Minor 24 + 4 = 28 20 to 35 planets Jupiter 48 + 4 = 52 52.0 0.0 Saturn 96 + 4 = 100 95.4 4.6 Uranus 192 + 4 = 196 191.9 4.1 Neptune 384 + 4 = 388 300.6 87.4 It will be seen, that, before the discovery of Neptune, the agreement was so close as to indicate that this was an actual law of the distances; but the discovery of this planet completely disproved its existence. [Illustration: Fig. 304.] 283. _The Satellite of Neptune._--Neptune is accompanied by at least one moon, whose orbit is shown in Fig. 304. The orbit of this satellite is inclined about thirty degrees to the plane of the ecliptic, and the motion of the satellite is retrograde, or from east to west. VII. COMETS AND METEORS. I. COMETS. General Phenomena of Comets. 284. _General Appearance of a Bright Comet._--Comets bright enough to be seen with the naked eye are composed of three parts, which run into each other by insensible gradations. These are the _nucleus_, the _coma_, and the _tail_. The _nucleus_ is the bright centre of the comet, and appears to the eye as a star or planet. The _coma_ is a nebulous mass surrounding the nucleus on all sides. Close to the nucleus it is almost as bright as the nucleus itself; but it gradually shades off in every direction. The nucleus and coma combined appear like a star shining through a small patch of fog; and these two together form what is called the _head_ of the comet. The _tail_ is a continuation of the coma, and consists of a stream of milky light, growing wider and fainter as it recedes from the head, till the eye is unable to trace it. [Illustration: Fig. 305.] The general appearance of one of the smaller of the brilliant comets is shown in Fig. 305. [Illustration: Fig. 306.] [Illustration: Fig. 307.] 285. _General Appearance of a Telescopic Comet._--The great majority of comets are too faint to be visible with the naked eye, and are called _telescopic_ comets. In these comets there seems to be a development of coma at the expense of nucleus and tail. In some cases the telescope fails to reveal any nucleus at all in one of these comets; at other times the nucleus is so faint and ill-defined as to be barely distinguishable. Fig. 306 shows a telescopic comet without any nucleus at all, and another with a slight condensation at the centre. In these comets it is generally impossible to distinguish the coma from the tail, the latter being either entirely invisible, as in Fig. 306, or else only an elongation of the coma, as shown in Fig. 307. Many comets appear simply as patches of foggy light of more or less irregular form. [Illustration: Fig. 308.]