PLANET 569 stars, and now applied to the eight primary members of the solar system, and by some astronomers to the asteroids. The planets of the ancient systems of astronomy were the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn ; and there can be no doubt that the week of seven days had its origin in this circumstance. (See WEEK.) The solar system may be divided into three distinct families : 1, the four planets, Mercury, Venus, the earth, and Mars, commonly called the terrestrial planets ; 2 (in order of distance), the asteroids or minor planets; 3, the four giant planets, Jupiter, Saturn, Uranus, and Neptune. In this classification we regard the moon, and the vari- ous satellites which attend on the four major planets, as so intimately associated with the planets to which they belong as not to need separate consideration. Nevertheless, it may be questioned whether our moon ought not to be regarded as a fifth member of the innermost family of planets, seeing that she differs much less from Mercury in size than Mercury differs from the earth. The first point to be noticed in the general survey of the sun's family is the scale of the various parts of the planetary sys- tem. It is the more necessary to dwell on this relation because in ordinary pictures of the orbits the point is overlooked. In fact, any picture professing to show the whole of the solar system must necessarily be inexact, sim- ply because the scale suited to show properly the orbits of the four outer planets is too small to show the paths of the four inner planets; and, a scale suited to these smaller orbits would be such that the paths of the outer planets could not be included within a sheet of reason- able compass. The relative distances of the eight primary planets are as follows, the earth's being taken as 1,000 : Mer- cury. 887 Venus. Earth. Mars. Jupiter. Saturn. Uranus. Neptune. 723 1,000 1,524 5,203 9,539 19,183 80,037 The distances of the asteroids range between 2,200 and 3,400 in this scale. There is a cer- tain uniformity in the progression of these dis- tances (omitting Neptune), which led astrono- mers to recognize as real the empirical law of Bode or Titius, at least until the discovery that Neptune's orbit does not correspond with the law. The law may be thus presented : Under the names of the planets in order set the num- ber 4; then write down in succession the numbers 0, 3, 6, 12, 24, 48, and so on, setting the under Mercury, the 3 under Venus, and so on. Adding the columns thus obtained, we get the following result : Mer- cury. Venus. Earth. Man. Aste- roids. Jupiter. Saturn. Uranus. Nep- tune.
8 6 12 24 48 96 4 192 4 384 4 7 10 16 28 52 100 196 888 The real distances given in the former table cor- respond very closely with these results. Di- vided severally by 100, they give the series: Mer- cury. 8'9 Venus. Earth. Mars. Aste- roids. Jupiter. Saturn. Uranus. Nep- tune. 7-2 10 15 2284 52 95 192 800 The statement of the law can be simplified, if we take the orbit of Mercury as an inner limit from which to measure the distances of the several planets. These distances, so measured, form a geometrical progression, doubling as we proceed outward ; only in the case of Neptune the multiplier suddenly changes from 2 to 1^. When we consider the dimensions and masses of the planets, we no longer find any uniform- ity of progression. The middle family or zone of asteroids is the least in point of mass ; the innermost family comes next in order ; and the outermost is by far the most massive. Taking the mass of the earth at 1,000, the combined mass of the outer family amounts to 419,975, that of the innermost to 2,068 ; and probably the combined mass of the zone of asteroids does not amount to 100 on this scale. The following table presents the relative masses of the eight primary planets, the earth's mass being taken as 1,000, and brings prominently into view the irregular distribution of matter within the solar system : PLANETS. Mass. PLANETS. Mass. Mercury 65 Jupiter 800,860 Venus 885 Saturn 89,692 1012 Uranus 12650 Mars 118 Neptune 16,778 To which add the sun's mass, 315,000,000, and the asteroidal family, less than 100. A rela- tion not commonly dealt with in treatises on astronomy may next be considered. Every orb in space bears sway, so to speak, over a certain region around it, in such sort that mat- ter within that region is more completely un- der the influence of the orb's attractive power than under any other attractive influence. It would not be easy to assign a perfectly satis- factory criterion for the extent of any orb's domain in this respect, simply because the control exercised by it depends to a great de- gree on the velocity of the attracted matter. But as a convenient statical measure of the controlling power of each planet, we may take the attraction exerted on a body at rest at any point, as compared with the sun's attraction on a body at that point ; and we may assign as the limit of a planet's domain the surface de- termined by the law that at any point of it the planet's attraction is exactly equal to the sun's. It is easy to determine this surface for any given planet. Thus let M be the sun's mass, I) the distance of a planet, m the mass of the planet, which may be supposed collected at the planet's centre, so far as such an inquiry as the
