Popular astronomy

Front matter

THE GREAT TELESCOPE OF THE UNITED STATES NAVAL OBSERVA- TORY, WASHINGTON.

CONSTRUCTED BY ALVAN CLARK AND SONS, 1873.

POPULAR ASTRONOMY.

Preface

To prevent a possible misapprehension in scientific quarters, the author desires it understood that the present work is not designed either to instruct the professional investigator or to train the special student of astronomy. Its main object is to present the general reading public with a condensed view of the history, methods, and results of astronomical research, especially in those fields which are of most popular and philosophic interest at the present day, couched in such language as to be intelligible without mathematical study. He hopes that the earlier chapters will, for the most part, be readily understood by any one having clear geometrical ideas, and that the later ones will be intelligible to all. To diminish the difficulty which the reader may encounter from the unavoidable occasional use of technical terms, a Glossary has been added, including, it is believed, all that are used in the present work, as well as a number of others which may be met with elsewhere.

Respecting the general scope of the work, it may be said that the historic and philosophic sides of the subject have been treated with greater fulness than is usual in works of this character, while the purely technical side has been proportionately condensed. Of the four parts into which it is divided, the first two treat of the methods by which the mo-

VI PREFACE.

tions and the mutual relations of the heavenly bodies have been investigated, and of the results of such investigation, while in the last two the individual peculiarities of those bodies are considered in greater detail. The subject of the general structure and probable development of the universe, which, in strictness, might be considered as belonging to the first part, is, of necessity, treated last of all, because it requires all the light that can be thrown upon it from every available source. Matter admitting of presentation in tabular form has, for the most part, been collected in the Appendix, where will be found a number of brief articles for the use of both the general reader and the amateur astronomer.

The author has to acknowledge the honor done him by several eminent astronomers in making his work more complete and interesting by their contributions. Owing to the great interest which now attaches to the question of the constitution of the sun, and the rapidity with which our knowledge in this direction is advancing, it was deemed desirable to present the latest views of the most distinguished investigators of this subject from their own pens. Four of these gentlemen — Kev. Father Secchi, of Kome ; M. Faye, of Paris ; Professor Young, of Dartmouth College ; and Professor Langley, of Allegheny Observatory — have, at the author's request, presented brief expositions of their theories, which will be found in their own language in the chapter on the sun.

An Addendum gives the basis of the remarkable modification of the theory of the solar spectrum proposed by Dr. Henry Draper, which appeared while the sheets were passing through the press.

Preface To The Second Edition

The great favor with which this work has been received by the public has led the author and publishers to give it a thorough revision, and to add the latest results of astronomical research. The new subjects treated comprise the results of Dr. Draper's discoveries in the solar spectrum, of Janssen's new methods of photographing the sun, and of the British observations of the late transit of Venus ; Professor Hall's discovery of the satellites of Mars, and the results of recent investigations of the motion of the moon. Alterations and additions have been made wherever necessary to bring the work fully up to date, and several errors which crept into the first edition have been corrected. It is hoped that the general reader will now find in it the fullest practicable explanation of every branch of the subject which can interest him.

Washington, Junev 1878.

Part I. The System Of The World Historically Developed

PAGE

Introduction 1

Chapter I

The Ancient Astronomy, or the Apparent Motions of the Heavenly Bodies 7

§ 1. The Celestial Sphere 7

§2. The Diurnal Motion 9

§3. Motion of the Sun among the Stars , 13

§4. Precession of the Equinoxes. — The Solar Year 19

§ 5. The Moon's Motion 21

Chapter II

The Copernican System, or the True Motions of the Heavenly.

Bodies 51

§ 2. Obliquity of the Ecliptic ; Seasons, etc. ; on the Copernican System Gl

§3. Tycho Brahe 6G

§4. Kepler. — His Laws of Planetary Motion 68

§5. From Kepler to Newton 71

X CONTENTS.

CHAPTER III. 1-agb Universal Gravitation 74

§ 2. Gravitation of Small Masses. — Density of the Earth 81

§3. Figure of the Earth 86

§5. The Tides 90

§ 6. Inequalities in the Motions of the Planets produced by their

Mutual Attraction 93

§ 7. Relation of the Planets to the Stars 103

Part II. Practical Astronomy

Introductory Remarks 105

CHAPTER I. The Telescope 108

§ 1. The First Telescopes 108

§2. The Achromatic Telescope 116

§3. The Mounting of the Telescope 120

§4. The Reflecting Telescope 123

§ 5. The Principal Great Reflecting Telescopes of Modern Times... 127

§6. Great Refracting Telescopes 137

§ 7. The Magnifying Powers of the Two Classes of Telescopes 141

CHAPTER II. Application of the Telescope to Celestial Measurements 148

§ 1. Circles of the Celestial Sphere, and their Relations to Positions

on the Earth 148

§2. The Meridian Circle, and its Use 154

§4. Mean, or Clock, Time 164

CONTENTS. xi

CHAPTER II. The Sun 241

§3. Periodicity of the Spots 2.54

§4. Law of Rotation of the Sun 25~>

§6. Physical Constitution of the Sun 2G4

§ 7. Views of Distinguished Students of the Sun on the Subject of

its Physical Constitution 271

Xll CONTENTS.

CHAPTER III. PAGE The Inner Group of Planets > 289

§1. The Planet Mercury ........ 289

§2. The Supposed Intra-Mercurial Planets 292

§6. The Planet Mars 326

CHAPTER IV. The Outer Group or Planets » 339

§1. The Planet Jupiter ? 339

§2. The Satellites of Jupiter 344

§8. Saturn and its System, Physical Aspect, Belts, Rotation 346,

§5. Constitution of the Ring 357

§6. The Satellites of Saturn 359

§7. Uranus and its Satellites 361

CHAPTER V. Comets and Meteors 373

§1. Aspects and Forms of Comets 373'

§ 2. Motions, Origin, and Number of Comets 377

§4. Encke's Comet, and the Resisting Medium 389

§5. Meteors and Shooting-stars 392

§6. Relations of Comets and Meteoroids 399

§ 7. The Physical Constitution of Comets 406

§8. The Zodiacal Light 413

PAET IV.

THE STELLAR UNIVERSE.

Introductory Remarks 415

CONTENTS. xiii

CHAPTER I. PAOT The Stars as they ark Seen 418

§ 1. Number and Orders of Stars and Nebula? 418

§2. Description of the Principal Constellations 425

§4. Double Stars 444

CHAPTER II. The Structure of the Universe 468

§ 1. Views of Astronomers before Herschel 469

§3. Probable Arrangement of the Visible Universe 486

§4. Do the Stars really form a System? 491

CHAPTER III. The Cosmogony •. 499

§ 1. The Modern Nebular Hypothesis 501

§2. Progressive Changes in our System 507

§3. The Sources of the Sun's Heat 513

§4. Secular Cooling of the Earth 519

§5. General Conclusions respecting the Nebular Hypothesis 522

§6. The Plurality of Worlds 524

Appendix

I. List of the Principal Great Telescopes of the World 529

II. List of the more Remarkable Double Stars 531

III. List of the more Interesting and Remarkable Nebulae and

Star Clusters 533

IV. Periodic Comets seen at more than One Return 535

XIV CONTENTS.

TAGE

V. Elements of the Orbits of the Eight Major Planets for 1850. 536

Elements of the Satellites of Jupiter 537

Elements of the Satellites of Saturn 537

Elements of the Satellites of Mars 537

Elements of the Satellites of Uranus 537

Elements of the Satellite of Neptune 537

VI. Elements of the Small Planets 538

VII. Determinations of Stellar Parallax 543

VIII. Synopsis of Papers on the Solar Parallax, 1854-77 546

IX. List of Astronomical Works, most of which have been consulted as Authorities in the Preparation of the Present

"Work 550

X. Glossary of Technical Terms of Frequent Occurrence in

Astronomical Works 557

Index 567

Explanation of the Star Maps 572

LIST OF ILLUSTRATIONS.

FIG. PAGi:

This Great Telescope of the United States Naval Observatory, Washington Frontispiece

1. Section of the Imaginary Celestial Sphere 8

2. Map illustrating the Diurnal Motion round the Pule 10

3. The Celestial Sphere and Diurnal Motion 12

4. Motion of the Sun past the Star Regulus 15

5. Showing the Sun to be farther than the Moon 22

6. Annular Eclipse of the Sun 26

7. Partial Eclipse of the Sun 2G

8. Eclipse of the Sun, the Shadow of the Moon falling on the

Earth 2G

9. Eclipse of the Moon, in the Shadow of the Earth . ... 27

10. Showing the Apparent Orbit of a Planet 38

11. Apparent Orbits of Jupiter and Saturn 39

12. Arrangement of the Seven Planets in the Ptolemaic System... 41

13. The Eccentric 42

14. Showing the Astrological Division of the Seven Planets

among the days of the week 46

15. Apparent Annual Motion of the Sun explained 55

1G. Showing how the Apparent Epicyclic Motion of the Planets

is accounted for 56

17. Relation of the Terrestrial and Celestial Poles and Equators. 62

18. Causes of Changes of' Seasons on the Copernican System 63

19. Enlarged View of the Earth, showing Winter in the North-

ern Hemisphere, and Summer in the Southern 6.">

20. Illustrating Kepler's First Two Laws of Planetary- Motion... 69

21. Illustrating the Fall of the Moon towards the Earth 78

22. Baily's Apparatus for determining the Density of the Earth. 83

23. View of Baily's Apparatus 84

24. Diagram illustrating the Attraction of Mountains 85

XVI LIST OF ILLUSTRATIONS.

FIG. PAGE

26. Attraction of the Moon tending to produce Tides 91

27. Armillary Sphere as described by Ptolemy 107

28. The Galilean Telescope 110

29. Formation of an Image by a Lens: Ill

30. Great Telescope of the Seventeenth Century 114

31. Refraction through a Compound Prism 116

32. Section of an Achromatic Objective 117

33. Section of Eye-piece of a Telescope 120

34. Mode of Mounting a Telescope 121

35. Speculum Bringing Rays to a Single Focus by Reflection 124

36. Herschelian Telescope 125

37. Horizontal Section of a Newtonian Telescope 125

38. Section of the Gregorian Telescope 126

39. Herschel's Great Telescope 129

40. Lord Rosse's Great Telescope 132

41. Mr. Lassell's Great Pour-foot Reflector 134

42. The New Paris Reflector 136

43. The Great Melbourne Reflector 138

44. Circles of the Celestial Sphere 149

45. The Washington Transit Circle 155

46. Spider Lines in Field of View of a Meridian Circle 156

47. Diagram illustrating Parallax 167

48. Diagram illustrating Parallax 168

49. Variation of Parallax with the Altitude 169

50. Apparent Paths of Venus across the Sun 178

51. Venus approaching Internal Contact on the Face of the Sun. 180

52. Internal Contact of Limb of Venus with that of the Sun.... 180

53. The Black Drop, or Ligament 181

54. Method of Photographing the Transit of Venus 188

55. Artificial Transit of Venus 190

56. Map of the Earth, showing the Areas of Visibility of the

Transit of 1874 193

57. Map of the World, showing the Regions in which the Tran-

sit of Venus will be visible on December 6th, 1882 197

58. Effect of Stellar Parallax 204

59. Aberration of Light 214

60. Revolving Wheel for measuring the Velocity of Light 218

61. Illustrating Foucault's Method of measuring the Velocity

of Light 220

62. Course of Rays through a Spectroscope 226

LIST OF ILLUSTRATIONS. xvii

fia.

63. Relative Size of Sun and Planets

64. Orbits of the Planets from the Earth outward 240

64a. Aspect of the Sun's Surface as Photographed by Janssen at

the Observatory of Meudon i'H

65. Man holding Telescope, to show Sun on Screen lM'.i

66. Solar Spot, after Secchi 250

67. Changes in the Aspect of a Solar Spot as it crosses the Sun's

Disk 252

68. Total Eclipse of the Sun, as seen at Des Moines, Iowa, Au-

69. Specimens of Solar Protuberances, as drawn by Secchi 262

70. The Sun, with its Chromosphere and Red Flames, on July

71. Illustrating Secchi's Theory of Solar Spots 275

72. Solar Spot, after Langley 287

73. Orbits of the Four Inner Planets, illustrating the Eccen-

tricity OF THOSE OF MERCURY AND MARS 289

74. Phases of Venus 297

75. Showing the Thickness of the Earth's Crust 305

76. Distribution of Auroras 308

77. View of Aurora 309

78. Spectrum of Two of the Great Auroras of 1871 311

79. Relative Size of Earth and Moon 312

80. View of Moon near the Third Quarter 319

81. Lunar Crater "Copernicus" 321

82. The Planet Mars on June 23d, 1875 328

83. Map of Mars 328

84. Northern Hemisphere of Mars 329

85. Southern Hemisphere of Mars .°,2'.t

85a. Apparent Orbits of the Satellites of Mars in 1877, as observed AND LAID DOWN BY PROFESSOR HALL 331

86. Jupiter, as seen with the Great Washington Telescope, March

87. View of Jupiter, as seen in Lord Rosse's Great Telescope,

February 27th, 1861 341

88. View of Saturn and his Rings 347

89. Specimens of Drawings of Saturn by Various Observers 351

90. Views of Encke's Comet in 1871 375

91. Head of Donati's Great Comet of 1858 37fi>

92. Parabolic and Elliptic Orbit of a Comet 378

9,

XV111 LIST OF ILLUSTRATIONS.

FIG. PAGE

93. Orbit or Halley's Comet 385

94. Great Comet op 1858 388

95. Meteor Paths, illustrating the Radiant Point 398

96. Orbit of November Meteors and the Comet of 1861 402

97. Orbit of the Third Comet of 1862 403

98. Measure of Position Angle of Double Star 446

99. Distance of Components of Double Star 446

100. Diagram to illustrate Position Angle 446

101. Telescopic View of the Pleiades 450

102. Cluster of 47 Toucani 452

103. Cluster w Centauri 452

104. The Great Nebula of Orion 454

105. The Annular Nebula in Lyra 456

106. The Omega Nebula : 458

107. Nebula Herschel 3722 459

108. The Looped Nebula; Herschel 2941 459

109. Herschel's View of the Form of the Universe 477

110. Illustrating Herschel's Orders of Distance of the Stars.... 479

111. Probable Arrangement of the Stars and Nebulae Visible

with the Telescope 489

112. Diagram illustrating Elliptic Elements of a Planet 560

STAK MAPS.

Map I. — The Northern Constellations within 50° "• of the Pole

" II. — Southern Constellations Visible in Autumn and Winter

" III. — Southern Constellations Visible in Winter and Spring

" IV. — Southern Constellations Visible in Spring and Summer

" V. — Southern Constellations Visible in Summer and Autumn J

► At End of Book.

POPULAR ASTRONOMY.

PART I. — TEE SYSTEM OF TEE WORLD HISTORICALLY DEVELOPED.

Introduction

Astronomy is the most ancient of the physical sciences, being distinguished among them by its slow and progressive development from the earliest ages until the present time. In no other science has each generation which advanced it been so much indebted to its predecessors for both the facts and the ideas necessary to make the advance. The conception of a globular and moving earth pursuing her course through the celestial spaces among her sister planets, which we see as stars, is one to the entire evolution of which no one mind and no one age can lay claim. It was the result of a gradual process of education, of which the subject was not an individual, but the human race. The great astronomers of all ages have built upon foundations laid by their predecessors; and when we attempt to search out the first founder, we find ourselves lost in the mists of antiquity. The theory of universal gravitation was founded by Newton upon the laws of Kepler, the observations and measurements of his French contemporaries, and the geometry of Apollonius. Kepler used as his material the observations of Tycho Brahe, and built upon the theory of Copernicus. When we seek the origin of the instruments used by Tycho, we soon find ourselves among the mediaeval Arabs. The discovery of the true system of the world by Copernicus was only possible by a careful study of the laws of apparent motion of the planets as expressed in the epicycles of Ptolemy and Hipparchus. Indeed, the more carefully one studies the great work of Copernicus, the more surprised he will be to find how completely Ptolemy furnished him both ideas and material. If we seek the teachers and predecessors of Hipparchus, we find only the shadowy forms of Egyptian and Babylonian priests, whose names and writings are all entirely lost. In the earliest historic ages, men knew that the earth was round ; that the sun appeared to make an annual revolution among the stars; and that eclipses were caused by the moon entering the shadow of the earth, or the earth that of the moon.

Indeed, each of the great civilizations of the ancient world seems to have had its own system of astronomy strongly marked by the peculiar character of the people among whom it was found. Several events recorded in the annals of China show that the movements of the sun and the laws of eclipses were studied m that country at a very early age. Some of these events must be entirely mythical ; as, for instance, the despatch of astronomers to the four points of the compass for the purpose of determining the equinoxes and solstices. But there is another event which, even if we place it in the same category, must be regarded as indicating a considerable amount of astronomical knowledge among the ancient Chinese. We refer to the tragic fate of Hi and Ho, astronomers royal to one of the ancient emperors of that people. It was part of the duty of these men to carefully study the heavenly movements, and give timely warning of the approach of an eclipse or other remarkable phenomenon. But, neglecting this duty, they gave themselves up to drunkenness and riotous living. In consequence, an eclipse of the sun occurred without any notice being given ; the religious rites due in such a case were not performed, and China was exposed to the anger of the gods. To appease their wrath, the unworthy astronomers were seized and summarily executed by royal command. Some historians have

INTRODUCTION. 3

gone so far as to fix the date of this occurrence, which is variously placed at from 2128 to 2159 years before the Christian era. If this is correct, it is the earliest of which profane history has left us any record.

In the Hindoo astronomy we see the peculiarities of the contemplative Hindoo mind strongly reflected. Here the imagination revels in periods of time which, by comparison, dwarf even the measures of the celestial spaces made by modern astronomers. In this, and in perhaps other ancient systems, we find references to a supposed conjunction of all the planets 3102 -years before the Christian era. Although we have every reason for believing that this conjunction was learned, not from any actual record of it, but by calculating back the position of the planets, yet the very fact that they were able to make this calculation shows that the motions of the planets must have been observed and recorded during many generations, either by the Hindoos themselves, or some other people from whom they acquired their knowledge. As a matter of fact, we now know from our modern tables that this conjunction was very far from being exact: but its error could not be certainly detected by the rude observations of the times in question.

Among a people so prone as the ancient Greeks to speculate upon the origin and nature of things, while neglecting the observation of natural phenomena, we cannot expect to find anything that can be considered a system of astronomy. But there are some ideas attributed to Pythagoras which are so frequently alluded to, and so closely connected with the astronomy of a subsequent age, that we may give them a passing mention. He is said to have taught that the heavenly bodies wTere set in a number of crystalline spheres, in the common centre of which the earth was placed. In the outer of these spheres were set the thousands of fixed stars which stud the firmament, while each of the seven planets had its own sphere. The transparency of each crystal sphere was perfect, so that the bodies set in each of the outer spheres were visible through all the inner ones. These spheres all rolled round on each other in a daily revolution, thus causing the rising and setting of the heavenly bodies. This rolling of the spheres on each other made a celestial music, the "music of the spheres," which filled the firmament, but was of too elevated a character to be heard by the ears of mortals.

It must be admitted that the idea of the stars being set in a hollow sphere of crystal, forming the vault of the firmament, was a very natural one. They seemed to revolve around the earth every day, for generation after generation, without the slightest change in their relative positions. If there were no solid connection between them, it does not seem possible that a thousand bodies could move around their vast circuit for such long periods of time without a single one of them varying its distance from one of the others. It is especially difficult to conceive how they could all move around the same axis. But when they are all set in a solid sphere, every one is made secure in its place. The planets could not be set in the same sphere, because they change their positions among the stars. This idea of the sphericity of the heavens held on to the -minds of men with remarkable tenacity. The fundamental proposition of the system, both of Ptolemy and Copernicus, was that the universe is spherical, the latter seeking to prove the naturalness of the spherical form by the analogy of a drop of water, although the theory served him no purpose whatever. Faint traces of the idea are seen here and there in Kepler, with whom it vanished from the mind of the race, as the image of Santa Clans disappears from the mind of the growing child.

Pythagoras is also said to have taught in his esoteric lectures that the sun was the real centre of the celestial movements, and that the earth and planets moved around it, and it is this anticipation of the Copernican system which constitutes his greatest glory. But he never thought proper to make a public avowal of this doctrine, and even presented it to his disciples somewhat in the form of an hypothesis. It must also be admitted that the accounts of his system which have reached us are so vague and so filled with metaphysical specn-

INTRODUCTION. 5

lation that it is questionable whether the frequent application of his name to the modern system is not more pedantic than justifiable.

The Greek astronomers of a later age not only rejected the vague speculations of their ancestors, but proved themselves the most careful observers of their time, and first made astronomy worthy the name of a science. From this Greek astronomy the astronomy of our own time may be considered as coming by direct descent. Still, were it not for the absence of historic records, we could probably trace back both their theories and their system of observation to the plains of Chaldea. The zodiac was mapped out and the constellations named many centuries before they commenced their observations, and these works marked quite an advanced stage of development. This prehistoric knowledge is, however, to be treated by the historian rather than the astronomer. If we confine ourselves to men whose names and whose labors have come down to us, we must concede to Hipparchus the honor of being the father of astronomy. Not only do his observations of the heavenly bodies appear to have been far more accurate than those of any of his predecessors, but he also determined the laws of the apparent motions of the planets, and prepared tables by which these motions could be calculated. Probably he was the first propounder of the theory of epicyclic motions of the planets, commonly called after the name of his successor, Ptolemy, who lived three centuries later.

Commencing with the time of Hipparchus, the general theory of the structure of the universe, or " s}Tsfem of the world," as it is frequently called, exhibits three great stages of development, each stage being marked by a system quite different from the other two in its fundamental principles. These are:

1. The so-called Ptolemaic system, which, however, really belongs to Hipparchus, or some more ancient astronomer. In this system the motion of the earth is ignored, and the apparent motions of the stars and planets around it are all regarded as real.

2. The Copernican system, in which it is shown that the sun is really the centre of the planetary motions, and that the earth is itself a planet, both turning on its axis and revolving round the sun.

3. The Newtonian system, in which all the celestial motions are explained by the one law of universal gravitation.

This natural order of development shows the order in which a knowledge of the structure of the universe can be most clearly presented to the mind of the general reader. We shall therefore explain this structure historically, devoting a separate chapter to each of the three stages of development which we have described. We commence witli what is well known, or, at least, easily seen by every one who will look at the heavens with sufficient care. We imagine the observer out-of-doors on a starlit night, and showT him how the heavenly bodies seem to move from hour to hour. Then, we show him what changes he will see in their aspects if he continues his watch through months and years. By combining the apparent motions thus learned, he forms for himself the ancient, or Ptolemaic, system of the world. Having this system clearly in mind, the passage to that of Copernicus is but a step. It consists only in showing that certain singular oscillations which the sun and planets seem to have in common are really due to a revolution of the earth around the sun, and that the apparent daily revolution of the celestial sphere arises from a rotation of the earth on its own axis. The laws of the true motions of the planets being perfected by Kepler, they are shown by Newton to be included in the one law of gravitation towards the sun. Such is the course of thought to which we first invite the reader.

THE CELESTIAL SPHERE.