CHAPTER XIII.

THE NINETEENTH CENTURY, §§ 272-320 354-409

§ 272. The three chief divisions of astronomy, observational, gravitational, and descriptive 354

§ 273. The great growth of descriptive astronomy in the nineteenth century 355

§ 274. =Observational Astronomy.= Instrumental advances: the introduction of photography 357

§ 275. The method of least squares: Legendre and Gauss 357

§ 276. Other work by Gauss: the Theoria Motus: rediscovery of the minor planet Ceres 358

§ 277. Bessel: his improvement in methods of reduction: his table of refraction: the Fundamenta Nova and Tabulae Regiomontanae 359

§ 278. The parallax of 61 Cygni: its distance 360

§ 279. Henderson’s parallax of α Centauri and Struve’s of Vega: later parallax determinations 362

§ 280. Star catalogues: the photographic chart 362

§§ 281-4. The distance of the sun: transits of Venus: observations of Mars and of the minor planets in opposition: diurnal method: gravitational methods, lunar and planetary: methods based on the velocity of light: summary of results 363

§ 285. Variation in latitude: rigidity of the earth 367

§ 286. =Gravitational Astronomy.= Lunar theory: Damoiseau, Poisson, Pontécoulant, Lubbock, Hansen, Delaunay, Professor Newcomb, Adams, Dr. Hill 367

§ 287. Secular acceleration of the moon’s mean motion: Adams’s correction of Laplace: Delaunay’s explanation by means of tidal friction 369

§ 288. Planetary theory: Leverrier, Gyldén, M. Poincaré 370

§ 289. The discovery of Neptune by Leverrier and Dr. Galle: Adams’s work 371

§ 290. Lunar and planetary tables: outstanding discrepancies between theory and observation 372

§ 291. Cometary orbits: return of Halley’s comet in 1835: Encke’s and other periodic comets 372

§ 292. Theory of tides: analysis of tidal observations by Lubbock, Whewell, Lord Kelvin, and Professor Darwin: bodily tides in the earth and its rigidity 373

§ 293. The stability of the solar system 374

§ 294. =Descriptive Astronomy.= Discovery of the minor planets or asteroids: their number, distribution, and size 376

§ 295. Discoveries of satellites of Neptune, Saturn, Uranus, Mars, and Jupiter, and of the crape ring of Saturn 380

§ 296. The surface of the moon: rills: the lunar atmosphere 382

§ 297. The surfaces of Mars, Jupiter, and Saturn: the canals on Mars: Maxwell’s theory of Saturn’s rings: the rotation of Mercury and of Venus 383

§ 298. The surface of the sun: Schwabe’s discovery of the periodicity of sun-spots: connection between sun-spots and terrestrial magnetism: Carrington’s observations of the motion and distribution of spots: Wilson’s theory of spots 385

§§ 299-300. Spectrum analysis: Newton, Wollaston, Fraunhofer, Kirchhoff: the chemistry of the sun 386

§ 301. Eclipses of the sun: the corona, chromosphere, and prominences: spectroscopic methods of observation 389

§ 302. Spectroscopic method of determining motion to or from the observer: Doppler’s principle: application to the sun 391

§ 303. The constitution of the sun 392

§§ 304-5. Observations of comets: nucleus: theory of the formation of their tails: their spectra: relation between comets and meteors 393

§§ 306-8. Sidereal astronomy: career of John Herschel: his catalogues of nebulae and of double stars: the expedition to the Cape: measurement of the sun’s heat by Herschel and by Pouillet 396

§ 309. Double stars: observations by Struve and others: orbits of binary stars 398

§ 310. Lord Rosse’s telescopes: his observations of nebulae: revival of the “island universe” theory 400

§ 311. Application of the spectroscope to nebulae: distinction between nebulae and clusters 401

§ 312. Spectroscopic classification of stars by Secchi: chemistry of stars: stars with bright-line spectra 401

§§ 313-4. Motion of stars in the line of sight. Discovery of binary stars by the spectroscope: eclipse theory of variable stars 402

§ 315. Observations of variable stars 403

§ 316. Stellar photometry: Pogson’s light ratio: the Oxford, Harvard, and Potsdam photometries 403

§ 317. Structure of the sidereal system: relations of stars and nebulae 405

§§ 318-20. Laplace’s nebular hypothesis in the light of later discoveries: the sun’s heat: Helmholtz’s shrinkage theory. Influence of tidal friction on the development of the solar system: Professor Darwin’s theory of the birth of the moon. Summary 406

LIST OF AUTHORITIES AND OF BOOKS FOR STUDENTS 411

INDEX OF NAMES 417

GENERAL INDEX 425

LIST OF ILLUSTRATIONS.

FIG. PAGE The moon Frontispiece

1. The celestial sphere 5

2. The daily paths of circumpolar stars To face p. 8

3. The circles of the celestial sphere 9

4. The equator and the ecliptic 11

5. The Great Bear To face p. 12

6. The apparent path of Jupiter 16

7. The apparent path of Mercury 17

8-11. The phases of the moon 30, 31

12. The curvature of the earth 32

13. The method of Aristarchus for comparing the distances of the sun and moon 34

14. The equator and the ecliptic 36

15. The equator, the horizon, and the meridian 38

16. The measurement of the earth 39

17. The eccentric 44

18. The position of the sun’s apogee 45

19. The epicycle and the deferent 47

20. The eclipse method of connecting the distances of the sun and moon 50

21. The increase of the longitude of a star 52

22. The movement of the equator 53

23, 24. The precession of the equinoxes 53, 54

25. The earth’s shadow 57

26. The ecliptic and the moon’s path 57

27. The sun and moon 58

28. Partial eclipse of the moon 58

29. Total eclipse of the moon 58

30. Annular eclipse of the sun 59

31. Parallax 60

32. Refraction by the atmosphere 63

33. Parallax 68

34. Jupiter’s epicycle and deferent 70

35. The equant 71

36. The celestial spheres 89

PORTRAIT OF COPPERNICUS To face p. 94

37. Relative motion 102

38. The relative motion of the sun and moon 103

39. The daily rotation of the earth 104

40. The solar system according to Coppernicus 107

41, 42. Coppernican explanation of the seasons 108, 109

43. The orbits of Venus and of the earth 113

44. The synodic and sidereal periods of Venus 114

45. The epicycle of Jupiter 116

46. The relative sizes of the orbits of the earth and of a superior planet 117

47. The stationary points of Mercury 119

48. The stationary points of Jupiter 120

49. The alteration in a planet’s apparent position due to an alteration in the earth’s distance from the sun 122

50. Stellar parallax 124

51. Uraniborg 133

52. Tycho’s system of the world 137

PORTRAIT OF TYCHO BRAHE To face p. 139

53. One of Galilei’s drawings of the moon „ 150

54. Jupiter and its satellites as seen on January 7, 1610 152

55. Sun-spots To face p. 154

56. Galilei’s proof that sun-spots are not planets 156

57. The differential method of parallax 165

PORTRAIT OF GALILEI To face p. 171

58. The daily libration of the moon 173

PORTRAIT OF KEPLER To face p. 183

59. An ellipse 185

60. Kepler’s second law 186

61. Diagram used by Kepler to establish his laws of planetary motion 187

62. The “music of the spheres” according to Kepler 190

63. Kepler’s idea of gravity 196

64. Saturn’s ring, as drawn by Huygens To face p. 200

65. Saturn, with the ring seen edge-wise „ 200

66. The phases of Saturn’s ring 201

67. Early drawings of Saturn To face p. 202

68. Mars in opposition 206

69. The parallax of a planet 206

70. Motion in a circle 214

71. The moon as a projectile 220

72. The spheroidal form of the earth 234

73. An elongated ellipse and a parabola 238

PORTRAIT OF NEWTON To face p. 240

PORTRAIT OF BRADLEY „ 258

74, 75. The aberration of light 262, 263

76. The aberrational ellipse 264

77. Precession and nutation 268

78. The varying curvature of the earth 277

79. Tobias Mayer’s map of the moon To face p. 282

80. The path of Halley’s comet 294

81. A varying ellipse 303

PORTRAIT OF LAGRANGE To face p. 305

PORTRAIT OF LAPLACE „ 307

PORTRAIT OF WILLIAM HERSCHEL „ 327

82. Herschel’s forty-foot telescope 329

83. Section of the sidereal system 333

84. Illustrating the effect of the sun’s motion in space 345

85. 61 Cygni and the two neighbouring stars used by Bessel 360

86. The parallax of 61 Cygni 361

87. The path of Halley’s comet 373

88. Photographic trail of a minor planet To face p. 377

89. Paths of minor planets 378

90. Comparative sizes of three minor planets and the moon 379

91. Saturn and its system 380

92. Mars and its satellites 381

93. Jupiter and its satellites 382

94. The Apennines and the adjoining regions } To face p. 383 of the moon }

95. Saturn and its rings „ 384

96. A group of sun-spots „ 385

97. Fraunhofer’s map of the solar spectrum „ 387

98. The total solar eclipse of 1886 „ 390

99. The great comet of 1882 „ 393

100. The nebula about η Argus „ 397

101. The orbit of ξ Ursae 399

102. Spiral nebulae To face p. 400

103. The spectrum of β Aurigae „ 403

104. The Milky Way near the cluster in Perseus „ 405

A SHORT HISTORY OF ASTRONOMY.