten years hence, with more accuracy than we can observe its position in the telescope.
18. Here we see the utility of the science, and how upon one branch of it, PHYSICAL ASTRONOMY, which deals with the laws of motion and the structure of the heavenly bodies, is founded another branch, PRACTICAL ASTRONOMY, which teaches us how their movements may be made to help mankind.
19. Let us first see what it does for our sailors and travellers. A ship that leaves our shore for a voyage round the world takes with it a book called the " Nautical Almanack," prepared beforehand— three or four years in
INTRODUCTION. 7
advance — by our Government astronomers. In this book the places the moon, sun, stars, and planets will occupy at certain stated hours for each day are given, and this information is all our sailors and travellers require to find their way across the pathless seas or unknown lands.
20. But we need not go on board ship or into new countries to find out the practical uses of Astronomy. It is Astronomy which teaches us to measure the flow of time, the length of the day, and the length of the year: without Astronomy to regulate them, clocks and watches would be almost impossible, and quite useless. It is Astronomy which divides the year into seasons for us, and teaches us the times of the rising and setting of the moon, which lights up our night. It is to Astronomy that we must appeal when we would inquire into the early history of our planet, or when we wish to map its surface.
21. Such, then, is Astronomy— the science which, as its name, derived from two Greek words (ao-n'jp, " star," and i/o/xof, " law ") implies, unfolds to us the laws of the stars.
CHAPTER I.
THE STARS AND NEBULA.
LESSON I.— MAGNITUDES AND DISTANCES OF THE STARS. SHAPE OF OUR UNIVERSE.
22. THE first thing which strikes us when we look at the stars is, that they vary very much in brightness. All of those visible to the naked eye are divided into six classes of brightness, called "magnitudes," so that we speak of a very brilliant one as "a star of the first magnitude:" of the feeblest visible, as a star of the sixth magnitude, and so on. The number of stars of all magnitudes visible to the naked eye is about 6,000 ; so that the greatest number visible at any one time — as we can only see one half of the sky at once — is 3,000. If we employ a small telescope this number is largely increased, as that instrument enables us to see stars too feeble to be perceived by the eye alone. For this reason such stars are called telescopic stars. The stars thus revealed to us still vary in brightness, and the classifi- cation into magnitudes is continued down to the I2th, I4th, i6th, or even higher magnitudes, according to the power of the telescope ; in powerful telescopes at least 20,000,000 stars down to the I4th magnitude are visible.
ro ASTRONOMY.
23. A star of the sixth magnitude is, as we have seen, the faintest visible to the naked eye. It has been esti- mated that the other stars are brighter than one of the sixth magnitude, by the number of times shown in the following table : —
Times.
A star of the 5th magnitude 2
„ 4th „ 6
„ 3d „ 12
„ 2d „ 25
„ ist „ 100
Sirius, the brightest of the )
ist magnitude stars . . ) 324
The Sun, the nearest star ^ ^
to us )
24. Now it is evident that these stars, as they all shine out with such different lights, one star differing from another star in glory, are either of the same size at very different distances, the furthest away being of course the faintest ; or are of different sizes at the same distance, the biggest shining the brightest ; or are of different sizes at different distances. Where the actual distances of the stars are known we can be certain ; but from other con- siderations it is most probable that the difference in brilliancy is due to difference of distance, and not to size.
25. The distances of the stars from us are so great that it scarcely conveys any impression on the mind to state them in miles ; some other method, therefore, must be used, and the velocity of light affords us a convenient one. Light travels at the rate of 186,000 miles in a second of time — that is to say, between the beats of the pendulum of an ordinary clock, light travels a distance equal to eight times round the earth.
THE STARS AND NEBULA. ir
26. In spite, however, of this great remoteness, the distances of some of them are known with considerable accuracy. Thus, leaving the sun out of the question, we find that the next nearest is situated at a distance which light requires three and a half years to traverse.
27. From the measurements already made, we may say that, on the average, light requires fifteen and a half years to reach us from a star of the first magnitude, twenty-eight years from a star of the second, forty-three years from a star of the third, and so on, until, for stars of the 1 2th magnitude, the time required is 3,500 years.
28. Winding among the stars, a beautiful belt of pale light spans the sky, and sometimes it is so situated, that we see that it divides the heavens into two nearly equal portions. This belt is the Milky Way; and the smallest telescope shows that it is composed of stars so faint, and apparently so near together, that the eye can only per- ceive a dim continuous glimmer.
29. We find the largest stars scattered very irregularly, but if we look at the smaller ones, we find that they gradually increase in number as their position approaches the portion of the sky occupied by the Milky Way. In fact, of the 20,000,000 stars visible, as we have stated, in powerful telescopes, at least 18,000,000 lie in and near the Milky Way. This fact must be well borne in mind.
30. Adding this fact to what has been said about the distances of the stars, we can now determine the shape of our universe. It is clear that it is most extended where the faintest stars are visible, and where they appear nearest together; because they appear faint in conse- quence of their distance, and because their close packing does not arise from their actual nearness to each other, but results from their lying in that direction at constantly increasing distances. Indeed, the stars which give rise to
12 ASTRONOMY.
the appearance of the Milky Way, because in that part of the heavens they lie behind each other to an almost infinite distance, are probably as far from each other as our sun is from the nearest star.
31. The Milky Way, then, indicates to us, and traces for us, the direction in which our universe has its largest dimensions; the absence of faint stars in the parts of the sky furthest from the Milky Way shows us that the limits of the universe in that direction are much sooner reached than in the direction of the Milky Way itself. We gather, therefore, that its thickness is small compared with its length and breadth. This flat stratum of stars is split, as we might split a round piece of thick cardboard, in those regions where we see the Milky Way divided into two branches, and here its edge is double. Our sun is situated near the point at which the mass of stars begins to divide itself into two portions ; and, as there are more stars on the south side of the Milky Way than there are on the north, we gather that our earth occupies a position somewhat to the north of the middle of its thickness.
32. But although the Milky Way thus enables us to get a rough idea of the shape of our universe, as we might get a rough idea of the shape of a wood from some point within it by seeing in which direction the trees appeared densest and thickest together, and in which direction it was most easy to pierce its limits, still what the telescope teaches us, and what we know of other similar universes, shows that its boundaries are most probably very irregular.
33. The MageUanic Clouds, called the Nubecula Major and Nubecula Minor, visible in the southern hemisphere, are two cloudy oval masses of light, and are very like portions of the Milky Way, but they are apparently unconnected with its general structure.
THE STARS AND NEBULA. 13
LESSON II. — THE CONSTELLATIONS. MOVEMENTS OF THE STARS. MOVEMENT OF OUR SUN.
34. We have in the last lesson considered our star- system as a whole ; we have discussed its dimensions, and given an idea of its shape. Before we proceed with a detailed examination of the stars of which it is composed, it will be convenient to state the groupings into which they have been arranged, and the way in which any particular star may be referred to.
35. The stars then, from the remotest antiquity, have been classified into groups called constellations, each constellation being fancifully named after some object which the arrangement of the stars composing it was thought to suggest.
36. The first classification is due to Ptolemy of Alexandria, who about the year 150 A.D., arranged the 1,022 stars observed by Hipparchus, the father of astro- nomy, at Rhodes, about one century before our era. His catalogue contains 48 constellations ; two were added by Tycho Brahe. and to these 50 (called the ancient] constellations have been added, in more modern times, 59, carrying the number up to 109.
37- The names of the ancient constellations and of the more important of the modern ones are as follow, begin- ning with those through which the sun passes in his annual round ; these are called the zodiacal constellations (very carefully to be distinguished, as we shall see further on (Art. 361), from the signs of the zodiac bearing the same name). In English and in rhyme these are as under :
" The Ram, the Bull, the Heavenly Twins, And next the Crab, the Lion shines, The Virgin and the Scales,
'4
ASTRONOMY.
The Scorpion, Archer, and He-goat, The Man that bears the watering-pot, And Fish with glittering tails."
And in Latin they run thus :
" Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces.
38. The constellations visible above the zodiacal con- stellations, called the northern constellations, are as follow :
Ursa Major.
Ursa Minor.
Draco.
Cepheus.
Bootes.
Corona Borealis.
Hercules.
Lyra.
Cygnus.
Cassiopea.
Perseus.
A uriga.
Serpentarius.
Serpens.
Sagitta.
Aquila.
Delphinus.
Equuleus.
Pegasus.
Andromeda.
Triangulum.
Cameleoparda Us.
Canes Venatici.
Vulpecula et Anser.
Cor Caroli.
The Great Bear (The Plough).
The Little Bear.
The Dragon.
Cepheus.
Bootes.
The Northern Crown.
Hercules.
The Lyre.
The Swan.
Cassiopea (The Lady's Chair).
Perseus.
The Waggoner.
The Serpent Bearer.
The Serpent.
The Arrow.
The Eagle.
The Dolphin.
The Little Horse.
The Winged Horse.
Andromeda.
The Triangle.
The Cameleopard.
The Hunting Dogs,
The Fox and the Goose.
Charles' Heart.
THE STARS AND NEBULAE.
39. The constellations visible below the zodiacal ones, called the southern constellations, are :
Get us.
Orion.
Eridanns.
Lepus.
Cants Major.
Can is Minor.
Argo Navis.
Hydra.
Crater.
Corvus.
Centaurus.
Lupus.
Ara.
Corona Austral is.
Piscis Australis.
Monoceros.
Columba Noachi.
Crux A ustralis.
The Whale.
Orion.
The River Eridanus.
The Hare.
The Great Dog.
The Little Dog.
The Ship Argo.
The Snake.
The Cup.
The Crow.
The Centaur.
The Wolf.
The Altar.
The Southern Crown.
The Southern Fish.
The Unicorn.
Noah's Dove.
The Southern Cross.
AO. The whole heavens, then, being portioned out into these constellations, the next thing to be done was to in- vent some method of referring to each particular star. The method finally adopted and now in use is to arrange all the stars in each constellation in the order of brightness, and to attach to them in that order the letters of the Greek alphabet, using after the letters the genitive of the Latin name of the constellation. Thus Alpha (a) Lyra denotes the brightest star in the Lyre ; a Ursa Minoris, the brightest star in the Little Bear. Some of the brightest stars are still called by the Arabian or other names they were known by in former times, thus, a Lyrce is known also as Vega, a Bootis as Arcturits, ft Orionis as Rigel, a Ursa Mtnoris as Polaris (the Pole star), &c.
16
ASTRONOMY.
4-1. All the constellations, and the positions of the prin- cipal stars, have been accurately laid down in Star-Maps and on Celestial Globes. With one or other of these the reader should at once make himself familiar. In star- maps the stars are laid down as we actually see them in the heavens, looking at them from the earth ; but in globes their positions are reversed, as the earth, on which the spectator is placed, is supposed to occupy the centre of the globe, while we really look at the globe from the outside. Consequently the positions of the stars are reversed. So if we suppose two stars, the brighter one of them to the right in the heavens, the brighter one will be shown to the right of the other on a star-map, but to the left of it on a globe.
4-2. The twenty brightest stars in the heavens, or first magnitude stars, are as follow: they are given in the order of brightness, and should be found on a map or globe.
|
Sirius, in the constellation |
Cam's Major. |
|
Can opus, „ |
Argo. |
|
Alpha, „ |
Centaur. |
|
Arcturus, „ |
Bootes. |
|
Rigel, |
Orion. |
|
Capella, „ |
Auriga. |
|
Vega, „ |
Lyra. |
|
Procyon, ,, |
Canis Minor. |
|
Betelgeuse, „ |
Orion. |
|
Achernar, „ |
Eridanus. |
|
Aldebaran, „ |
Taurus. |
|
Beta, Centauri, „ |
Centaur. |
|
Alpha. Crucis, „ |
Crux. |
|
An tares, ., |
Scorpio. |
|
Atair, „ |
Aquila. |
|
Spica, „ |
Virgo. |
THE STARS AND NEBULAE. J?
Fomalhaut, in the constellation Piscis Australis. Beta Crucis, „ Crux.
Pollux, „ Gemini.
Regulus, „ Leo.
43. Now, although the stars, and the various constel- lations, retain the same relative positions as they did in ancient times, all the stars are, nevertheless, in motion ; and in some of them nearest to us, this motion, called proper motion, is very apparent, and it has been measured. Thus Arcturus is travelling at the rate of at least fifty-four miles a second, or three times faster than our Earth travels round the sun, which is one hundred times faster than an ordinary railway train.
44. Nor is our Sun, which be it remembered is a star, an exception ; it is approaching the constellation Hercules at the rate of four miles in a second, carrying its system of planets, including our Earth, with it Here, then, we have an additional cause lor a gradual change in the positions of the stars, for a reason we shall readily understand, if. when we walk along a gas-lit street, we notice the distant lamps. We shall find that the lamps we leave behind close up, and those in front of us open out as we approach them : in fact, the stars which our system is approaching are slowly opening out, while those we are quitting are closing up, as our distance from them is Increasing.
45. The real motions of the stars, — called, as we have seen, their proper motions, — and the one we have just pointed out, however, are to be gathered only from the most careful observation, made with the most accurate instruments. There are apparent motions, which may be detected in half an hour by the most careless observer.
46. These apparent motions are caused, as we
C
1 8 ASTRONOMY.
shall fully explain by and by (Chap. IV.), by the two real motions of the Earth, first round its own axis, and secondly round the Sun.
LESSON III.— DOUBLE AND MULTIPLE STARS. VARIABLE STARS.
47- A careful examination of the stars with powerful telescopes, reveals to us the most startling and beautiful
appearances. Stars which ap- pear single to the unassisted eye, appear double, triple, and quadruple, and in some in- stances the number of stars revolving round a centre com- mon to all is even greater. Be- cause our Sun is an isolated star, and because the planets
. -Orbit of a Double Star. ^ nQW dark bodies> mstead
of shining, like the Sun, by their own light, as they once must have done, it is difficult, at first, to realize such phenomena, but they are among the most firmly-estab- lished facts of modern astronomy. A beautiful star in the constellation of the Lyra will at once give an idea of such a system, and of the use of the telescope in these inquiries. The star in question is (e) Lyras, and to the naked eye appears as a faint single star. A small telescope, or opera-glass even, suffices to show it double, and a power- ful instrument reveals the fact that each star composing this double is itself double ; hence it is known as " the Double-double." Here, then, we have a system of four suns, each pair, considered by itself, revolving round
THE STAXS AND NEBULAE.
Fit?. 2. --The Double-double Star in the constellation Lyra. i. As seen in an opera-glass. 2. As seen in a small telescope. 3. As seen in a telescope of great power.
a point situated between them; while the two pairs considered as two single stars, perform a much larger journey round a point situ- ated between them.
It may be stated roundly that the wider pair will com- plete a revolution in 2,000 years ; the closer one in half that time ; and possibly both double systems may revolve round the point lying be- tween them in something less than a million of years.*
4-8. More than 6,000 double stars are now known, and of these motion has already been detected in nearly 700, the motion in some cases being very rapid. In some cases the brilliancy of the component stars is nearly equal, but in others the light is very unequal. For instance, a first magnitude star may have a companion of the four- teenth magnitude. Sirius has, at least, one such com- panion. Here is a-list of some double stars, showing the time in which a complete revolution is effected :
Years.
Zeta (f) Her cults ;'" 36
Eta (77) Cor once Boreal is ... 43
Zeta (£) Cancri 60
Alpha (a) Centauri 75
Omega (a>) Leonis .......... ,r . 82
Gamma (7) Corona Boreal is 100
Delta (5) Cygni 1 78
Beta (/3) Cygni 500
Gamma (y) Leonis 1 200
* Admiral Sniyih.
C 2
20 ASTRONOMY.
49. Here, then, there can be no doubt that the stars are connected, and such pairs are called physical couples, to distinguish them from the optical couples, in which the component stars are really distant from each other, and have no real connexion ; their apparent nearness to each other being an appearance caused by their lying in the same straight line, as seen from the Earth.
50. Where the distance of a physical double star is known, we can determine the dimensions of the orbit of one star round the other, as we can determine the Earth's orbit round the Sun. Thus we know that the distance between the two stars of 61 Cygni is 4,275,000,000 miles, and yet the two stars seem as one to the naked eye.
51. The stars are not only of different magnitudes (Art. 22), but the brilliancy of some particular stars changes from time to time. If the variation in the light is, as it is generally, slow, regular, and within certain limits, stars in which this is noticed are called variable stars, or shortly, variables. In some cases, however, the increase and decrease have been sudden, and in others the limits of change have been unknown ; and hence we read of new stars, lost stars, and temporary stars, in addition to the more regular variables. There is little doubt, however, that all these phenomena are the same in kind, though different in degree.
52. The variation is, of course, determined by the different magnitudes of the stars at different times, and the amount of variability is measured by the extreme magnitudes. The period of the variability is the time that elapses between two successive greatest bright- nesses.
53. We give a table of a few variable stars, in order that the foregoing may be clearly understood :
THE STARS AND NEBULA 21
Change of Magnitude Period of
from to Change.
17 Argus ... I .... 4 ... 46 years.
R Cephei ... 5 .... n ... 73 years.
~ . , x- (lower than ) *f A^
R CasstopecE . . 6 . > . 435 days-
i or 2 . jlo^r^than
j.33,
$ Cancri ... 8 .... 10^. . . 10
2i . 4 2
54-. The fourth star on our list is a very interesting one, for at its period of greatest brightness it sometimes reaches the first magnitude, sometimes the second ; but among the acknowledged variables )3 Persei is perhaps the most interesting, as its period is so short, and, unlike o Ceti— (called also Mira, or the Marvellous) — it is never invisible to the naked eye. The star in question shines as a star of the second magnitude for two days and thirteen hours and a half, and then suddenly loses its light and in three hours and a half falls to the fourth magnitude ; its brilliancy then increases again, and in another period of three hours and a half it reattains its greatest bright- ness— all the changes being accomplished in less than three days.
55. Among the new, or temporary stars, those ob- served in 1572 and last year (1866) are the most notice- able. The first appeared suddenly in the sky and was visible for seventeen months ; its light at first was equal to that of the planets at their greatest brilliancy ; so bright was it indeed, that it was clearly visible at noonday. Now it is not a little curious that in the years 945 and 1264 something similar was observed in the same region of the sky (in Cassiopea) in which this star appeared. If then
22 ASTRONOMY.
we assume all these phenomena to be due to the fact that we have here a long-period variable star which is very bright at its maximum and fades out of view at its mini- mum, we may expect a reappearance of the star in the year 1885.
56. We now come to the new star which broke upon our sight last year, in the constellation of Corona Bo- r?alis, and which was observed with much minuteness ami with powerful methods of research not employed before. This star was recorded some years ago as one of the ninth magnitude. In May, however, it suddenly flashed up, and on the I2th of that month shone as a star of the second magnitude. On the 14th it had descended to the third magnitude, — the decrease of brightness was for some time at the rate of about half a magnitude a day, and towards the end of May it was less rapid. There is good reason to believe that this increased brilliancy was due to the sudden ignition of hydrogen gas in the star's atmo- sphere. Here we have a fact which must prove of the highest importance, although we are not yet in a position to do much more than speculate upon it.
57. The question of variable stars is one of the most puzzling in the whole domain of astronomy. Mr. Balfour Stewart, from his researches on the Sun — which is doubt- less a variable star — thinks that " we are entitled to conclude that, in our own system, the approach of a planet to the Sun is favourable to increased brightness, and especially in that portion of the Sun which is next the planet." In the case of variable stars, the hypothesis which was formerly thought to give the best explanation of the phenomenon is that which assumes rotation on an axis, while it is supposed that the body of the star is not equally luminous on every part of its surface
58. If, instead of this, we suppose such a star to have a large planet revolving round it at a small distance, then,
THE STARS AND NEBULA. 23
according to Mr. Stewart's theory, that portion of the star which is near the planet will be more luminous than that which is more remote ; and this state of things will revolve round as the planet itself revolves, presenting to a distant spectator an appearance of variation, with a period equal to that of the planet.
59. If we suppose the planet to have a veiy elliptical orbit, then for a long period of time it will be at a great distance from its primary, while, for a comparatively short period, it will be very near. We should, therefore, expect a long period of darkness, and a comparatively short one of intense light — precisely what we have in temporary stars.
LESSON IV.— COLOURED STARS. APPARENT SIZE. THE STRUCTURE OF THE STARS. CLUSTERS OF STARS.
6O. The stars shine out with variously coloured lights; thus we have scarlet stars, red stars, blue and green stars, and indeed stars so diversified in hue that ob- servers attempt in vain to define them, so completely do they shade into one another. Of large stars oi different colours we may give the following table, founded on Mr. Ennis's observations : —
Red Stars . . . Aldebaran. Antares. Betelgeuse. Blue Stars . . . Capella. Rigel. Bellatrix. Pro-
cyon. Spica.
Green Stars . . Sirius. Vega. A fair. Deneb. Yellow Stars . . Arcturus. White Stars . . Regulus. Denebola. Fomalhaut.
Polaris.
24 ASTRONOMY.
61. In the double and multiple stars, however, we meet with the most striking colours and contrasts ; Iota (i) Cancri, and Gamma (y) Andromeda;, may be instanced. In Eta (77) Cassiopece we find a large white star with a rich ruddy purple companion. Some stars occur of a red colour, almost as deep as that of blood. What wondrous colouring must be met with in the planets lit up by these glorious suns, especially in those belonging to the compound systems, one sun setting, say in clearest green, another rising in purple or yellow or crimson ; at times two suns at once mingling their variously coloured beams ! A remarkable group in the Southern Cross pro- duced on Sir John Herschel "the effect of a superb piece of fancy jewellery." It is composed of over 100 stars, seven of which only exceed the tenth magnitude ; among these, two are red, two green, three pale green, and one greenish blue.
62. The colours of the stars also change. If we go back to the times of the ancients, we read that Sirius, which is now green, was red ; that Capella, which is now pale blue, was also red.
63. In some variable stars the changes of colours observed are very striking. In the new star of 1572, Tycho Brahe observed changes from white to yellow, and then to red ; and we may add that generally when the brightness decreases the star becomes redder.
64-. The size or diameter of the stars cannot be deter- mined by our most powerful instruments ; but we know that, as seen from the Earth, they are, in consequence of their distance, mere points of light, so small as to be be- yond all our most delicate measurement. The Moon, which travels very slowly across the sky, sometimes (as we shall see by and by) gets before, or eclipses, or occults, some of them ; but they vanish in a moment —which they would not do if they were not as small as we have stated.
THE STARS AND NEBULA. 25
65. We will now pass on to what is known of the physical constitution of the stars. In the first place we know that the stars, of whatever their interiors may be composed, present to us on their exteriors a bright surface, which is called the photosphere; outside this photo- sphere, as outside the surface of our earth, is an atmo- sphere composed of vapours. The materials of the photospheres are at an intense heat : so hot are they, that, although they consist of metals and other substances, they exist in a liquid or vaporous state — these states being the effect of heat.
66. We can render this intelligible by taking water and iron as instances : when both are in a solid state we get ice and hard iron ; if we apply heat we melt both ice and hard iron into water and molten iron — we know that it requires more heat to melt iron than it does to melt ice. Having got both into a liquid state, additional heat will turn the water into steam and the molten iron into iron- vapour ; but again the heat required to vaporize the iron is vastly greater than that required to turn the water into steam — how much greater may be gathered from the fact that while ice melts at o° of the centigrade thermometer, iron only melts at 2,000°: the heat required to produce iron-steam or vapour is not known.
67. The degree of heat therefore present in the photo- spheres of the stars exceeds our measurement. Do we know anything of the substances which throw out this heat and therefore light ? Yes, a little. For instance : —
Beta O) Pcgasi contains sodium, magnesium, and perhaps
barium. Sirius „ sodium, magnesium, iron, and
hydrogen.
Alpha (a) Lyra (Vega) sodium, magnesium, and iron. Pollux contains sodium, magnesium, and iron.
26 ASTRONOMY.
68. Now the vapours produced in the photospheres ascend to form the atmospheres, and these atmospheres absorb the light given out by the photospheres. A piece of coloured glass will teach us what absorption is. Thus a green glass is green because it absorbs all other light but the green ; it is a sort of sieve, which stops every ray of light except the green ones. So on with glasses, solids, vapours, or liquids of other colours. Now the colours of the stars may be influenced not only by the degree of heat of their photospheres, but in the amount of absorp- tion produced by their atmospheres. Our Sun at setting, for instance, seems sometimes blood red, in consequence of the absorption of our atmosphere ; if the absorption were in his own atmosphere, he would be blood red at noonday. Concerning the causes which produce the changes, both in colour and brightness, we must confess that, after all, we are yet ignorant.
69. It is remarkable that the elements most widely diffused among the stars, including hydrogen, sodium, magnesium, and iron, are some of those most closely connected with the living organisms of our globe.
We shall be able, when we come to examine the struc- ture of the nearest star— the Sun — to obtain a more detailed knowledge of the structure of the stars generally.
70. Having now dealt with the peculiarities of indi- vidual stars, — that is to say, their distance, arrangement, colour, variability, and structure, — we next come to the various assemblages or companies of stars observed in various parts of the heavens.
71. In the double and multiple systems (Art. 47) we saw the first beginnings of the tendency of the stars to group themselves together. In some parts of our system this tendency is exhibited in a very remarkable manner, the beautiful group of the Pleiades affording a familiar instance. The six or seven stars visible to the naked eye
Plate I.
STAR-CLUSTERS. i. The Cluster in Hercules. 2. The Crab Cluster.
THE STARS AND NEBULA. 29
become 60 or 70 when viewed in the telescope. The Hyades, in the constellation Taurus, and the Prsesepe, or " Beehive," in Cancer, may also be mentioned. In other cases the groups consist of an innumerable number of suns apparently closely packed together. That in the constellation Perseus is among the most beautiful objects in the heavens ; but many others, scarcely less stupen- dous, though much fainter by reason of the greater distance, are revealed by the telescope.
72. Assemblages of stars are divided into—
1. Irregular group*, generally more or less visible
to the naked eye.
2. Star-clusters, invisible to the naked eye, but
which, in the most powerful telescopes, are seen to consist of separate stars. These are sub-divided into ordinary clusters, and globular clusters.
73. Clusters and nebulae are designated by their num- ber in the catalogues which have been made of them by different astronomers. The most important of these catalogues have been made by Messier, Sir William Herschel, and Sir John Herschel. A catalogue recently published by the latter contains 5,079 objects.
74-. We have already given some examples of star- groups. The magnificent star-clusters, in the constel- lations Hercules, Libra, and Aquarius, may be instanced as among those which are best seen in moderate telescopes ; but some of the clusters which lie out of our universe, and which we must regard as other universes, are at such immeasurable distances, and are therefore so faint, that in the most powerful telescopes the real shape and boun-
30 AXTKONOMY.
clary are not seen, and there is a gradual fading away at the edge, the last traces of which appear either as a luminous mist, or cloud-like filament, which become finer till they cease altogether to be seen. The Dumb-Bell cluster, in Vulpecula, and the Crab cluster, in Taurus, both of which have been resolved into Stars, are instances of this.
75. In some of these star-clusters the increase of bright- ness from the edge to the centre is so rapid that it would appear that the stars are actually nearer together at the centre than they are near the edge of the cluster ; in fact, that there is a real condensation towards the centre.
LLSSON V. — NEBUL/E. CLASSIFICATION AND DESCRIPTION.'
76. We now come to the Nebulae. " Nebula " is a Latin word signifying a cloud, and for this reason the name has been given to everything which appeared cloud- like to the naked eye or in a telescope. The group in Perseus, for instance, appears like a nebula to the naked eye ; in the smallest telescope, however, it is separated into stars.
77. Every time a telescope larger than any formerly used has been made use of, however, numbers of what were till then called nebulae, and about which as nebulas nothing was known, have been found to be nothing but star-clusters, some of them of very remarkable forms, so distant that even in telescopes of great power they could not be resolved,— that is to say, could not be separated into distinct stars.
Plate ft.
i. The Nebula in Orion
NEBUL/E.
2. Spiral Nebula in Canes Venatici.
THE STARS AND NEBULAE. 33
78. Now, this is what has happened ever since the dis- covery of telescopes. Hence it was thought by some that all the so-called nebulae were, in reality, nothing but distant star-clusters.
79. One of the most important discoveries of modern times, however, has furnished evidence of a fact long ago conjectured by some astronomers, — namely, that some of the nebulae are something different from masses of stars, and that the cloud-like appearance is due to something else besides their distance and the still comparatively small optical means one can at present bring to bear upon them.
80. This discovery is so recent that there has not yet been time to sort out the real nebulae from those which, by reason of their great distance, appear like nebulae. We are compelled, therefore, in this book to accept as nebulae all formerly classed as such which up to this time have not been resolved into stars.
81. Nebulae, then, may be divided into the following classes: —
1.— Irregular netulse.
2.— Ring- nebulae and Elliptical nebulse.
3.— Spiral, or Whirlpool nebulae.
4-.— Planetary nebulae.
5. Nebulae surrounding stars.
82. Some of the irregular nebulae — those in the constellations Orion and Andromeda, for example — are visible to the naked eye on a dark night.
83- The great nebula of Orion is situated in the part of the constellation occupied by the sword-handle and sur- rounding the multiple-star Theta (6). The nebulosity near the stars is fiocculent, and of a greenish white tinge. There seems no doubt that the shape of this nebula and the position of its brightest portions are changing. One part of it appears, in a powerful telescope, startlingly like
D
34 ASTRONOMY.
the head of a fish. On this account it has been termed the Fish-mouth nebula.
84-, Two other fine irregular nebulae are visible in the Southern hemisphere : one is in the constellation Dorado, the other surrounds Eta (rj) Argus. The latter occupies a space equal to about five times the apparent area of the Moon.
85. We have classed the ring-nebulae and elliptical nebulae together because probably the latter are, in several instances, ring-nebulae looked at sideways. The finest ring-nebula is the 57th in Messier's catalogue (written 57 M. for short). It is in the constellation Lyra. The finest elliptical nebula is the one in Andromeda to which we have before referred. This nebula, the 3ist of Messier's catalogue (31 M.), when viewed in large instruments, shows several curious black streaks running in the direc- tion in which the npbula is longest.
86. The spiral or whirlpool nebulae are repre- sented by that in the constellation of Canes Venatici (51 M.). In an ordinary telescope this presents the ap- pearance of two globular clusters, one of them surrounded by a ring at a considerable distance, the ring varying in brightness, and being divided into two in a part of its length. But in a larger instrument the appearance is en- tirely changed. The ring turn. s. into a spiral coil of nebulous matter, and the outlying mass is seen connected with the main mass by a curved band. 33 M. Piscium, and 99 M. Virginis, are other examples of this strange phenomenon, which indicate to us the action of stupendous forces of a kind unknown in our own universe,
87. The fourth class, or planetary nebulae, were so named by Sir John Herschel, as they shine with a planetary and often bluish light, and are circular or slightly elliptical in form. 97 M. Ursae Majoris and 46 M. Argus may be taken as specimens.
THE STARS AND NEBULA. 35
88. We come lastly to the nebuhe surrounding stars, or nebulous stars. The stars thus surrounded are apparently like all other stars, save in the fact of the presence of the appendage ; nor does the nebulosity give any signs of being resolvable with our present telescopes. Iota (i) Orionis, Epsilon (*) Orionis, 8 Canum Venati- corum, and 79 M. Ursae Majoris, belong to this class.
LESSON VI. — NEBULAE (continued}. THEIR FAINT- NESS. VARIABLE NEBULAE. DISTRIBUTION IN SPACE. THEIR STRUCTURE. NEBULAR HYPOTHESIS.
89. Having stated and described the several classes into which nebulae may be divided, their general features and structure have next to be considered.
90. Like the stars, they are of different brightnesses, but as yet they have not been divided into magnitudes. This, however, has been done in a manner by determining what is termed the space-penetrating power or light-g rasping power of the telescope powerful enough to render them visible. Thus, supposing nebulae to con- sist of masses of stars, it has been estimated that Lord Rosse's great Reflector, the most powerful instrument as yet used in such inquiries, penetrates 500 times further into space than the naked eye can ; that is, can detect a nebula or cluster 500 times further off than a star of the sixth magnitude.
91. Now, if we suppose that a sixth magnitude star is 12 times further off than a star of the first magnitude — and this is within the mark — and that, as we have seen in Art. 27, light requires 1 20 years to reach us from such a star, the telescope we have referred to penetrates so pro- foundly into space that no star can escape its scrutiny,
D 2
36 ASTRONOMY.
" unless at n remoteness that would occupy light in over- spanning it sixty thousand years."
92. An idoa of the extreme faintness of the more dis- tant nebulas may be gathered from the fact, that the light of some of those visible in a moderately-large instrument has been estimated to vary from y^ to siroo~o °f ^e light of a single sperm candle consuming 158 grains of material per hour, viewed at the distance of a quarter of a mile: that is, such a candle a quarter of a mile off is 20,000 times more brilliant than the nebula !
93. The phenomenon of variable, lost, new, and tempo- rary stars has its equivalent in the case of the nebulae, the light of which, it has been lately discovered, is in some cases subject to great variations.
94-. In 1 86 1 it was found that a small nebula, dis- covered in 1856 in Taurus, near a star of the tenth magni- tude, had disappeared, the star also becoming dimmer. In the next year the nebula increased in brightness again. The compressed nebula 80 M. in May 1860, appeared as a star of the seventh magnitude. During the next month it recovered its nebulous appearance.
95. In Art. 30 the marked character of the distribution of the stars of our universe, giving rise to the appearance of the Milky Way, was pointed out. The distribution of the nebulae, however, is very different ; in general they lie out of the Milky Way, so that they are either less con- densed there, or the visible universe (as distinguished from our own stellar one) is less extended in that direction. They are most numerous in a zone which crosses the Milky Way at right angles, the constellation Virgo being so rich in them that a portion of it is termed the nebulous region of Virgo. In fact, not only is the Milky Way the poorest in nebulae, but the parts of the heavens furthest away from it are richest.
THE STARS AND NEBULA. 37
96. We now come to the question, What is a Nebula ? Theanswer is— A true nebula is a mass of glowing or incandescent gas, and there are indications that the gases in question are nitrogen and hydrogen. This fact, the fruit of the brilliant discovery which has been before alluded to (Art. 79), for ever sets at rest the ques- tion so long debated as to the existence or non-existence of a nebulous fluid in space.
97. When therefore we see, in what we know to be a true nebula, closely associated points of light, we must not regard the appearance as an indication of resolvability into true stars. These luminous points, in some nebulas at least, must be looked upon as themselves gaseous bodies, denser portions probably of the great nebulous mass. It has been suggested that the apparent perma- nence of general form in a nebula is kept up by the continual motions of these denser portions.
98. The nebular hypothesis, given to the world before the existence of a nebulous fluid was proved, supposes that all the countless suns which are distributed through space once existed in the condition of nebulous matter. It may take long years to prove, or disprove, this hypothesis ; but it is certain that the tendency of recent observations is to show its correctness.
CHAPTER II.
THE SUN.
LESSON VII. — ITS RELATIVE BRIGHTNESS, ITS SIZE, DISTANCE, AND WEIGHT.
99. WE will now consider the star nearest to us — the Sun, which dazzles the whole family of planets by its brightness, supports their inhabitants by its heat, and keeps them in bounds by its weight.
100. The relative brilliancy of the centre of our system, compared to that of the stars, is, as we saw in Art. 23, so great that it is difficult at first to look upon it as in any way related to those feeble twinklers. This difficulty, however, is soon dispelled when we consider how near it is to us. Thus, to give another instance, though we receive 10,000,000,000 times more light from the Sun than we do from Alpha (a) Lyra, that star is more than a million times further from us. There is reason to believe, indeed, that our Sun is, after all, by no means a large star compared with others ; for if we assume that the light given out by Sirius, for instance, is no more brilliant than is our sunshine, that star would be equal in bulk to more than 3,000 suns.
101. Astronomers now know the distance of the Sun from the Earth. It is about 91,000,000 miles ; and it is
Plate IE
p
m
a. < I
O
o a
o
THE SUN. 39
easy, therefore, as we shall see by and by (Chap. VIII.), to determine its size ; and here again, as in the case of the distances of the stars, we arrive at figures which convey scarcely any ideas to the mind. The distance from one side of the Sun to the other, through its centre — or, in other words, the diameter of the Sun, — is 853,380 miles. Were there a railway round our earth, a train, going at the rate of 30 miles an hour, would accomplish the journey in a month • a railway journey round the Sun, going at the same rate, would require more than nine years. In this way we may also obtain the best idea of the Sun's distance from us — a distance travelled over by light in eight and a half minutes. A train going at the speed we have named, and starting on the 1st of January, 1867, would not arrive at the Sun till about the middle of the year 2,205 •
102. Such then are the distance and size of the centre of our system. If we represent the Sun by a globe about two feet in diameter, a pea at the distance of 430 feet will represent the Earth ; and let us add, the nearest fixed star would be represented by a similar globe placed at the distance of 9,000 miles.
103. More than 1,200,000 Earths would be required to make one Sun. Astronomers express this by saying that the volume of the Sun is 1,200,000 times greater than that of the Earth ; but as the matter of which the Sun is composed weighs only one quarter as much, bulk for bulk, as do the materials of which the Earth is made up, taken together, 300,000 Earths only would be required in one scale of a balance to weigh down the Sun in the other. That is, the mass, or weight of the Sun, is 300,000 times greater than that of our Earth.
104. The Sun, like the Earth or a top when spinning, turns round, or rotates, on an axis; this rotation was discovered by observing the spots on its surface, about which we shall have much to say in the next Lesson. It
40 ASTRONOMY.
is found that the spots always make their first appear- ance on the same side of the Sun ; that they travel across it in about fourteen days ; and that they then disappear on the other side. This is not all : if they be observed in June, they go straight across the sun's face or disc with a dip downwards ; if in September, they then cross in a curve ; while in December they go straight across again, with a dip upwards r and in March their paths are again curved, this time with the curve in the opposite direction.
105. Now it is important that we make this perfectly clear. We know that the Earth goes round the Sun once a year. It has been found also that its path is so level — that is to say, the Earth in its journey does not go up or down, but always straight on — that we might almost imagine the Earth floating round the Sun on a boundless ocean, both Sun and Earth being half immersed in it. We shall see further on that this level — this plane— called the plane of the Ecliptic — is used by astronomers in precisely the same way as we commonly use the sea level. We say, for instance, that such a mountain is so high above the level of the sea. Astronomers say that such a star is so high above the plane of the ecliptic.
106. Well then, we have imagined the Earth and Sun to be floating in an ocean up to the middle — which is the meaning of half immersed. Now, if the Sun were quite upright, the spots would always seem at the same distance above the level of our ocean. But this we have not found to be the case. From two opposite points of the Earth's path (the points it occupies in June and December) the spots are seen to describe straight lines across the disc, while midway between these points (September and March) their paths are observed to be sharply curved, in one case with the convex side upwards, in the other with the con- vex side downwards. A moment's thought will show that
THE SUN. 41
these appearances can only arise from a dipping down of the Sun's axis of rotation. Now this we find to be the
September. December. March.
J''ig: 3. — Position of the Sun's axis, and apparent paths of the spots across the disc, as seen from the Earth at different times of the year. The arrows show the direction in which the Sun turns round.
case. The Sun's axis inclines towards the point occu- pied by the Earth in September. When we come to deal with the Earth and the other planets, we shall find that their axes also incline in different directions.
1O7. It has been found that the spots, besides having an apparent motion, caused by their being carried round by the Sun in its rotation, have a motion of their own. This proper motion, as distinguished from their apparent motion, has recently been investigated in the most com- plete manner by Mr. Carrington. What he has dis- covered shows that there need be no wonder that different observers have varied so greatly in the time they have assigned to the Sun's rotation. As we have already shown (Art. 104), this rotation has been deduced from the time taken by the spots to cross the disc ; but it now seems that all sun-spots have a movement of their own, and that the rapidity of this movement varies regularly with their distance from the solar equator,— that is, the region half-way between the two poles of rotation. In
42 ASTRONOMY.
fact, the spots near the equator travel faster than those away from it, so that if we take an equa- torial spot we shall say that the Sun rotates in about twenty-five days ; and if we take one situated halt-way between the equator and the poles, in either hemisphere, we shall say that it rotates in about twenty-eight days.
1O8. We have now considered the distance and size of the Sun ; we have found that it, like our Earth, rotates on its axis, and we have determined the direction in which the axis points. We must next try to learn something of its appearance and of its nature, or, as it is called, its physical constitution. Here we confess at once that our knowledge on this subject is not yet complete. This, however, is little to be wondered at. We have done so much, and gleaned so many facts, at distances the very statement of which is almost meaningless to us, so stupen- dous are they, that we forget that our mighty Sun, in spite of its brilliant shining and fostering heat, is still some 91,000,000 miles removed ;— that its diameter is 100 times that of our Earth ; and that the chasm we call a sun- spot is yet large enough to swallow us up, and half a dozen of our sister planets besides ; while, if we employ the finest telescope, we can only observe the various phenomena as we should do with the naked eye at a distance of 180,000 miles.
To look at the Sun through a telescope, without proper appliances, is a very dangerous affair. Several astro- nomers have lost their eyesight by so doing, and our readers should not use even the smallest telescope without proper guidance.
Plat*
SUN-SPOTS (the great Sun-Spot of 1865).
The spot entering the Sun's disc, Oct. 7th (foreshortened \iew). 2. Oct iDth. 3. Oct. i4th : central view, showing the formation of a bridge, and the nucleus. 4. Oct. i6th.
THE SUN. 45
LESSON VIII.— TELESCOPIC APPEARANCE OF THE SUN- SPOTS. PENUMBRA, UMBRA, NUCLEUS. FACUL/E. GRANULES. RED FLAMES.
1O9. We have already said that the first things which strike us on the Sun's surface, when we look at it with a powerful telescope, are the spots. In Plate IV. we give drawings of a very fine one, visible on the Sun in 1865. We shall often refer to them in the following description. The spots are not scattered all over the Sun's disc, but are generally limited to those parts of it a little above and below the Sun's equator, which is represented by the middle lines in Fig. 3. The arrows show the direction in which the spots, carried round by the Sun's rotation, appear to travel across the disc.
HO. The spots float, as it were, in what, as we have already seen in the case of the stars, is called the photo- sphere; the half-shade shown in the spot is called the penumbra (that is, half shade) ; inside the penumbra is a still darker shade, called the umbra, and inside this again is the nucleus. Diagrams 3 and 4 of Plate IV. will render this perfectly clear. The white surface repre- sents the photosphere; the half tones the penumbra; the dark, irregular central portions the umbra; and the blackest parts in the centre of these dark portions, the nucleus.
ill- Sun-spots are cavities, or hollows, eaten into the photosphere, and these different shades represent different depths.
112. Diligent observation of the umbra and penumbra, with powerful instruments, reveals to us the fact that change is going on incessantly in the region of the spots.
46 ASTRONOMY.
Sometimes changes are noticed, after the lapse of an hour even : here a portion of the penumbra is seen setting sail across the umbra ; here a portion of the umbra is melting from sight ; here, again, an evident change of position and direction in masses which retain their form. The enormous changes, extending over tens of thousands of square miles of the Sun's surface, which took place in the great sun-spot of 1865, are shown in Plate IV.
113. Near the edge of the solar disc, and especially about spots approaching the edge, it is quite easy, even with a small telescope, to discern certain very bright streaks of diversified form, quite distinct in outline, and either entirely separate or uniting in various ways into ridges and network. These appearances, which have been termed faculee, are the most brilliant parts of the Sun. Where, near the edge, the spots become invisible, undu- lated shining ridges still indicate their place — being more remarkable thereabout than elsewhere, though everywhere traceable in good observing weather. Faculae may be of all magnitudes, from hardly visible, softly-gleaming, narrow tracts 1,000 miles long, to continuous compli- cated and heapy ridges 40,000 miles and more in length, and 1,000 to 4,000 miles broad. Ridges of this kind often surround a spot, and hence appear the more con- spicuous ; such a ridge is shown in Fig i, Plate IV. ; but sometimes there appears a very broad white platform round the spot, and from this the white crumpled ridges pass in various directions.
114-. So much for the more salient phenomena of the Sun's surface, which we can study with our telescopes. There is much more, however, to be inquired into ; and here we may remark that the Sun himself has bestowed a great boon upon observational Astronomy ; and, whether brightly shining or hid in dim eclipse, now tells his own story, and prints his image on a retina which never forgets, and withal
THE SUN. 47
so docilely, that each day he is visible at the Kew Obser- vatory a young lady 'takes observations which surpass immeasurably in value those made by the hardest-headed astronomers of bygone times.
115. We may begin by saying, that the whole surface of the Sun, except those portions occupied by the spots, is coarsely mottled; and, indeed, the mottled appearance requires no very large amount of optical power to render it visible : in a large instrument, it is seen that the surface is principally made up of luminous masses— described by Sir William Herschel as corrugations. The luminous masses present to different observers almost every variety of irregular form : they have been stated to resemble "rice grains," "granules or granulations," and so on.
116. The word "willow-leaf " very well paints the appearance of the minute details sometimes observed in the penumbrae of spots,
which occasionally are made up apparently of elongated masses of unequal bright- ness, so arranged that for the most part they point like so many arrows to the centre of the nucleus, giving to the penumbra a radiated F*e* 4.— Part of a Sun-spot.
annparanrp Atnthprtim^ " Willow- leaves" detaching them-
ce. At ( es, se|ves from the penumbra. A very
and occasionally in the same faint one at F- spot, the jagged edge of the penumbra projecting over the nucleus has caused the interior edge of the penumbra to be likened to coarse thatching with straw.
117- There are darker or shaded portions between the granules, often pretty thickly covered with dark dots, like stippling with a soft lead-pencil ; these are what have been called "pores" by Sir John Herschel, and
48 ASTRONOMY.
"punc tu lat ions" by his father. Some of these arc almost black, and are like excessively small eruptive spots. 118. When the Sun is totally eclipsed,— that is, as will be explained by and by, when the Moon comes exactly between the Earth and the Sun, — other appsarances are unfolded to us, which the extreme brightness of the Sun prevents our observing under ordinary circumstances : the Sun's atmosphere is seen to contain red masses of fan- tastic shapes, some of them quite disconnected from the Sun ; to these the names of " red-flames " and " promi- nences " have been given. Now, as these bodies appear much brighter than the surrounding atmosphere, we con- clude that they are hotter than the atmosphere, as a bright fire is hotter than a dim one.
LESSON IX. — EXPLANATION OF THE APPEARANCES ON THE SUN'S SURFACE. THE SUN'S LIGHT AND HEAT. SUN-FORCE. THE PAST AND FUTURE OF THE SUN.
119. We are now familiar with the appearances presented to us on the Sun's surface in a powerful telescope. Let us see if we can account for them. As the spots break out and close up with great rapidity, as changes both on the large and small scale are always going on on the surface, we can only infer that the photosphere of the Sun, and therefore of the stars, is of a cloudy nature ; but while our clouds are made up of particles of water, the clouds on the Sun must be composed of particles of various metals and other substances in a state of intense heat — how hot we shall see by and by. The photosphere is surrounded by an atmosphere composed of the vapours of the bodies which are incandescent in the photosphere. It seems.
THE SUN. 49
also, that not only is the visible surface of the Sun en- tirely of a cloudy nature, but that the atmosphere is a highly-absorptive one. Thus when the clouds are highest they appear brightest — we see facula — because they extend high into that atmosphere, and consequently there is less atmosphere to obscure our view there than elsewhere. Spots may be due either to the absorption of a greater thickness of atmosphere, as they are hollows in the cloudy surface, or to the whole of the cloudy surface being cleared off in those parts from a something which emits less light than the clouds. The more minute features — the granules — are most probably the dome-like tops of the smaller masses of the clouds, bright for the same reason that the faculae are bright, but to a less degree ; and the fact that these granules lengthen out as they approach a spot and descend the slope of the pen- umbra, may possibly be accounted for by supposing them to be elongated by the current which causes their down- rush into a spot, as the clouds in our own sky are lengthened out when they are drawn into a current.
120. Some spots cover millions of square miles, and remain for months ; others are only visible in powerful instruments, and aie of very short duration. There is a great difference in the number of spots visible from time to time ; indeed, there is a minimum period, when none are seen for weeks together, and a maximum period, when more are seen than at any other time. The interval between two maximum periods, or two minimum periods, is about eleven years.
121. Now as we must get less light from the Sun when he is covered with spots than when there are none, we may look upon him as a variable star, with a period of eleven years. Mr. Balfour Stewart has shown recently that this period is in some way connected with the action of the planets on the photosphere. It is also known that the
E
50 ASTRONOMY.
magnetic needle has a period of the same length, its greatest oscillations occurring when there are most sun- spots. Auroras, and the currents of electricity which traverse the Earth's surface, are affected by a similar period.
122. Of the theories by which various astronomers have attempted to account for sun-spots we shall in this little book say nothing, as recent discoveries have shown that the old ones must be reconsidered, and those lately put forward are not yet sufficiently established.
123. We have before seen (Art. 67) what substances exist in a state of incandescence in some of the stars. In the case of the Sun we are acquainted with a greater number. Here is the list :—
Elements in the Sun.
Sodium. Zinc. Gold, probable.
Iron. Calcium. Cobalt, doubtful.
Magnesium. Chromium. Strontium, ditto.
Barium. Nickel. Cadmium, ditto.
Copper. Hydrogen, probable. Potassium, ditto.
The atmosphere of the Sun, like the atmosphere of the stars, consists of the vapours of these and of other — yet unknown — substances, and extends to a height exceeding 80,000 miles above the visible surface.
124. Now let us inquire into some of the benign in- fluences spread broadcast by the Sun. We all know that our Earth is lit up by its beams, and that we are warmed by its heat ; but this by no means exhausts its benefits, which we share in common with the other planets which gather round its hearth.
125. And first, as to its light. We have already com- pared its light with that which we receive from the stars, but that is merely its relative brightness; we want
THE SUN. 51
now to know its actual, or, as it is otherwise called, its intrinsic brightness. Now it is clear, at once, that no number of candles can rival this brightness ; let us therefore compare it with one of the brightest lights that we know of — the lime-light. The lime-light proceeds from a ball of lime made intensely hot by a flame composed of a mixture of hydrogen and oxygen playing on it. It is so bright, that we cannot look on it any more than we can look on the Sun ; but if we place it in front of the Sun, and look at both through a dark glass, the lime- light, though so intensely bright, looks like a black spot. In fact, Sir John Herschel has found that the Sun gives out as much light as 146 lime-lights would do if each ball of lime were as large as the Sun and gave out light from all parts of its surface.
126. Then, as to the Sun's heat. The heat thrown out from every square yard of the Sun's surface is as great as that which would be produced by burning six tons of coal on it each hour. Now, we may take the surface of the Sun roughly at 2,284,000,000,000 square miles, and there are 3,097,600 square yards in each square mile. How many tons of coal must be burnt, therefore, in an hour, to represent the Sun's heat ?
127. But the Sun sends out, or radiates, its light and heat in all directions; it is clear, therefore, that as our Earth is so small compared with the Sun, and is so far away from it, the light and heat the Earth can inter- cept is but a very small portion of the whole amount ; in fact, we only grasp the ^2TSTfW{F7itn Part of it. That is to say, if we suppose the Sun's light and heat to be divided into two hundred and twenty- seven million parts, we only receive one of them.
E 2
52 ASTRONOMY.
128. But this is not all. There is something else be- sides light and heat in the Sun's rays, and to this some- thing we owe the fact that the Earth is clad with verdure ; that in the tropics, where the Sun shines always in its might, vegetable life is most luxuriant, and that with us the spring time, when the Sun regains its power, is marked by a new birth of flowers. There comes from the Sun, besides its light and heat, another force, chemical force, which separates carbon from oxygen, and turns the gas which, were it to accumulate, would kill all men and animals, into the life of plants. Thus, then, does the Sun build up the vegetable world.
129. Now, let us think a little. The enormous engines which do the heavy work of the world ; the locomotives which take us so smoothly and rapidly across a whole continent; the mail-packets which take us so safely across the broad ocean ; owe all their power to steam, and steam is produced by heating water by coal. We all know that coal is the remains of an ancient vegetation ; we have just seen that vegetation is the direct effect of the Sun's action. Hence, without the Sun's action in former times we should have had no coal. The heavy work of the world, therefore, is indirectly done by the Sun.
130. Now for the light work. Let us take man. To work a man must eat. Does he eat beef? On what was the animal which supplied the beef fed ? On grass. Does he eat bread ? What is bread ? Corn. In both these, and in all cases, we come back to vegetation, which is, as we have already seen, the direct effect of the Sun's action. Here again, then, we must confess that to the Sun is due man's power of work. All the world's work, therefore, with one trifling exception (tide-work, of which more presently), is done by the Sun, and man himself, prince or peasant, is but a little engine, which directs merely the energy supplied by the Sun.
THE SUN. 53
131. Will the Sun, then, keep up for ever a supply of this force? It cannot, if it be not replenished, any more than a fire can be kept in unless we put on fuel; any more than a man can work without food. At present, philoso- phers are ignorant of any means by which it is replenished. As, probably, there was a time when the Sun existed as matter diffused through infinite space, the coming together of which matter has stored up its heat, so, probably, there will come a time when the Sun, with all its planets welded into its mass, will roll, a cold, black ball, through infinite space.*
132. Such, then, is our Sun — the nearest star.- Although some of the stars do not contain those elements which on the earth are most abundant — a Orionis and (3 Pegasi, for instance, are worlds without hydrogen — still we see that, on the whole, the stars differ from each other, and from our Sun, only by the lower order of differences of special modification, and not by the more important differences of distinct plans of structure. There is, there- fore, a probability that they fulfil an analogous purpose ; and are, like our Sun, surrounded with planets, which by their attraction they uphold, and by their radiation illu- minate and energize. As has been previously pointed out, the elements most widely diffused through the host of stars are some of those most closely connected with the constitution of the living organisms of our globe, including hydrogen, sodium, magnesium, and iron.
The probable past and future of the Sun are, therefore, the probable past and future of every star in the firmament of heaven.
* Sir W. Thomson
CHAPTER HI. THE SOLAR SYSTEM.
LESSON X.— GENERAL DESCRIPTION. DISTANCES OF THE PLANETS FROM THE SUN. SIZES OF THE PLANETS. THE SATELLITES. VOLUME, MASS, AND DENSITY OF THE PLANETS.
133. From the Sun we now pass to the system of bodies which revolve round it; and here1, as elsewhere in the heavens, we come upon the greatest variety. We find planets — of which the Earth is one — differing greatly in size, and situated at various distances from the Sun. We find again a ring of little planets clustering in one part of the system ; these are called asteroids, or minor planets : and we already know of at least two masses or rings of smaller planets still, some of them so small that they weigh but a few grains : these give rise to the appear- ances called meteors, bolides, or shooting-stars. We find also comets, some of which break in, as it were, upon us from all parts of space ; and then, passing round our Sun, rush back again : we find others so little erratic that they may be looked upon as members of the solar household. Besides these there is another ring which is rendered visible to us by the appearance called the Zodiacal Light.
THE SOLAR SYSTEM. 55
134-. The Solar System, then, consists of the follow- ing:—
Eight large Planets, as follow, in the order of distance from the Sun :—
1. Mercury. 5. Jupiter.
2. Venus. 6. Saturn.
3. EARTH. 7. Uranus.
4. Mars. 8. Neptune.
Ninety-seven small Planets revolving round the Sun Between the orbits of Mars and Jupiter. Their names are given in the Appendix.
Meteoric bodies, which at times approach near the larth's orbit, and occasionally reach the Earth's surface.
Comets.
The Zodiacal Light. A ring of apparently nebu- lous matter, the exact nature and position oi which in the system are not yet determined.
135. Let us begin by getting some general notions of this system. In the first place, all the planets travel round the Sun in the same direction, and that direction, looking down upon the system from the northern side of it, is from west to east, or, in other words, in the opposite direction to that in which the hands of a clock or a watch move. Secondly, the forms of the paths of all the planets and of many of the comets are elliptical, but some are very much more elliptical than others.
136. Next let the reader turn back to Article 105, in which we have attempted to give an idea of the plane of the Ecliptic. Now, the larger planets keep very nearly to this level, which is represented in the following figure.
ASTRONOMY.
Fig. 5. — Section, or side view, of the plane of the Ecliptic, showing that the orbits of the large planets are nearly in the plane ; that the orbit of Pallas has the greatest dip or inclination to it ; and that the orbits of the comets are inclined to it in all directions.
The straight line we suppose to represent the Earth's orbit looked at edgeways, as we can look at a hoop edgeways. The others represent the orbits of some of the planets and of some of the comets seen edgeways in the same manner. The orbits of Mars, Jupiter, Saturn, Uranus, and Neptune lie so nearly in the plane of the ecliptic, that in our figure, the scale of which is very small, they may be supposed to lie in that plane. With some of the smaller planets and comets we see the case is very different. The latter especially plunge as it were down into the surface of our ideal sea, or plane of the ecliptic, in all directions, instead of floating on, or revolving in it.
137. Again, as we thus find planets travelling round the Sun, so also do we find other bodies travelling round some of the planets. These bodies are called Moons, or Satellites; The Earth, we know, has one Moon ; Jupiter has four, Saturn eight, Uranus four, and Neptune, according to our present knowledge, one.
138. As we have before stated, all the planets revolve round the Sun in one direction, i.e. from west to east. All the planets rotate, or turn on their axes, in the same
THE SOLAR SYSTEM. 57
direction, and so do the satellites, with one exception. This exception is found in the motion of the satellites of the planet Uranus, which move from east to west.
139. Let us next inquire into the various distances of the planets from the Sun, bearing in mind, that as the orbits are elliptical, the planets are sometimes nearer to the Sun than at other times. This will be explained by and by ; in the meantime we may say, that the average or mean distances are as follow; the times of revo- lution are also given : —
Period of revolution round Distance in Miles. the Sun.
D. H. M.
Mercury. . . . 35,393,o°o • • 87 23 15
Venus .... 66,130,000 . . 224 16 48
EARTH. . . . 91,430,000 . . 365 6 9
Mars .... 139,312,000 . . 686 23 31
Jupiter .... 475,693,000 . . 4332 14 2
Saturn .... 872,135,000 . . 10759 5 16
Uranus . . . 1,752,851,000 . . 30686 17 21
Neptune . . .2,746,271,000 . . 60118 o o
140. Let us next see what are the sizes of the different planets. Their diameters are as follow : —
Diameter in Miles.
Mercury 2,962
Venus 7>5io
EARTH 7,901
Mars 4,000
Jupiter . . 85,390
Saturn 7i?9O4
Uranus 33,024
Neptune 36,620
141. We have before attempted to give an idea of the comparative sizes of the Earth and Sun, and of the dis- tance between them ; let us now complete the picture. Still
5B ASTRONOMY.
taking a globe some two feet in diameter to represent the Sun, Mercury would now be proportionately represented by a grain of mustard-seed, revolving in a circle 164 feet in diameter ; Venus, a pea, in a circle of 284 feet in diameter ; the Earth also a pea, at a distance of 430 feet ; Mars, a rather large pin's head, in a circle of 654 feet ; the smaller planets by grains of sand, in orbits of from 1,000 to 1,200 feet; Jupiter, a moderate sized orange, in a circle nearly half a mile across ; Saturn, a small orange, in a circle of four-fifths of a mile ; Uranus, a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half; and Neptune, a good- sized plum, in a circle about two miles and a half in diameter.*
14-2. As the planets revolve round the Sun at vastly dif- ferent distances, so do the satellites revolve round their primaries. Our solitary Moon courses round the Earth at a distance of 240,000 miles, and its journey is performed in a month. The first satellite of the planet Saturn is only about one-third of this distance, and its journey is performed in less than a day. The first satellite of Uranus is about equally near, and requires about two and a half days. The first satellite of Jupiter is about the same distance from that planet as our Moon is from us, and its revolution is accomplished in one and three- quarters of our days. The only satellite which takes a longer time to revolve round its primary than our Moon, is Japetus, the eighth satellite of Saturn. We have seen above (Art. 140), that the diameter of the smallest planet — leaving the asteroids out of the question — is 2,962 miles. We find that among the satellites we have three bodies — the third and fourth satellites of Jupiter, and the sixth moon of Saturn — of greater dimensions than one of the
* Sir John Herschel.
THE SOLAR SYSTEM. 59
large planets, Mercury, and nearly as large as another, Mars.
It is not necessary in this place to give more details concerning the distances and sizes of the planets and satellites. A complete statement will be found in Tables II. and III. of the Appendix.
14-3- The relative distances of the planets from the Sun was known long before their absolute dis- tances— in the same way as we might know that one place was twice or three times as far away as another without knowing the exact distance of either. When once the distance of the Earth from the Sun was known, astro- nomers could easily find the distance of all the rest from the Sun, and therefore from the Earth. Their sizes were next determined, for we need only to know the distance of a body and its apparent size, or the angle under which we see it, to determine its real dimensions.
14-4-. In the case of a planet accompanied by satellites we can at once determine its weight, or mass, for a reason we shall state by and by (Chap. IX.); and when we have got its weight, having already obtained its size or volume, we can compare the density of the materials of which the planet is composed with those we are familiar with here ; having first also obtained experimentally the density of our own Earth.
14-5. Let us see what this word density means. To do this, let us compare platinum, the heaviest metal, with hydrogen, the lightest gas. The gas is, to speak roughly, a quarter of a million times lighter than the metal ; the gas is therefore the same number of times less dense : and if we had two planets of exactly the same size, one composed of platinum and the other of hydrogen, the latter would be a quarter of a million times less dense than the former. Now, if it seems absurd to talk of a hydrogen planet, we must remember that if the materials
60 ASTRONOMY.
of which our system, including the Sun, is composed, once existed as a great nebulous mass extending far be- yond the orbit of Neptune, as there is reason to believe, the mass must have been more than 200,000,000 times less dense than hydrogen !
146. Philosophers have found that the mean density of the Earth is a little more than five and a half times that of water, that is to say, our Earth is five and a half times heavier than it would be if it were made up of water. If we now compare the density of the other planets with it, we find that they almost regularly increase in density as we approach the Sun ; Mercury being the most dense ; Venus, the Earth, and Mars, having densities nearly alike, but less than that of Mercury; while Saturn and Uranus are the least dense.
147. Here is a Table showing the volumes, masses, and densities of the planets ; those of the Earth being taken as 100 : —
Volume or Mass-or -n^r,*:*™
Size. Weight. Density.
Mercury . . 5 . 7 ... 124
Venus .... 80 . 79 ... 90
EARTH ... 100 . 100 . . . 100
Mars ... 14 . 12 ... 96
Jupiter . . . 138,700 . 30,000 ... 20
Saturn . . . 74,600 . 9,000 ... 12
Uranus . . . 7,200 . 1,300 ... 18
Neptune . . 9,400 . 1,700 ... 17
148. To sum up, then, our first general survey of the Solar System, we find it composed of planets, satellites, comets, and several rings or masses of meteoric bodies ; the planets, both large and small, revolving round the Sun in the same direction, the satellites revolving in a similar manner round the planets. We have learned the mean distances of the planets from the Sun, and we have
THE SOLAR SYSTEM. 61
compared the distances and times of revolution of some of the satellites. We have also seen that the volumes, masses, and densities of the various planets have been determined. There is still much more to be learnt, both about the system generally, and the planets particularly; but it will be best, before we proceed with our general examination, to inquire somewhat minutely into the move- ments and structure of the Earth on which we dwell.
LESSON XI. — THE EARTH. ITS SHAPE. POLES. EQUATOR. LATITUDE AND LONGITUDE. DIAMETER.
149. As we took the Sun as a specimen of the stars, because it was the nearest star to us, and we could there- fore study it best, so now let us take our Earth, with which we should be familiar, as a specimen of the planets.
150. In the first place, we have learned that it is round. Had we no proof, we might have guessed this, because both Sun and Moon, and the planets observable in our telescopes, are round. But we have proof. The Moon, when eclipsed, enters the shadow thrown by the Earth ; and it is easy to see on such occasions, when the edge of the shadow is thrown on the bright Moon, that the shadow is circular.
151. Moreover, if we watch the ships putting out to sea, we lose first the hull, then the lower sails, until at last the highest parts of the masts disappear. Similarly the sailor, when he sights land, first catches the tops of mountains, or other high objects, before he sees the beach or port. If the surface of the Earth were an extended plain, this would not happen ; we should see the nearest things and the biggest things best : but as it is, every point
62 ASTRONOMY.
of the Earth's surface is the top, as it were, of a flattened dome ; such a dome therefore is interposed between us and every distant object. The inequalities of the land render this fact much less obvious on terra firma than on the surface of the sea.
152. On all sides of us we see a circle of land, or sea, or both, on which the sky seems to rest : this is called the sensible horizon. If we observe it from a little boat on the sea, or on a plain, this circle is small ; but if we look out from the top of a ship's mast or from a hill, we find it largely increased — in fact, the higher we go the more is the horizon extended, always however retaining its circular form. Now, the sphere is the only figure which, looked at from any external point, is bounded by a circle; and as the horizons of all places are circular, the Earth is a sphere, or at all events nearly so.
153- The Earth is not only round, but it rotates, or turns round on an axis, as a top does when it is spinning ; and the names of north pole and south pole are given to those points on the Earth where the axis would come to the surface if it were a great iron rod instead of a mathematical line. Half-way between these two poles, there is an imaginary line running round the Earth, called the equator or equinoctial line- The line through the Earth's centre from pole to pole is called the polar diameter; the line through the Earth's centre from any point in the equator to the opposite point is called the equatorial diameter, and one of these, as we shall see, is longer than the other.
154. We owe to the ingenuity of a French philosopher, M. Le*on Foucault, two experiments which render the Earth's rotation visible to the eye. For although, as we shall presently see, it is made evident by the apparent motion of the heavenly bodies and the consequent suc- cession of day and night, we must not forget that these
THE SOLAR SYSTEM. 63
effects might be, and for long ages were thought to be, produced by a real motion of the Sun and stars round the Earth. The first method consists in allowing a heavy weight, suspended by a fine wire, to swing backwards and forwards like the pendulum of a clock. Now, if we move the beam or other object to which such a pendulum is suspended, we shall not alter the direction in which the pendulum swings, as it is more easy for the thread or wire, which supports the weight, to twist than for the heavy weight itself to alter its course or swing when once in motion in any particular direction. Therefore, in the experiment, if the earth were at rest, the swing of the pendulum would always be in the same direction with regard to the support and the surrounding objects.
155. M. Foucault's pendulum was suspended from the dome of the Panthdon in Paris, and a fine point at the bottom of the weight was made to leave a mark in sand at each swing. The marks successively made in the sand showed that the plane of oscillation varied with regard to the building. Here, then, was a proof that the building, and therefore the Earth, moved.
156. Such a pendulum swinging at either pole would make a complete revolution in 24 hours, and would serve the purpose of a clock were a dial placed below it with the hours marked. As the Earth rotates at the north pole from west to east, the dial would appear to a spectator, carried like it round by the Earth, to move under the pendulum from west to east, while at the south pole the Earth and dial would travel from east to west : midway between the poles, that is, at the equator, this effect, of course, is not noticed, as there the two motions in opposite directions meet.
157- The second method is based upon the fact, that when a body turns on a perfectly true and symmetrical axis, and is left to. itself in such a manner that gravity is
64 ASTRONOMY.
not brought into play, the axis maintains an invariable position ; so that if it be made to point to a star, which is a thing outside the Earth and not supposed to move, it will continue to point to it. A gyroscope is an instrument so made that a heavy wheel set into very rapid motion shall be able to rotate for a long period, and that all disturbing influences, the action of gravity among them, are prevented.
158. Now, if the Earth were at rest, there would be no apparent change in the position of the axis, however long the wheel might continue to turn ; but if the Earth moves and the axis remain at rest, there should be some differ- ence. Experiment proves that there is a difference, and just such a difference as is accounted for by the Earth's rotation. In fact, if we so arrange the gyroscope that the axis of its rotation points to a star, it will remain at rest with regard to the star, while it varies with regard to the Earth. This is proof positive that it is the Earth which rotates on its axis, and not the stars which revolve round it ; for if this were the case the axis of the gyroscope would remain invariable with regard to the Earth, and change its direction with regard to the star.
159. If we look at a terrestrial globe, we find that the equator is not the only line marked upon it. There are other lines parallel to the equator, — that is, lines which are the same distance from the equator all round, — and other lines passing through both poles, and dividing the equator into so many equal parts. These lines are for the purpose of determining the exact position of a place upon the globe, and they are based upon the fact, that all circles are divided into 360 degrees (marked °), each degree into 60 minutes ('), and each minute into 60 seconds (").
160. We have first the equator midway between the poles, so that from any part of the equator to either pole is one quarter round the P^arth, or 90 degrees. On either
THE SOLAR SYSTEM. 6$
side of the equator there are circles parallel to it ; that is to say, at the same distance from it all round, dividing the distance to the poles into equal parts. Now, it is necessary to give this distance from the equator some name. The term latitude has been chosen. North latitude from the equator towards the north pole ; south latitude from the equator towards the south pole.
161. This, however, is not sufficient to define the exact position of a place, it only defines the distance from the equator. This difficulty has been got over by fixing upon Greenwich, our principal astronomical observatory, and supposing a circle passing through the two poles and that place, and then reckoning east and west from the circle as we reckon north and south from the equator. To this east and west reckoning the term longitude has been applied.
162. On the terrestrial Globe we find what are termed parallels of latitude, and meridians of longitude, at every 10° or 15°. Besides these, at 23 y on either side of the equator, are the Tropics : the north one the tropic of Cancer, the southern one the tropic of Capricorn; and at the same distance from either pole, we find the arctic and antarctic circles. These lines divide the Earth's surface into five zones — one torrid, two temperate, and two frigid zones.
163. The distance along the axis of rotation, from pole to pole, through the Earth's centre, is shorter than the distance through the Earth's centre from any one point in the equator to the opposite one. In other words, the diameter from pole to pole (the polar diameter) is shorter than the one in the plane of the equator (the equatorial diameter), and their lengths are as follow : —
Feet.
Polar diameter .... 41,848,380
Equatorial diameter . . 41,708,710
F
66 ASTRONOMY.
Now turn these feet into miles : the difference after all is small; but still it proves that the Earth is not a sphere, it is what is called an oblate spheroid.
LESSON XII.— THE EARTH'S MOVEMENTS. ROTATION. MOVEMENT ROUND THE SUN. SUCCESSION OF DAY AND NIGHT.
1 64. The Earth turns on its axis, or polar diameter, in 23 h. 56m. In this time we get the succession of day and night, which succession is due therefore to the Earth's rotation. Before we discuss this further we must return to another of the Earth's movements. We know also that it goes round the Sun, and the time in which that revolution is effected we call a year.
165. Let us now inquire into this movement round the Sun. We stated (Art. 135) that the planets travelled round the centre of the system in ellipses. We will here state the meaning of this. If the orbits were circular, the planet would always be the same distance from the Sun, as all the diameters of a circle are equal ; but an ellipse is a kind of flattened circle, and some parts of it are nearer the centre than others.
166. In Fig. 6 the outermost ring is a circle, which can be' easily constructed with a pair of compasses, or by sticking a pin into paper, throwing a loop over it, keeping the loop tight by means of a pencil, and letting the pencil travel round. The two inner rings are ellipses. It is seen at once that one is very like the circle, and the other unlike it. The points D E and F G are called the foci of the two ellipses, and the shape of the ellipse depends upon the distance these points are apart. We can see this for
THE SOLAR SYSTEM. 67
ourselves if we stick two pins in a piece of paper, pass a loop of cotton over them, tighten the cotton by means of a pencil, and, still keeping the cotton tight, let the pencil mark the paper, as in the case of the circle. The
6. — Showing the difference between a circle and ellipses of different eccentricities, and how they are constructed.
pencil will draw an ellipse, the shape of which we may vary at pleasure (using the same loop) by altering the distance between the/oct.
167. Now the Sun does not occupy the centre of the ellipse described by the Earth, but one of the foci. It results from this, that the Earth is nearer the Sun at one time than another. When these two bodies are nearest
F 2
68
ASTRONOMY.
together, we say the Earth is in perihelion.* When they are furthest apart, we say it is in aphelion, f Let us now make a sketch of the orbit of the Earth as we should see it if we could get a bird's-eye view of it, and determine the points the Earth occupies at different times of the year, and how it is presented to the Sun.
Fig. 7. — The Earth's path round the Sun.
168. Now refer back to Art. 106, in which we spoke of the position of the Sun's axis. We found that the Sun was not floating uprightly in our sea, the plane of the ecliptic :
* wept, at or near to ; rjXiov, the Sun. f OTTO, from, and »,Aio?.
THE SOLAR SYSTEM. 69
it was dipped down in a particular direction. So it is with our Earth. The Earth's axis is inclined in the same manner, but to a much greater extent. The direction of the inclination, as in the case of the Sun, is, roughly speaking, always the same.
169. We have then two completely distinct motions — one round the axis of rotation, which, roughly speaking, remains parallel to itself, performed in a day; — one round the Sun, performed in a year. To the former motion we owe the succession of day and night; to the latter, combined with the inclination of the Earth's axis, we owe the seasons.
170. In Fig. 7 is given a bird's-eye view of the system. It shows the orbit of the Earth, and how the axis of the Earth is inclined — the direction of the dip being such that on the 2ist of June the axis is directed towards the Sun, the inclination being 23 J°. Now, if we bear in mind that the Earth is spinning round once in twenty-four hours, we shall immediately see how it is we get day and night. The Sun can only light up that half of the Earth turned towards it; consequently, at any moment, one-half of our planet is in sunshine, the other in shade ; the rotation of the Earth bringing each part in succession from sunshine to shade.
171. But it will be asked, " How is it that the days and nights are not always equal ?" For a simple reason. In the first place, the days and nights are equal all over the world on the 22d of March and the 22d of September, which dates are called the vernal and autumnal equinoxes for that very reason— equinox being the Latin for equal night. But to make this clearer, let us look at the small circle we have marked on the Earth — it is the arctic circle. Now let us suppose ourselves living in Greenland, just within that circle. What will happen ? At the spring equinox (it will be most convenient to follow the order of
70 ASTRONOMY.
the year) we find that circle half in light and half in shade. One-half of the twenty-four hours (the time of one rotation), therefore, will be spent in sunshine, the other in shade : in other words, the day and night will be equal, as we before stated. Gradually, however, as we approach the summer solstice (going from left to right), we find the circle coming more and more into the light, in consequence of the inclination of the axis, until, when we arrive at the solstice, in spite of the Earth's rotation, we cannot get oui of the light. At this time we see the midnight sun due north ! The Sun, in fact, does not set. The solstice passed, we approach the autumnal equinox, when again we shall find the day and night equal, as we did at the vernal equinox. But when we come to the winter solstice, we get no more midnight suns : as shown in the figure, all the circle is situated in the shaded portion ; hence, again in spite of the Earth's rotation, we cannot get oitt of the darkness, and we do not see the Sun even at noonday.
172. Now, these facts must be well thought of. If this be done there will be no difficulty in understanding how it is that at the poles (both north and south) the years consist of one day of six months' duration, and one night of equal length. To comprehend our long summer days and short nights in England, we have only to take a part about half-way between the arctic circle and the equator, as marked on the plate, and reason in the same way as we did for Greenland. At the equator we shall find the day and night always equal.
173. Here is a Table showing the length of the longest days in different latitudes, from the equator to the poles. We see that the Earth's surface on either side the equator may be divided into two zones, in one of which the days and nights are measured by hours, and in the other by months : —
THE SOLAR SYSTEM.
o O
16
4i
61
63 64
|
o (Equator) |
Hours. 12 |
0 |
48 . |
Hours. . . 22 |
|
AA |
66 |
21 .... |
||
|
4.8 |
66 |
24. |
||
|
24. . |
I c |
|||
|
2 |
16 |
67 |
27 |
Month.,. j |
|
17 |
**/ 60 |
|||
|
27 |
. . 18 |
vy |
4.O |
•» |
|
IQ . |
IQ |
78 |
II..*. |
4. |
|
2O |
81 |
c |
||
|
SO . |
21 |
QO |
o (Pole) |
6 |
What we have said about the northern hemisphere applies equally to the southern one, but the diagram will not hold good, as the northern winter is the southern summer, and so on ; and moreover, if we could look upon our Earth's orbit from the other side, the direction of the motions would be reversed. The reader should construct a diagram for the southern hemisphere for himself.
LESSON XIII. — THE SEASONS.
175. So much, then, for the succession of day and night. The seasons next demand our attention. Now, the changes to which we inhabitants of the temperate zones are accustomed, the heat of summer, the cold of winter, the medium temperatures of spring and autumn, depend simply upon the height to which the Sun attains at mid-day. The proof of this lies in the facts that on the equator the Sun is never far from the zenith, and we have perpetual summer : near the poles, — that is, in the frigid zones, — the Sun never gets very high, and we have per-
72 ASTRONOMY.
petual winter. How, then, are the changing seasons in the temperate zones caused ?
176. In Fig. 7 we were supposed to be looking down upon our system. We will now take a section from solstice to solstice through the Sun, in order that we may have a side view of it. Here, then, in Fig. 8, we have the Earth in two positions, and the Sun in the middle.
Fig. 8. — Explanation of the apparent altitude. of the Sun, as seen from London, in Summer and Winter.
On the left we have the winter solstice, where the axis of rotation is inclined away from the Sun to the greatest possible extent. On the right we have the summer sol- stice, when the axis of rotation is inclined towards the Sun to the greatest possible extent. The line ab in both represents the parallel of latitude passing through London. The dotted line from the centre through b shows the direction of the zenith — the direction in which our body points when we stand upright. We see that this line forms a larger angle with the line leading to the Sun, or the two lines open out wider, at the winter solstice, than they do at the summer one. Hence we see the Sun in winter at noon, low down, far from the zenith, while in summer we are glad to seek protection from his beams nearly overhead. Tlie reader should now make a
THE SOLAR SYSTEM.
73
similar diagram to represent the position of the Sun at the equinoxes ; he will find that the axis is not then in- clined either to or from the Sun, but sideways, the result being that the Sun itself is seen at the same distance from the point overhead in spring and autumn, and hence the temperature is nearly the same, though Nature ap- parently works very differently at these two seasons ; in one we have the sowing-time, in the other the fall of the leaf.
Fig. 9. — The Earth, as seen from the Sun at the Summer Solstice (noon at London).
177. Perhaps the Sun's action on the Earth, in giving rise to the seasons, will be rendered more clear by in- quiring how the Earth is presented to the Sun at the four seasons — that is, how the Earth would be seen by an
74 ASTRONOMY.
observer situated in the Sun. First, then, for summer and winter. Figs. 9 and 10 represent the Earth as it would be seen from the Sun at noon in London, at the summer and winter solstices. In the former, England is seen well down towards the centre of the disc, where the Sun is vertical, or overhead ; its rays are therefore most felt, and we enjoy our summer. In the latter, England is so near
Fig. 10. — The Earth, as seen from the Sun at the Winter Solstice (noon at London).
the northern edge of the disc that it cannot be properly represented in the figure. It is therefore furthest from the region where the Sun is overhead ; the Sun's rays are consequently feeble, and we have winter.
178. In Figs. II and 12, representing the Earth at the two equinoxes, we see that the position of England, with
THE SOLAR SYSTEM. 75
regard to the centre of the disc, is the same— the only difference being that in the two figures the Earth's axis is inclined in different directions. Hence there is no differ- ence in temperature at these periods.
179. These figures should be well studied in connexion with Fig. 7, and also with Art. 170, in which the cause of the succession of day and night is explained. All these
Fig. ii. — The Earth, as seen from the Sun at the Vernal Kquii.ox (noon at London).
drawings represent London on the meridian which passes through the centre of the illuminated side of the Earth. It must therefore be noon at that place, as noon is half-way between sunrise and sunset. All the places represented on the western border have the Sun rising upon them ; all the places on the eastern border have the Sun setting. As,
76 ASTRONOMY.
therefore, at the same moment of absolute time we have the Sun rising at some places, overhead at others, and setting at others, we cannot have the same time, as measured by the Sun, at all places alike.
Fig. 12. — The Earth, as seen from the Sun at the Autumnal Equinox (noon at London).
18O. In fact, as the Earth, whose circumference is divided into 360° (Art. 159), turns round once in twenty- four hours, the Sun appears to travel 15° in one hour from east to west. One degree of longitude, therefore, makes a difference of four minutes of time, and vice versd.
THE SOLAR SYSTEM. 77
LESSON XIV.— STRUCTURE OF THE EARTH. THE EARTH'S CRUST. INTERIOR HEAT OF THE EARTH. CAUSE OF ITS POLAR COMPRESSION. THE EARTH ONCE A STAR.
181. Having said so much of the motions of our Earth — we shall return to them in a subsequent Lesson — let us now turn to its structure, or physical constitution.
We all of us are acquainted with the present appear- ance of our globe, how that its surface is here land, there water ; and that the land is, for the most part, covered with soil which permits of vegetation, the vege- tation varying according to the climate ; while in some places meadows and wood-clad slopes give way to rugged mountains, which rear their bare or ice-clad peaks to heaven.
182. Taking the Earth as it is, then, the first question that arises is, Was it always as it is at present? The answer given by Geology and Physical Geography, two of the kindred sciences of Astronomy, is that the Earth was not always as we now see it, and that for millions of years changes have been going on, and are going on still.
183. It has been found, that what is called the Earth's crust— that is, the outside of the Earth, as the peel is the outside of an orange — is composed of various rocks of different kinds and of different ages, all of them how- ever belonging to two great classes : —
CLASS I. Rocks that have been deposited by water : these are called stratified or sedimentary rocks.
CLASS II. Rocks that once were molten : these are called igneous rocks.
ASTRONOMY.
184. Now, the sedimentary rocks have not always existed, for when we come to examine them closely it is found that they are piled one over the other in suc- cessive layers : the newer rocks reposing upon the older ones. The order in which these rocks have been deposited by the sea is as follows :—
List of Stratified Rocks. Cainozoic, or Tertiary .
•\ f Alluvium.
I Upper ' Drift. | ( Crag.
Lower N Upper
Eocene.
Cretaceous.
Mesozoic, or Secondary . I
f Lower -s Lias.
I Trias.
Palaeozoic, or Primary . .
S Permian. Carboniferous. N Devonian.
( Silurian.
Lower \ Cambrian. ( Laurentian.
185. That these beds have been deposited by water, and principally by the sea, is proved by the facts — first, that in their formation they resemble the beds being deposited by water at the present time ; and, secondly, that they nearly all contain the remains of fishes, reptiles, and shell-fish in great abundance — indeed, some of the beds are composed almost entirely of the remains of animal life.
186. It must not be supposed that the stratified bed;
THE SOLAR SYSTEM. 79
of which we have spoken are everywhere met with as they are shown in the Table ; each bed could only have been deposited on those parts of the Earth's crust which were under water at the time ; and since the earliest period of the Earth's history, earthquakes and changes of level have been at work, as they are at work now — but much more effectively, either because the changes were more decided and sudden, or because they were at work over immense periods of time.
187- It is found, indeed, that the sedimentary rocks have been upheaved and worn away again, bent, con- torted, or twisted to an enormous extent ; instead of being horizontal, as they must have been when they were originally formed at the bottom of the sea, they are now seen in some cases upright, in others dome-shaped, over large areas.
188. Had this not been the case the mineral riches of the Earth would for ever have been out of our reach, and the surface of the Earth would have been a monotonous plain. As it is, although it has been estimated that the thickness of the series of sedimentary rocks, if found complete in any one locality, would be 14 miles, each member of the series is found at the surface at some place or other.
189. The whole series of the sedimentary rocks, from the most ancient to the most modern, have been disturbed by eruptions of volcanic materials, similar to those thrown up by Vesuvius, and other volcanoes active in our own time, and intrusions of rocks of igneous origin proceeding from below ; of which igneous rocks, granite, which in conse- quence of its great hardness is so largely used for paving and macadamizing our streets, may here be taken as one example out of many. These rocks are extremely easy to distinguish from the stratified ones, as they have no ap- pearance of stratification, contain no fossils, and their
So ASTRONOMY.
constituents are different, and are irregularly distributed throughout the mass.
190. If we strip the Earth, then, in imagination, of the sedimentary rocks, we come to a kernel of rock, the constituents of which it is impossible to determine, but which may be imagined to be analogous to the older rocks of the granitic series, and to have been part of the original molten sphere which must have been both hot and luminous, in the same way that molten iron is both hot and luminous. Doubtless there was a time when the surface of our earth was as hot and luminous as the surfaces of the sun and stars are still.
191. Now, suppose we have a red-hot cannon-ball ; what happens ? The ball gradually parts with, or radiates away, its heat, and gets cool, and as it cools it ceases to give out light ; but its centre remains hot long after the surface in contact with the air has cooled down.
192. So precisely has it been with our earth ; indeed we have numerous proofs that the interior of the earth is at a high temperature at present, although its surface has cooled down. Our deepest mines are so hot that, without a perpetual current of cold fresh air, it would be impos- sible for the miners to live down them. There are hot springs coming from great depths, and the water which issues from them is, in some cases, at the boiling tempe- rature— that is, 100° of the centigrade thermometer. In the hot lava emitted from volcanoes we have evidence again of this interior heat, and how it is independent of that at the surface ; for among the most active volcanoes with which we are acquainted are Hecla in Iceland, and Mount Erebus in the midst of the icy deserts which surround the south pole.
193. It has been calculated that the temperature of the earth increases as we descend at the rate of J° (cen-
THE SOLAR SYSTEM. 81
tigrade) in about thirty yards. We shall therefore have a temperature of —
Centigrade. Miles.
1 00° or the temperature of ) , ., r
, .,. fat a depth of . . 2
boiling water . . )
400° or the temperature of ) ,
red-hot iron ... $
i, 000° or the temperature of / g
melted glass ... 1
1,500° or the temperature at^ which everything ! with which we are I acquainted would [ "
be in a state of i fusion J
194. If this be so, then the Earth's crust cannot exceed 28 miles in thickness — that is to say, the yi^-th part of the radius (or of half the diameter), so that it is com- parable to the shell of an egg. But this question is one on which there is much difference of opinion, some philo- sophers holding that the liquid matter is not continuous to the centre, but that, owing to the great pressure, the centre itself is solid. Evidence also has recently been brought forward to show that the Earth may be a solid or nearly solid globe from surface to centre.
195. The density of the Earth's crust is only about half of the mean density of the Earth taken as a whole. This has been accounted for by supposing that the ma- terials of which it is composed are made denser at great depths than at the surface, by the enormous pressure of the overlying mass; but there are strong reasons for believing that the central portions are made up of much
G
82
ASTRONOMY.
denser bodies, such as metals and their metallic com- pounds, than are common at the surface.
196. It was prior to the solidification of its crust, and while the surface was in a soft or fluid condition, that the Earth put on its present flattened shape, the flattening being due to a bulging out at the equator, caused by the Earth's rotation. If we arrange a hoop, as shown in
**£• X3- — Explanation of the Spheroidal form of the Earth.
Fig. 13, and make it revolve very rapidly, we shall see that that part of the hoop furthest from the fixed points, and in which the motion is most rapid, bulges out as the Earth does at the equator.
197. The form of the Earth, moreover, is exactly that which any fluid mass would take under the same circum- stances. M. Plateau has proved this by placing a mass of oil in a transparent liquid exactly of the same density as the oil. As long as the oil was at rest it took the form of a perfect sphere floating in the middle of the fluid, exactly as the Earth floats in space ; but the moment a slow motion of rotation was given to the oil by means of a piece of wire forced through it, the spherical form was changed into a spheroidal one, like that of the Earth.
198. The tales told by geology, the still heated state of the Earth, and the shape of the Earth itself, all show that long ago the sphere was intensely heated, and fluid.
THE SOLAR SYSTEM 83
LESSON XV. — THE EARTH (continued). THE ATMO- SPHERE. BELTS OF WINDS AND CALMS. THE ACTION OF SOLAR AND TERRESTRIAL RADIATION. CLOUDS. CHEMISTRY OF THE EARTH. THE EARTH'S PAST AND FUTURE.
199. Having said so much of the Earth's crust, we must now, in order to fully consider our Earth as a planet, pass on to the atmosphere, which may be likened to a great ocean, covering the Earth to a height which has not yet been exactly determined. This height is generally sup- posed to be 45 or 50 miles, but there is evidence to show that we have an atmosphere of some kind at a height of 400 or 500 miles.
200. The atmosphere, as we know, is the home of the winds and clouds, and it is with these especially that we have to do, in order to try to understand the appearances presented by the atmospheres of other planets. Although in any one place there seems no order in the production of winds and clouds, on the Earth treated as a whole we find the greatest regularity ; and we find, too, that the Sun's heat and the Earth's rotation are, in the main, the causes of all atmospheric disturbances.
201. If we examine a map showing the principal move- ments and conditions of the atmosphere, we shall find, belting the Earth along the equator, a belt of equatorial calms and rains. North of this we get a broad region, a belt of trade-winds, where the winds blow from the north-east ; to the south we find a similar belt, where the prevailing winds are south-east. Polewards from these
G 2
84 ASTRONOMY.
belts to the north and south respectively lie the calms of Cancer and the calms of Capricorn. Still further to- wards the poles, we find the counter- trades, in regions where the winds blow from the equator to the poles : i.e. in the northern hemisphere they blow south and west, and in the southern hemisphere north and west ; and at the poles themselves we find a region of polar calms.
202, Now if the Earth did not rotate on its axis we should still get the trade-winds, but both systems would blow from the pole to the equator ; but as the Earth does rotate, the nearer the winds get to the equator the more rapidly is the Earth's surface whirled round underneath them ; the Earth, as it were, slips from under them in an easterly direction, and so the northern trade-winds appear to come from the north-east, and the southern ones from the south-east. Similarly, the counter-trades, which blow towards the poles, appear to come, the northern ones from the south-west and the southern ones from the north-west. The nearer they approach the poles the slower is the motion of the Earth under them, compared with the regions nearer the equator ; consequently, they are travelling to the eastward faster than the parallels at which they successively arrive, and they appear to come from the westward.
203. Now, how are these winds set in motion ? The tropics are the part of the Earth which is most heated, and, as a consequence, the air there has a tendency to ascend, and a surface-wind sets in towards the equator on both sides, to fill up the gap, as it were ; when it gets there it also is heated ; the two streams join and ascend, and flow as upper-currents towards either pole. Where the two streams meet in the region of equatorial calms some 4° or 5° broad, we have a cloud-belt, and daily rains. The counter-trades are the upper-currents referred to above, which in the regions beyond the calms of Cancer and
THE SOLAR SYSTEM. 85
Capricorn descend to the Earth's surface, and form surface currents.
2O4-. We see, therefore, that it is the Sun which sets all this atmospheric machinery in motion, by heating the equatorial regions of the Earth ; and as the Sun changes its position with regard to the equator, oscillating up and down in the course of the year, so do the calm-belts and trade- winds. The belt of equatorial calms follows the Sun northwards from January to July, when it reaches 25° N. lat. and then retreats, till at the next January it is in 25° S. lat.
205. So much for the Sun's direct action, and one of its effects on our rotating planet — the prevailing wind- currents, which are set in motion by the ^7iyJ-(nnn> Part of the Sun's radiation into space, which represents an amount of heat that would daily raise 7,513 cubic miles of water from the freezing to the boiling point.
206. To the radiation from the Earth, combined with the existence of the vapour of water in the air, must be ascribed all the other atmospheric phenomena. Aqueous vapour is the great mother of clouds. When it is chilled by a cold wind or a mountain top, it parts with its heat, is condensed and forms a cloud; and then mist, rain, snow, or hail, is formed : when it is heated by the direct action of the Sun, or by a current of warm air, it absorbs all the heat and expands, and the clouds disappear.
207. We now come to the materials of which our planet, including the Earth's crust and atmosphere, is composed. These are 64 in number ; they are called the chemical elements. These consist of—
N on- metallic f Nitrogen, oxygen, hydrogen, chlorine,
elements; bromine, iodine, fluorine, silicon,
or boron, carbon, sulphur, selenium,
Metalloids. | tellurium, phosphorus, arsenic.
86 ASTRONOMY.
Metals of the alkalies : — Potassium, sodium, caesium, rubidium, lithium. Metallic J Metals of the alkaline earths:— Cal- eiementa. | cium, strontium, barium.
Other metals: — Aluminium, zinc, iron, tin, tungsten, lead, silver, gold, £c.
The elements which constitute the great mass of the Earth's crust are comparatively few — aluminium, calcium, carbon, chlorine, hydrogen, magnesium, oxygen, potas- sium, silicon, sodium, sulphur. Oxygen combines with many of these elements, and especially with the earthy and alkaline metals ; indeed, one-half of the ponderable matter of the exterior parts of the globe is composed of oxygen in a state of combination. Thus sandstone, the most common sedimentary rock, is composed of silica, which is a compound of silicon and oxygen, and is half made up of the latter ; granite, a common igneous rock, composed of quartz, felspar, and mica, is nearly half made up of oxygen in a state of combination in those substances.
2O8. The chemical composition, by weight, of 100 parts of the atmosphere at present is as follows :—
Nitrogen .... 77 parts. Oxygen 23 „
Besides these two main constituents, we have —
Carbonic acid . quantity variable with the locality. Aqueous vapour . quantity variable with the tem- perature and humidity. Ammonia ... a trace.
We said at present, because, when the Earth was molten, the atmosphere must have been very different.
THE SOLAR SYSTEM. 87
We had, let us imagine, close to the still glowing crust — composed perhaps of acid silicates —a dense vapour, com- posed of compounds of the materials of the crust which were volatile only at a high temperature ; the vapour of chloride of sodium, or common salt, would be in large quantity ; above this, a zone of carbonic acid gas ; above this again a zone of aqueous vapour, in the form of steam ; and lastly, the nitrogen and oxygen.*
As the cooling went on, the lowest zone, composed of the vapour of salt, and other chlorides, would be con- densed on the crust, covering it with a layer of these sub- stances in a solid state. Then it would be the turn of the steam to condense too, and form water ; it would fall on the layer of salt, which it would dissolve, and in time the oceans and seas would be formed, which would conse- quently be salt from the first moment of their appearance. Then, in addition to the oxygen and nitrogen which still remain, we should have the carbonic acid, which, in the course of long ages, was used up by its carbon going to form the luxurious vegetation, the pressed remains of which is the coal which warms us, and does nearly all our work.
2O9. Now it is the presence of vapour in our lower atmosphere which renders life possible. When the surface of the Earth was hot enough to prevent the formation of the seas, as the water would be turned into steam again the instant it touched the surface, there could be no life. Again, if ever the surface of the Earth be cold enough to freeze all the water and all the gaseous vapour in the atmo- sphere, life — as we have it — would be equally impossible. If this be true, all the Earth's history with which we are acquainted, from the dawn of life indicated in what geologists call the oldest rocks, down to our own time, and perhaps onwards for tens of thousands of years,
* David Forbes, in the Geological Magazine, vol. iv. p. 439.
88 ASTRONOMY.
is only the history of the Earth between the time at which its surface had got cold enough to allow steam to turn into water, and that at which its whole mass will be so cold that all the water on the surface, and all the vapour of water in the atmosphere, will be turned into ice.
210. The nebular hypothesis here comes in and shows us how, prior to the Earth being in a fluid state, it existed dissolved in a vast nebula, the parent of the Solar System ; how this nebula gradually contracted and condensed, throwing off the planets one by one ; and how the central portion of the nebula, condensed perhaps to the fluid state, exists at present as the glorious heat-giving Sun.
Although, therefore, we know that stars give out light because they are white-hot bodies, and that planets are not self-luminous because they are comparatively cold bodies, we must not suppose that planets were always cold bodies, or that stars will always be white-hot bodies. Indeed, as we have shown, there is good reason for sup- posing that all the planets were once white-hot, and gave out light as the Sun does now.
LESSON XVI. — THE MOON : ITS SIZE, ORBIT, AND MOTIONS : ITS PHYSICAL CONSTITUTION.
211. The Moon, as we have already seen, is one of the satellites, or secondary bodies ; and although it ap- pears to us at night to be so infinitely larger than the fixed stars and planets, it is a little body of 2,153 miles in diameter ; so small is it, that 49 moons would be required to make one earth, 300,000 earths being required, as we have seen, to make one sun !
212. Its apparent size, then, must be due to its near-
THE SOLAR SYSTEM. 89
ness. This we find to be the case. The Moon revolves round the Earth in an elliptic orbit, as the Earth revolves round the Sun, at an average distance of only 238,793 miles, which is equal to about 10 times round our planet. As the Moon's orbit is elliptical, she is sometimes nearer to us than at others. The greatest and least distances are 251,947 and 225,719 miles: the difference is 26,228. When nearest us, of course she appears larger than at other times, and is said to be in perigee (irepi near, and yrj the Earth) ; when most distant, she is said to be in apogee (airo from, and yrj).
213. The Moon travels round the Earth in a period of 27 d. 7h. 43m. u^s. As we shall see presently, she re- quires more time to complete a revolution with respect to the Sun, which is called a lunar month, lunation, or synodic period.
214-. The Moon, like the planets and the Sun, rotates on an axis ; but there is this peculiarity in the case of the Moon, namely, that her rotation and her revolution round the Earth are performed in equal times, that— is, in 27 d. 7h. 43m. Hence we only see one side of our satellite. But, as the Moon's axis is inclined i° 32' to the plane of its orbit, we sometimes see the region round one pole, and sometimes the region round the other. This is termed the libration in latitude. There are also a libration in longitude, arising from the fact, that though its rotation is uniform, its rate of motion round the Earth varies, so that we sometimes see more of the western edge and sometimes more of the eastern one ; and a daily libration, due to the Earth's rotation, carrying the ob- server to the right and left of a line joining the centres of the Earth and Moon. When on the right, or west, of this line, we should of course see more of the western edge of the Moon; when to the left, in the case of an eastern position, we should see more of the eastern edge.
90 ASTRONOMY.
215. The plane in which the Moon performs her journey round the Earth is inclined 5° to the plane of the ecliptic, or the plane in which the Earth performs her journey round the Sun (Art. 105) . The two points in which the Moon's orbit, or the orbit of any other celestial body, intersects the Earth's orbit, are called the nodes. The line joining these two points is called the line of nodes. The node at which the body passes to the north of the ecliptic is called the ascending node, the other the descending node.
216. The motions of the Moon, as we shall see by and by, are very complicated. We may get an idea of its path round the Sun if we imagine a wheel going along a road to have a pencil fixed to one of its spokes, so as to leave a trace on a wall : such a trace would consist of a series of curves with their concave sides downwards, and such is the Moon's path with regard to the Sun.
217. Besides the bright portion lit up by the Sun, we sometimes see, in the phases which immediately precede and follow the New Moon, that the obscure part is faintly visible. This appearance is called the " Earth shine" (Lumen incinerosum, Lat.; Lumiere cendree, Fr.), and is due to that portion of the Moon reflecting to us the light it receives from the Earth. When this faint light is visible — when the " Old Moon "is seen in the "New Moon's arms " — the portion lit up by the Sun seems to belong to a larger moon than the other. This is an effect of what is called irradiation, and is explained by the fact that a bright object makes a stronger impression on the eye than a dim one, and appears larger the brighter it is.
218. The average of four estimations gives the Moon's light as -nrfVn? °f tnat °f tne ^un> so we should want 547,513 full moons to give as much light as the Sun does ; and as there would not be room to place such a large
THE SOLAR SYSTEM. 91
number in the one-half of the sky which is visible to us, as the new Moon covers ^$m of it:> * follows that the light from a sky full of moons would not be so bright as sunshine.
219. At rising or setting, the Moon sometimes appears to be larger than it does when high up in the sky. This is a delusion, and the reverse of the fact ; for, as the Earth is a sphere, we are really nearer the Moon by half the Earth's diameter when the part of the Earth on which we stand is underneath it ; as at moonrise or moonset we are situated, as it were, on the side of the Earth, half-way between the two points nearest to and most distant from the Moon. Let the reader draw a diagram, and reason this out.
220. Now a powerful telescope will magnify an object i, coo times ; that is to say, it will enable us to see it as if it were a thousand times nearer than it is: if the Moon were 1,000 times nearer, it would be 240 miles off, consequently astronomers can see the Moon as if it were situated at a little less than that distance, as it is measured from centre to centre, and we look from surface to surface. In con- sequence of this comparative nearness, the whole of the surface of our satellite turned towards us has been studied and mapped witli considerable accuracy.
221. With the naked eye we see that some parts of the Moon are much brighter than others ; there are dark patches, which, before large telescopes were in use, were thought to be, and were named, Oceans, Gulfs, and so on. The telescope shows us that these dark markings are smooth plains, and that the bright ones are ranges of mountains and hill country broken up in the most tremen- dous manner by volcanoes of all sizes. A further study convinces us that the smooth plains are nothing but oki sea-bottoms. In fact, once upon a time the surface of the Moon, like our Earth, was partly covered with water, and
92 ASTRONOMY.
the land was broken up into hills and fertile valleys ; as on the Earth we have volcanoes, so did it once happen in the Moon, with the difference that there the size of the volcanoes and the number and activity of them were far beyond anything we can imagine : our largest volcanic mountain Etna is a mere dwarf compared with many on the Moon.
222. The best way of seeing how the surface of our satellite is broken up in this manner, is to observe the terminator — the name given to the boundary between the lit-up and shaded portions : along this line the moun- tain peaks are lit up, while the depressions are in shade, and the shadows of the mountains are thrown the greatest distance on the illuminated portion. The heights of the mountains and depths of the craters have been measured by observing the shadows in this manner.
223. Besides the mountain-ranges and crater-moun- tains, there are also walled plains, isolated peaks, and curious markings, called rilies. The principal ranges, craters, and walled plains have been named after distin- guished philosophers, astronomers, and travellers.
224. The diameter of the walled plain Schickard, near the south-east limb or edge of the Moon, is 133 miles. Clavius and Grimaldi have diameters of 142 and 138 miles respectively. Here is a table of the height of some of the peaks, with that of some of our terrestrial ones, for comparison : —
Feet
Dorfel 26,691
Ramparts of ( measured from the ) g
Newton ( floor of the crater }
Eratosthenes (central cone) 15)75°
Mont Blanc 15,870
Snowdon 3>5°°
Beer and Madler have measured thirty-nine mountains
THE SOLAR SYSTEM. 93
higher than Mont Blanc. It must be recollected also that as the Moon is so much smaller than the Earth, the proportion of the height of a mountain to the diameter is much greater in the former.
225. As far as we know, with possibly one exception, which is not yet established, the volcanoes are now all extinct ; the oceans have disappeared ; the valleys are no longer fertile ; nay, the very atmosphere has apparently left our satellite, and that little celestial body which pro- bably was once the scene of various forms of life now no longer supports them.
This may be accounted for by supposing (see Art. 191) that, on account of the small mass of the Moon, its original heat has all been radiated into space (as a bullet will take less time to get cold than a cannon-ball).
226. The rilles, of which 425 are now known, are trenches with raised sides more or less steep. Besides the rilles, at full Moon, bright rays are seen, which seem to start from the more prominent mountains. Some of these rays are visible under all illuminations ; one which, emanating from Tycho, crosses a crater on the north- east of Fracastorius, is not only distinctly visible when the terminator grazes the west edge of Fracastorius, but is even brighter as the terminator approaches it. Those emanating from Tycho are different in their character from those emanating from Copernicus, while those from Proclus form a third class.
Mr. Nasmyth has been able to produce somewhat similar appearances on a glass globe by filling it with cold water, closing it up, and plunging it into warm water. This causes the inclosed cold water to expand very slowly, and the globe eventually bursts, its weakest point giving way, forming a centre of radiating cracks in a similar manner to the fissures, if they be fissures, in the Moon.
94 ASTRONOMY.
227. We say that the Moon has apparently no atmo- sphere ; (i) because we never see any clouds there, and (2) because, when the Moon's motion causes it to travel over a star, or to occult it, as it is called, the star disap- pears at once, and does not seem to linger on the edge, as it would do if there were an atmosphere.
228. As the Moon rotates on her axis, as we do, only more slowly, the changes of day and night occur there as here ; but instead of being accomplished in 24 hours, the Moon's day is 29^ of our days long, so that each portion of the surface is in turn exposed to, and shielded from, the Sun for a fortnight ; as there is no atmosphere either to shield the surface or to prevent radiation, it has been conjectured that the surface is, in turn, hotter than boiling water, and colder than anything we have an idea of.
In Plate VI. we give a view of the crater Copernicus, one of the most prominent objects in the Moon. The details of the crater itself, and of its immediate neighbour- hood, represented in the drawing, reveal to us unmis- takeable evidences of volcanic action. The floor of the crater is seen to be strewn over with rugged masses, while outside the crater-wall (which on the left-hand side casts a shadow on the floor, as the drawing was taken soon after sunrise at Copernicus, and the Sun is to the left) many smaller craters, those near the edge forming a regular line, are distinctly visible. Enormous unclosed cracks and chasms are also distinguishable. The depth of the crater floor, from the top of the wall, is 1 1,300 feet ; and the height of the wall above the general surface of the moon is 2,650 feet. The irregularities in the top of the wall are well shown in the shadow. The scale of miles attached to the drawing shows the enormous propor- tions of the crater.
LI \ \h < RATE]
COl'KRNICUS
Tlale'VL
.Tp Tn«v. Ma Tn-yth F, fl rr, RrnT*
96 ASTRONOMY.
lying to the right, as seen from the Earth ; at D we shall see the lit-up portion lying to the left, looking from the Earth. These positions are those occupied by the Moon at the First Quarter and Last Quarter respectively. When the Moon is at E and F we shall see but a small
Fig. 14. — Phases of the Moon.
part of the lit-up portion, and we shall get a crescent Moon, the crescent in both cases being turned to the Sun. At G and H the Moon will be gibbous.
231. So that the history of the phases is as follows : —
New Moon. The Moon is invisible to us, because the Sun is lighting up one side and we are on the other.
THE SOLAR SYSTEM. 97
Crescent Moon. We just begin to see a little of the illumi- nated portion, but the Moon is still so nearly in a line with the Sun, that we only catch, as it were, a glimpse of the side turned towards the Sun, and see the Moon herself for a short time after sunset.
First Quarter. As seen from the Moon, the Earth and Sun are at right angles to each other. When the Sun sets in the west, the Moon is south. Hence, as the illumination is sideways, the right hand side of the Moon is lit up.
Gibbous Moon. The Moon is now more than half lighted up on the right-hand side.
Full Moon. The Earth is now between the Sun and Moon, and therefore the entire half of the Moon which is illuminated is visible.
232. From Full Moon we return through the Second Quarter and other similar phases to the New Moon, when the cycle recommences. So that, from New Moon the illuminated portion of our satellite waxes, or increases in size, till Full Moon, and then wanes, or diminishes, to the next New Moon ; the illuminated portion, except at Full Moon, being separated from the dark one by a semi-ellipse, called, as we have seen (Art. 222), the terminator.
233. In Fig. 14 we supposed that the Moon's motion was performed in the plane of the ecliptic. Our readers now know (Art. 215) that this is not the case : if it were so, every Full Moon would put out the Sun ; and as the Earth, and every body through which light cannot pass, both on the Earth and off it, casts a shadow, every New Moon would be hidden in that shadow. These appear- ances are called eclipses, and they do happen sometimes.
H
98
ASTRONOMY.
Let us see if we can show under what circumstances they do happen. One-half of the Moon's journey is performed above the plane of the ecliptic, one-half below it ; hence at certain times — twice in each revolution — the Moon is in that plane, at those parts of it called the nodes. Now, if the Moon at that time happens to be new or full — that is, in line with the Earth and Sun — in one case we shall have an eclipse of the Sun, in the other we shall have an eclipse of the Moon. This will be rendered clear by the accompany- ing figure. We have the Sun at bottom, the Earth at top, and the Moon in two positions marked A and B, the level of the page representing the level, or plane, of the ecliptic. We suppose therefore in both cases that the Moon is at a node, — that is, on that level, neither above nor below it.
234. At A, therefore, the Moon stops the Sun's light, its shadow falls on a part of the Earth ; and the people, therefore, who live on that particular part of it where the shadow falls cannot see the Sun, because the Moon is in the way. Hence we shall have what is called an eclipse of the Son.
235. At B the Moon is in the shadow of the Earth cast by the Sun ; therefore the Moon cannot receive any light from the Sun, because the Earth is in the way. Hence we shall have what is called an eclips^ if the Moon.
SUN
/•' g 15.— Eclipses of the Sun and Moon.
THE SOLAR SYSTEM. 99
236. It will easily be seen from the figure, that whereas the eclipse of the Moon by the shadow of the much larger Earth will be more or less visible to the whole side of the Earth turned away from the Sun, the shadow cast by the small Moon in a solar eclipse is, on the contrary, so limited, that the eclipse is only seen over a small area.
237. In the figure two kinds of shadows are shown, one much darker than the other ; the latter is called the umbra, the former the penumbra. If the Sun were a point of light merely, the shadow would be all umbra ; but it is so large, that round the umbra, where no part of the Sun is visible, there is a belt where a portion of it can be seen ; hence we get a partial shadow, which is the meaning of penumbra. This will be made quite clear if we get two candles to represent any two opposite edges of the Sun, place them rather near together, at equal distances from a wall, and observe the shadow they cast on the wall from any object ; on either side the shadow thrown by both candles will be a shadow thrown only by one.
238. In a total eclipse of the Moon, as the Moon travels from west to east, we first see the eastern side of the Moon slightly dim as she enters the penumbra ; this is the first contact with the penumbra, spoken of in almanacs. At length, when the real umbra is reached, the eastern edge becomes almost invisible ; we have the first contact with the dark shadow; the circular shape of the Earth's shadow is distinctly seen, and at last she enters it entirely. When the Moon passes, how- ever, into the shadow of the Earth, it is scarcely ever quite obscured; the Sun's light j° bent by the Earth's atmo- sphere towards the Moon, and sometimes tinges it with a ruddy colour.
A total eclipse of the Moon may last about if hours. When the Moon again leaves the umbra we have the last
II 2
loo ASTRONOMY.
contact with the dark shadow; and after the last contact with the penumbra, the eclipse is over.
239. If the Moon is not exactly at a node, we shall only get a partial eclipse of the Me on, the degree of eclipse depending upon the distance from the node. For instance, if the Moon is to the north of the node, the lower limb may enter the upper edge of the penumbra or of the umbra ; if to the south of the node, the upper portion will be obscured.
LESSON XVI II. — ECLIPSES (continued}. ECLIPSES OF THE SUN. TOTAL ECLIPSES AND THEIR PHENO- MENA. CORONA. RED-FLAMES.
24-O. In a total eclipse of the Sun, the diameter of the shadow which falls on the Earth is never large, averaging about 150 miles ; and as the Moon, which throws the shadow, revolves from west to east in a month, while the Earth's surface, on which it falls, rotates from west to east in a day, the shadow travels more slowly than the surface, and so appears to sweep across it from east to west with great rapidity. The longest time an eclipse of the Sun can be total at any place is seven minutes, and of course it is only visible at those places swept by the shadow. Hence, in any one place total eclipses of the Sun are of very rare occurrence ; in London, for instance, there has been no total eclipse of the Sun since 1715.
24-1. In eclipses of the Sun there are no visible effects of umbra and penumbra seen on the Sun itself; we have the real (though invisible) Moon eating into the real and visible Sun, the western edge of the Sun in the case of total eclipses being first obscured. The obscuration
THE SOLAR SYSTEM. roi
increases until the Moon covers all the Sun, and soon afterwards the western edge of the Sun reappears.
24-2. As in the case of the Moon, there are other eclipses besides total ones. To explain this we must give a few figures. As both Sun and Moon are round, or nearly so, the shadow from the latter is round ; and as the Sun is larger than the Moon, the shadow ends in a point. The shape of the shadow is. in fact, that of a cone — hence the term " cone of shadow." Now, the length of this cone varies with the Moon's distance from the Sun : when nearest, the Moon will of course throw the shortest shadow. The lengths are about as follow :— ^.}^
When the Moon and Sun are nearest together . 230,000 „ „ „ furthest apart . . . 238,000
But in Art. 212 we saw that the distance between the Earth and Moon varied as follows : —
Miles.
When the Moon and Earth are nearest together . 225,719 „ „ „ furthest apart . .251,947
Hence, when the Moon is furthest from the Earth, or in apogee, the shadow thrown by the Moon is not long enough to reach the Earth ; at such times the Moon looks smaller than the Sun, and if she be at a node, we shall have an annular eclipse — that is, there will be a ring (annulus, Lat. ring) of the Sun visible round the Moon when the eclipse would otherwise have been total.
243. There may be partial eclipses of the Sun, for the same reason as we have partial eclipses of the Moon ; only as the Moon is not exactly at the node in one case, she does not get totally eclipsed, because she does not pass quite into the shadow of the Earth ; and in the other, the Sun does not get totally eclipsed, because the Moon does not pass exactly between us and the Sun.
24-4.. The nodes of the Moon are not stationary, but move backwards upon the Moon's orbit, a complete revo-
102 ASTRONOMY.
lution taking place with regard to the Moon in 18 year* 2 19 days, nearly. The Moon in her orbit, therefore, meets the same node again before she arrives at the same place with regard to the Sun again, one period being 27 d. 5h.6m., called the nodical revolution of the Moon; and the other, 29 d. I2h. 44m., called the synodical revolution of the Moon, in which it regains the same position with regard to the Sun. The node is in the same position with regard to the Sun after an interval of 346 d. I4h. 52111. This is called a synodic revolution of the node. Now it so hap- pens that nineteen synodic.il revolutions of the node, after which period the Sun and node would be alike situated ; are equal to 223 synodical revolutions of the Moon, after which period the Sun and Moon would be alike situated : so that, if we have an eclipse at the beginning of the period, we shall have one at the end of it, the Sun, Moon, and node having got back to their original positions. This period of 18 years 219 days is a cycle of the Moon, known to the ancient Chaldeans and Greeks under the name of Saros, and by its means eclipses were roughly predicted before astronomy had made much progress.
24-5. A total eclipse of the Sun is at once one of the most awe-inspiring and grandest sights it is possible for man to witness. As the eclipse advances, but before the totality is complete, the sky grows of a dusky livid, or purple, or yellowish crimson, colour, which gradually gets darker and darker, and the colour appears to run over large portions of the sky, irrespective of the clouds. The sea turns lurid red. This singular colouring and darken- ing of the landscape is quite unlike the approach of night, and gives rise to strange feelings of sadness. The Moon's shadow is seen to sweep across the surface of the Earth, and is even seen in the air ; the rapidity of its motion and its intenseness produce a feeling that something material is sweeping over the Earth at a speed perfectly frightful.
arren Ee La.Eue."F.E.S.
THE SOLAR SYSTEM. 103
All sense of distance is lost, the faces of men assume a livid hue, fowls hasten to roost, flowers close, cocks crow, and the whole animal world seems frightened out of its usual propriety.
24-6. A few seconds before the commencement of the totality the stars burst out ; and surrounding the dark Moon on all sides is seen a glorious halo, generally of a silver-white light; this is called the corona. It is slightly radiated in structure, and extends sometimes beyond the Moon to a distance equal to our satellite's diameter. Besides this, rays of light, called aigrettes, diverge from the Moon's edge, and appear to be shining through the light of the corona. In some eclipses some parts of the corona have reached to a much greater distance from the Moon's edge than in others.
247. It is supposed that the corona is the Sun's atmo- sphere, which is not seen when the Sun itself is visible, owing to the overpowering light of the latter.
24-8. When the totality has commenced, apparently close to the edge of the Moon, and therefore within the corona, are observed fantastically- shaped masses, full lake- red, fading into rose-pink, variously called red-flames and red-prominences. Two of the most remarkable of these hitherto noticed were observed in the eclipse of 1851. They have thus been described by the Rev. W. R. Dawes :—
" A bluntly triangular pink body [was seen] suspended, as it were, in the corona. This was separated from the Moon's edge when first seen, and the separation increased as the Moon advanced. It had the appearance of a large conical protuberance, whose base was hidden by some intervening soft and ill-defined substance. ... To the north of this appeared the most wonderful phenomenon of the whole : a red protuberance, of vivid brightness and very deep tint, arose to a height of perhaps if' when first
104 ASTRONOMY.
seen, and increased in length to 2' or more, as the Moon's progress revealed it more completely. In shape it somewhat resembled a Turkish cimeter, the northern edge being convex, and the southern concave. Towards the apex it bent suddenly to the south, or upwards, as seen in the telescope. ... To my great astonishment, this marvellous object continued visible for about five seconds, as nearly as I could judge, after the Sun began to reappear."
24-9. It has been definitely established that these pro- minences belong to the Sun, as those at first visible on the eastern side are gradually obscured by the Moon, while those on the western are becoming more visible, owing to the Moon's motion from west to east over the Sun. The height of some of them above the Sun's sur- face is upwards of 40,000 miles.
25O. It is thought that these red-prominences are incandescent clouds floating in the Sun's atmosphere, or resting upon the photosphere ; but this has not as yet been definitely established.
LESSON XIX. — THE OTHER PLANETS COMPARED WITH THE EARTH. PHYSICAL DESCRIPTION OF MARS.
251. We are now in a position to examine the other planets of our system, and to bring the facts taught us by our own to bear upon them. In the case of all the planets we have been able to ascertain the facts necessary to de- termine the elements of their revolution round the Sun ; that is to say, the time in which the complete circuit round the common luminary is accomplished, and the shape and position of their orbits with regard to our own. Now the
Plate T2ZZ"
Warren De La Rue del
S.RusseH Sculp.
THE SOLAR SYSTEM. 105
shape of the orbit depends upon the degree of its ellipticity — for all are elliptical — and its position upon the distance of the planet from the Sun, and the degree in which the plane of each orbit departs from our own. When we have, in addition to these particulars, the position of the nodes — the points in which the orbit intersects the plane of our orbit — and the position of the perihelion points, we have all the materials necessary for calculation or for making a diagram of the planet's path.
252. Still, however satisfactory our examination of the planets has been with regard to their revolution round the Sun, we are compelled to state that when we wish to in- quire into their rotations on their axes, the length of their days, their seasons, and their physical constitutions, the knowledge as yet acquired by means of the telescope is far from complete. Thus, of the planets Mercury and Venus we have nothing certain to tell on these matters ; they are both so lost in the Sun's rays, and so refulgent in consequence of their nearness to that body, that our ob- servation of them, of Mercury especially, has been baffled. Of the same class of facts in the case of Uranus and Neptune we are equally ignorant, but for a different reason. At our nearest approach to Uranus we are up- wards of 1,700,000,000 miles away from that planet ; at our nearest approach to Neptune we are upwards of 2,600,000,000 miles away, and we cannot be surprised that our telescopes fail us in delicate observations at such dis- tances. Still, in the case of Uranus, we have been able to assume some facts from the motions of his moons.
253. With regard however to Mars, Jupiter, and Saturn, the planets whose orbits are nearest to our own, our information is comparatively full and complete. For instance, we can for these planets give the following in- formation in addition to that stated in Arts. 139 and 140, and Table II. of the Appendix.
106 ASTRONOMY.
Length of Day. Inclination of
H. M. S. Axis.
Mars . . . 24 37 22 . . . 28° 51' o" Jupiter .. 95521... 3 40 Saturn . . 10 29 17 . . . 28 10 o
The first column requires no explanation. We see, however, at once that the day in Mars is nearly equal to our own, while in the large planets, Jupiter and Saturn, it is not half so long. Now the revolutions of these planets round the Sun are accomplished as follow : —
Mars in 686 of our days. Jupiter in 4-333 „ » Saturn in 10,759 » »
We can therefore easily find the number of days ac- cording to the period of rotation of each planet, which go to make each planetary year : thus in Saturn's year there are 24,584 Saturnian days, or 67 times more days than in our own.
254. In the second column the inclination of the planets' axes of rotation is given. It will be recol- lected that the inclination of our own is 23^°, and that it is on this inclination that our seasons depend. It will be seen at once therefore that Mars and Saturn are much like the Earth in this respect, and that Jupiter is a planet almost without seasons, for the inclination of its axis is only 3°, while that of Uranus is 100°. The axis of rotation of Uranus, in fact, lies almost in the plane of its orbit.
255. As in the case of the Earth, we find in many in- stances the axis of rotation, or polar diameter, of the other planets shorter than the equatorial diameter. The amount of polar compression, — that is, the amount of flattening, by which the polar diameter is less than the equatorial one, — measured in fractions of the latter, is as follows : —
THE SOLAR SYSTEM. 107
Mercury Venus Earth Mars .
Jupiter . Saturn Uranus Neptune
From this Table we learn that if the equatorial dia- meter of Mercury be taken as 29, the polar one is only' 28 : in the cases of Jupiter and Saturn, the diameters are as 17 to 1 6 and 9 to 8, respectively. In these two last the rotation is very rapid (Art. 253) ; and this great flattening is what we should expect from the reasoning in Art. 196
256. We now come to what we can glean of the physical structure of the planets as seen in the telescope when they are nearest the Earth. Let us begin with Mars. We give in Plate IX. two sketches, taken in the year 1862. Here at once we see that we have something strangely like the Earth. The shaded portions represent water, the lighter ones land, and the bright spot at the top of the drawings is probably snow lying round the south pole of the planet, which was then visible.
257. The two drawings represent the planet as seen in an astronomical telescope, which inverts objects so that the south pole of the planet is shown at top. The upper drawing was made on the 25th of September, the lower one on the 23d. In the upper one a sea is seen on the left, stretching down northwards ; while, joined on to it, as the Mediterranean is joined on to the Atlantic, is a long narrow sea, which widens at its termination.
In the lower drawing this narrow sea is represented on the left. The coast-line on the right strangely reminds one of the Scandinavian peninsula, and the included Baltic Sea.
258. It will be now easy to understand how we have been able to determine the length of the day and the inclination of the axis. We have only to watch how long
308 ASTRONOMY.
it takes one of the spots near the equator of each planet to pass from one side to the other, and the direction it takes, to get at both these facts.
259. Mars not only has land and water and snow like us, but it has clouds and mists, and these have been watched at different times. The land is generally reddish when the planet's atmosphere is clear ; this is due to the ab- sorption of the atmosphere, as is the colour of the setting Sun with us. The water appears of a greenish tinge.
260. Now, if we are right in supposing that the bright portion surrounding the pole be ice and snow, we ought to see it rapidly decrease in the planet's summer. This is actually found to be the case, and the rate at which the thaw takes place is one of the most interesting facts to be gathered from a close study of the planet. In 1862 this decrease was very visible. The summer solstice of Mars occurred on the 3oth of August, and the snow-zone was observed to be smallest on the nth of October, or forty- two of our days after the highest position of the Sun. This very rapid melting may be ascribed to the inclination of the axis, which is greater than with us; to the greater eccentricity of the planet's orbit ; and to the fact that the summer time of the southern hemisphere occurs when the planet is near perihelion,
261. Far a reason that will be easily understood when we come to deal with the effect of the Earth's revolution round the Sun on the apparent positions and aspects of the planets, we sometimes see the north pole, and some- times the south pole of Mars, and sometimes both : when either pole only is visible, the features, which appear to pass across the planet's disc in about twelve hours — that is, half the period of the planet's rotation — describe curves with the concave side towards the visible pole. When both poles are visible they describe straight lines, exactly as in the case of the Sun (Art. 106). These changes enable all
Plate IX.
MARS in 1862.
THE SOLAR S YSTEM. \ \ \
the surface to be seen at different times, and maps of Mars have been constructed, the exact position of the features of the planet being determined by their latitude and longitude, as in the case of the Earth.
262. But although we see in Mars so many things that remind us of our planet, and show us that the extreme temperatures of the two planets are not far from equal, a distinction must be drawn between them. In conse- quence of the great eccentricity of the orbit of Mars, the lengths of the various seasons are not so equal as with us, and, owing to the longer year, they are of much greater extent. In the northern hemisphere of the planet they are as under : —
days. hrs.
Spring lasts .... 191 8
Summer „ . . . . 181 o
Autumn „ .... 149 8
Winter „ . . . . 147 o
As we must reverse the seasons for the southern hemi- sphere, spring and summer, taken together, are 76 days longer in the northern hemisphere than in the southern.
LESSON XX. — THE OTHER PLANETS COMPARED WITH THE EARTH (continued). JUPITER: HIS BELTS AND MOONS. SATURN : GENERAL SKETCH OF HIS
SYSTEM.
263. Let us now pass on to Jupiter, by far the largest planet in the system, and bright enough sometimes, in spite of its great distance, to cast a shadow like Venus. The first glance at the drawing (Plate X. Fig. i) will show us that we have here something very unlike Mars ;
H2 ASTRONOMY.
and this is the case The planet Jupiter is surrounded by an atmosphere so densely laden with clouds, that of the actual planet itself we know nothing.
What are generally known as the belts of Jupiter are dusky streaks which cross a brighter background in direc- tions generally parallel to the planet's equator. And for the most part, the largest belts are situated on either side of it, in exactly the same way as the two belts of Trade-Winds on the Earth lie on either side of the belt of Equatorial Calms and rains. Outside these, again, we get represen- tatives of the Calms of Cancer and Capricorn, although these are not so regularly seen, the portion of the planet's surface polewards of the two belts being liable to great changes of appearance, sometimes in a very short time. The portions of the atmosphere representing the terres- trial calm-belts sometimes exhibit a beautiful rosy tint, the equatorial one especially.
264. The variations of this cloudy atmosphere lend great variety to the appearance of the planet at different times; the belts are sometimes seen in large numbers, and extend almost to the poles. Besides the belts, some- times bright spots, sometimes dark ones, are seen, which have enabled us to determine the period of the planet's rotation, which, as we have seen, is very rapid — so rapid, that on the equator an observer would be carried round at the rate of 467 miles a minute, instead of 17 as on the Earth. We can easily understand that this rapid rotation would break the cloudy surface into belts more than with us, or as is the case of Mars ; in the latter planet, indeed, no trace of cloud-belts has as yet been detected ; their absence is perhaps due to its slow rotation and small size.
265. Although all astronomers do not agree that the surface of the planet is never seen, there are many strong reasons why it should not be seen. In the first place,
JUPITER,
ratten DC- -
SATITRK.
THE SOLAR SYSTEM. 113
Mars and the Earth, whose atmospheres are nearly alike, have nearly the same densities (Art. 145), while, in the case of Jupiter and Saturn — the belts of which latter planet, as far as we can observe them, resemble Jupiter's — the density, as calculated on the idea that what we see is all planet, is only about one-fifth that of the Earth ; and as the density of the Earth is 5^ times that of water, it follows that the densities of the two planets in question are not far off that of water.
266. Now, if we suppose that the apparent volume of Jupiter (and similarly of Saturn) is made up of a large shell of cloudy atmosphere and a kernel of planet, there is no reason why the density of the real Jupiter (and of the real Saturn) should vary very much from that of the Earth or Mars, and this would save us from the water- planet hypothesis. Moreover, a large shell of cloudy atmosphere is precisely what our own planet was most probably enveloped in, in one of the early stages of its history (Art. 208).
267- In addition to the changing features of Jupiter itself, the telescope reveals to us four moons, which as they course along rapidly in their orbits, and as these orbits lie nearly in the plane of the planet's orbit, lend a great additional interest to the picture. In the various positions in their orbits the satellites sometimes appear at a great distance from the primary ; sometimes they
come between us and the Fig l6._Juplter and his Moons planet, appearing now as (general view).
bright and now as dark spots on its surface. At other times they pass between the planet and the Sun, throwing their shadows on the planet's disc, and causing, in fact, eclipses of the Sun. They also enter
T
H4 ASTRONOMY.
into the shadow cast by the planet, and are therefore eclipsed themselves ; and sometimes they pass behind the planet, and are said to be o c c u 1 1 e d. Of these appearances we shall have more to say by and by (Lesson XXXVI).
268. Referring to the sizes of these moons and their distances from the planet, in Table III. of the Appendix, it may be here added that, like our Moon, they rotate on their axes in the same time as they revolve round Jupiter. This has been inferred from the fact that their light varies, and that they are always brightest and dullest in the same positions with regard to Jupiter and the Sun.
269. In Plate X. the black spot is the shadow of a satel- lite, and the satellite itself is seen to the left ; the passage of either a satellite or shadow is called a transit. In a solar eclipse, could we observe it from Venus, we should see a similar spot sweeping over the Earth's surface.
270. We now come to Saturn; and here again, as in the case of Jupiter, we come upon another departure from Mars and the Earth. The planet itself, which is
belted like Jupi- ter, is surrounded not only by eight moons, but by a series of rings, one of which, the inner one, is trans- parent! The belts have been before referred to (Art.
Fig, 17. — Saturn and his Moons (general view). ~£.c\ fV, A
not, therefore, detain us here ; and we may dismiss the satellites — as their distances from Saturn, &c. are given in Table III. of the Appendix — with the remark, that as the equator of Saturn, unlike that of Jupiter, is greatly
THE SOLAR SYSTEM. ill
inclined to the ecliptic; transits, eclipses, and occulta- tions of the satellites, the orbits of which for the most part lie in that plane, happen but rarely.
271. It is to the rings that most of the interest of this planet attaches. We may well imagine how sorely puzzled the earlier observers, with their very imperfect telescopes, were, by these strange appendages. . The planet at first was supposed to resemble a vase ; hence the name Ansce, or handles, given to the rings in certain positions of the planet. It was next supposed to consist of three bodies, the largest one in the middle. The true nature of the rings was discovered by Huyghens in 1655, who announced it in this curious form : — "aaaaaaa ccccc d eeeee g h iiiiiii 1111 mm
nnnnnnnnn oooo pp q rr s ttttt uw.uu," which letters, transposed, read : —
" annulo cingitur, tcniti piano, nusquam cohaerente, ad cclipticam inclinato"
There is nothing more encouraging in the history of astronomy than the way in which eye and mind have bridged over the tremendous gap vvhich separates us from this planet. By degrees the fact that the appearance was due to a ring was determined ; then a separation was noticed dividing the ring into two ; the extreme thinness of the ring came out next, when Sir William Herschel observed the satellites "like pearls strung on a silver thread;" then an American astronomer, Bond, discovered that the number of rings must be multiplied we know not how many fold. Next followed the making out of the transparent ring by Dawes and Bond, in 1852 ; then the transparent ring was discovered to be divided as the whole system had once been thought to be ; last of all comes evidence that the smaller divisions in the various rings are subject to change, and that the ring-system itself is pro- bably increasing in breadth, and approaching the planet.
I 2
ii6 ASTRONOMY.
LESSON XXL— THE OTHER PLANETS COMPARED WITH THE EARTH (continued). DIMENSIONS OF SATURN AND HIS RINGS. PROBABLE NATURE OF THE RINGS. EFFECTS PRODUCED BY THE RINGS ON THE PLANET. URANUS. NEPTUNE: ITS DISCOVERY.
The lower figure of Plate X. will give an idea of the appearance presented by the planet and its strange and beautiful appendage. It will be shown in the sequel (Chap. IV.) how we see, sometimes one surface, and some- times another, of the ring, and how at other times the edge of it is alone visible.
272. The ring-system is situated in the plane of the planet's equator, and its dimensions are as follow : —
Miles.
Outside diameter of outer ring . . . 166,920
Inside „ „ . . . 147,670
Distance from outer to inner ring . . 1,680
Outside diameter of inner ring . . . 144,310
Inside „ „ ... 109,100
Inside „ dark ring . . . 91,780
Distance from dark ring to planet . . 9,760
Equatorial diameter of planet . . . 72,250
So that the breadths of the three principal rings, and of the entire system, are as follo\v : —
Miles.
Outer bright ring 9,625
Inner bright ring 17,605
Dark ring 8,660
Entire system 37,57°
and the distance between the two bright rings is 1,680 miles.
THE SOLA R S YS TEM. 1 1 7
In spite of this enormous breadth, the thickness of the rings is not supposed to exceed 100 miles.
273. Of what, then, are these rings composed ? There is great reason for believing that they are neither solid nor liquid ; and the idea now generally accepted is that they are composed of myriads of satellites, or little bodies, moving independently, each in its own orbit, round the planet ; giving rise to the appearance of a bright ring when they are closely packed together, and a very dim one when they are most scattered. In this way we may account for the varying brightness of the different parts, and for the haziness on both sides of the ring near the planet (shown in Fig. 38 ), which is supposed to be due to the bodies being drawn out of the ring by the attraction of the planet.
274-. Although Saturn appears to resemble Jupiter in its atmospheric conditions, unlike that planet, and like our own Earth, its year, owing to the great inclination of its axis, is sharply divided into seasons, which however are here indicated by something else than a change of tem- perature ; we refer to the effects produced by the presence of the strange ring appendage. To understand these effects, its appearance from the body of the planet must first be considered. As the plane of the ring lies in the plane of the planet's equator, an observer at the equator will only see its thickness, and the ring therefore will put on the appearance of a band of light passing through the east and west points and the zenith. As the ob- server, however, increases his latitude either north or south, the surface of the ring- system will begin to be seen, and it will gradually widen, as in fact the observer will be able to look down upon it ; but as it increases in width it will also increase its distance from the zenith, until in lat 63° it is lost below the horizon, and between this latitude and the poles it is altogether invisible.
n8 ASTRONOMY.
275- Now the plane of the ring always remains parallel to itself, and twice in Saturn's year — that is, in two opposite points of the planet's orbit — it passes through the Sun. It follows, therefore, that during one-half of the revolution of the planet one surface of the rings is lit up, and during the remaining period the other surface. At night, there- fore, in one case, the ring-system will be seen as an illumi- nated arch, with the shadow of the planet passing over it, like the hour-hand over a dial ; and in the other, if it be not lit up by the light reflected from the planet, its position will only be indicated by the entire absence of stars.
276. But if the rings eclipse the stars at night, they can also eclipse the Sun by day. In latitude 40° we have morning and evening eclipses for more than a year, gradually extending until the Sun is eclipsed during the whole day — that is, when its apparent path lies entirely in the region covered by the ring ; and these total eclipses continue for nearly seven years : eclipses of one kind or another taking place for 8 years 292 days. This will give us an idea how largely the apparent phenomena of the heavens, and the actual conditions as to climates and seasons, are influenced by the presence of the ring.
As the year of Saturn is as long as thirty of ours, it follows that each surface of the rings is in turn deprived of the light of the Sun for fifteen years.
277- We have now finished with the planets known to the ancients; the remaining ones, Uranus and Neptune, have been discovered in modern times — the former in 1781, by Sir Wm. Herschel, and the latter in 1846, inde- pendently, by Professor Adams and M. Le Verrier.
278. Both these planets are situated at such enormous distances from the Sun, and therefore from us, that Uranus is scarcely, and Neptune not at all, visible to the naked eye. Owing to this remoteness, nothing is known of their physical peculiarities. We have already stated,
THE SOLAR SYSTEM. 119
ho \vever, that the motion of the satellites of Uranus is in the opposite direction to that of all the other planetary members of the system.
279. The discovery of the planet Neptune is one of the most astonishing facts in the history of Astronomy. As we shall see in the sequel, every body in our system affects the motions of every other body ; and after Uranus had been discovered some time, it was found that on taking all the known causes into account, there was still something affecting its motion ; it was suggested that this something was another planet, more distant from the Sun than Uranus itself. And the question was, where was this planet, if it existed ?
When we come to consider the problem in all its grandeur, we need not be surprised that two minds, who felt themselves competent to solve it, should have inde- pendently undertaken it. As far back as July 1841, we find Mr. Adams determined to investigate the irregularities of Uranus: early in September 1846, the new planet had fairly been grappled. We find Sir John Herschel remarking, " We see it as Columbus saw America from the shores of Spain. Its movements have been felt trembling along the far-reaching line of our analysis with a certainty hardly inferior to ocular demonstration."
On the 29th July, 1846, the large telescope of the Cam- bridge Observatory was first employed to search for the planet in the place where Professor Adams's calculations had assigned it. M. Le Verrier, in September, wrote to the Berlin observers, stating the place where his calcula- tion led him to believe it would be found : his theoretical place and Professor Adams's being not a degree apart. At Berlin, thanks to their star-maps, which had not yet been published, Dr. Galle found the planet the same evening, very near the position assigned to it by both Astronomers.
120 ASTRONOMY.
LESSON XXII.— THE ASTEROIDS, OR MINOR PLANETS. BODE'S-LAW. SIZE OF THE MINOR PLANETS: THEIR ORBITS : HOW THEY ARE OBSERVED.
28O. If we write down —
o 3 6 12 24 48 96
and add 4 to each, we get
4 7 10 16 28 52 100
and this series of numbers represents very nearly the distances of the ancient planets from the Sun, as fol- lows i—- Mercury, Venus, Earth, Mars, — , Jupiter, Saturn.
This singular connexion was discovered by Titius, and is known by the name of Bode's-law, We see that the fifth term has apparently no representative among the planets. This fact acted so strongly on the imagination of Kepler that he boldly placed an undiscovered one in the gap. Up to the time of the discovery of Uranus the undiscovered planet did not reveal itself: when it was found, however, that the actual position of Uranus was very well represented by the next term of the series, 196, it was determined to make an organized search for it, and for this purpose a society of astronomers was formed ; the zodiac was divided into 24 zones, each zone being confided to a member of the society. On the first day of the present century a planet was discovered and named Ceres, which, curiously enough, filled up the gap. But the discovery of a second, third, and fourth, named respec- tively Pallas, Juno, and Vesta, soon followed, and up to the present time (February 1868) no less than 97 of
THE SOLAR SYSTEM. 121
these little bodies have been detected. A list of them, with their symbols, will be found in the Appendix, Table I.
281. None of these planets, except occasionally Ceres and Vesta, can be seen by the naked eye ; and this brings ir. at once to their chief characteristic — the largest minor planet is but 228 miles in diameter, and many of the smaller ones are less than 50.
282. The orbits of those hitherto discovered, for the most part, lie nearer to Mars than Jupiter, and the orbits in some cases are so elliptical, that if we take the extreme distances into account, they occupy a zone 240,000,000 miles in width — the distance between Mars and Jupiter being 336,000,000. The planet nearest the Sun