Nebulae, classification of, 28 Nebulae, distance and size of, 31, 71, 72 extra-galactic, 28, 30, 70, 73, 82, 204 ff. motions of, 82 rotation of, 205 weights of, 71, 204, 208, 215 _M_ 31, _see_ Andromeda, Great Nebula in _M_ 33, 71, 210 and Plate XX (p. 213) N.G.C. 4594, 71, 72, 204, 208 and Plate XV (p. 204) Nebulae, galactic, 29 and Plates III (p. 29), VI (p. 37), VII (p. 44) Nebulae, planetary, 29, 273, 321, 323 and Plate II (p. 28) Nebulae, spiral, 30, 236 Nebular hypothesis of Laplace, 230, 231, 232 ff. Neptune, 16, 17, 18, 19 birth of, 240, 243, 244 rotation of, 225 satellite of, 240, 243 New stars (novae), 61, 351 Newton, Sir Isaac, (cosmogony), 197, 203 (distances of stars), 33 (law of gravitation), 44, 45, 46, 161, 197 (light and optics), 50, 105 Nicholas of Cusa, 3 Nishina, Y., 146, 148 Novae, 61, 351 Nucleus, atomic, _see_ Atomic ο Ceti, 58, 272, 280 ο₂ Eridani, 268, 278, 280 _Ο_-type stars, 51, 321, 323 Obscuring matter in galactic system, 24, 30 Oort, J. H., 67, 84, 214 Opacity of stellar matter, 309 Orbit, of binary systems, 47, 167 ff., 224 ff. of earth, 34, 36, 227 of moon, 34, 43, 226, 250 of planets, 45, 46, 237, 239, 245 of stars, 211, 213, 216 Oresme, 2 Orion, constellation of, 32, 39, 173 Paneth, F., 247 Parallactic measures of stellar distances, 33, 61, 63 Parallactic motion, defined, 33 Parallax, spectroscopic, 62, 303 Pearce’s star (H.D. 1337), 49, 54, 317 Period-luminosity law (Cepheid variables), 59-61 Perrin, J., 159, 184 Perseid shooting-stars, 251 Phases of moon and inner planets, 4, 5 Philolaus, 2 Phosphoros, 17 Photo-electric action, 128 Photography in astronomy, 37 Piazzi, G., 17 Planck, M., 123, 124, 126, 255 “Planet X,” 19 Planetary motions, 36, 45, 46, 78 Planetary nebulae, 29, 273, 321, 323 and Plate II (p. 28) Planetary orbits, 45, 46, 237, 245 Planetesimal Hypothesis, 235, 236 Planets, arrangement of, 16, 19, 240 birth of (Laplace), 231, 234 birth of (Tidal Theory), 237, 240, 244 in the universe, 341, 349 motions and orbits of, 36, 45, 46, 78, 237, 239, 245 Plaskett, J. S., 67, 214 Plaskett’s star (B.D. 6° 1309), 49, 55, 269, 270, 272 Pleiades, 32, 39, 40, 173, 216 Plummer, H. C., 223 Pluto, 16, 18, 19, 20, 239, 240, 241, 243 birth of, 240, 243 temperature of, 20 Poincaré, H., 66, 285, 338 Poincaré’s theorem, 285, 291 Pressure in a gas, 97 in a star, 306 of radiation, 120 of radiation in a star, 290, 306 Primaeval chaos, evolution from, 202, 205, 229, 230 Proctor, R. A., 235 Procyon, 48, 266, 278, 279, 280 Proton, 119, 121 Proxima Centauri, 36, 40, 264, 278 Ptolemy, 2, 3, 4, 5, 6, 8, 31