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In astronomy, the group of heavenly bodies, comprising the sun and the bodies which move around the sun as a centre of attraction, of which the Earth is one. These bodies may be classified as follows: first the Sun, distinguished as containing much the greater part of all the matter composing the system, being more than 600 times as massive as all the other bodies combined. It is this great mass which makes it the central one of the system. It is also, so far as is known, the only incandescent body of the system, and therefore the only one that shines by its own light. Secondly, planets. The bodies of this class consist of eight major planets moving round the sun at various distances, and of an unknown number of minor planets, much smaller than the major planets, forming a separate group. Thirdly, satellites, or secondary planets revolving around the major planets, and therefore accompanying them in their revolutions around the sun. A fourth class of bodies, the constitution of which is still in some doubt, comprises comets and meteors. These differ in that comets are visible either in a telescope or to the naked eye, and seem to be either wholly or partially of a nebulous or gaseous character, while meteors are, individually at least, invisible to us except as they become incandescent by striking the atmosphere of the earth. It is, however, an open question whether a comet is other than an accumulation of meteoric bodies (see Comet).

The major planets are separated into two groups of four each, between which the minor planets, for the most part, revolve. The arrangement of the major planets, with the numbers of their respective satellites thus far known, in the order of distance from the sun, is as follows: The first group in order - the smaller major planets - comprises: Mercury, with no known satellite; Venus, ?,?, with no known satellite; The Earth, ®, with one satellite, the moon; Mars, with two satellites.

Outside of this group lies the zone of minor planets or asteroids.

The outer group of major planets comprises: Jupiter, 24,24, with seven satellites; Saturn, T?, with ten satellites; Uranus, a or 1, with four satellites; Neptune, with one satellite.

The distances separating the individual orbits in each group seem to approximate to a certain order of progression, expressed in Bode's law (see Bode). But there is an obvious gap between the two groups of major planets which is filled by the group of minor planets. Taking the mean distance of this group as that of a planet, the distance of the major planets closely approximates to Bode's law, except in the case of Neptune.

A remarkable feature of the solar system, which distinguishes it from all other known systems in the universe, is the symmetry of arrangement and motion of its greater bodies. All the major planets and many of the minor planets revolve in elliptic g FIG. 6.--Seed -vessel (pyxidium) of Henbane ( Hyoscyamus niger) opening by circumscissile dehiscence.

orbits so nearly circular in form that the unaided eye would not notice the deviation from that form. But as the orbits are not centred on the sun, which is in a focus of each, the displacement of the seeming circle would be readily seen in the case of Mercury and of Mars. The same statements are true of the orbits of the satellites around their primaries. The major planets all move around the sun in the same direction, from west to east, in orbits but little inclined to each other. All the known minor planets have the same common direction, but their orbits generally have a greater eccentricity and mutual inclination. The general rule is that the satellites also move round in the same direction, and in orbits of moderate inclination. Exceptions occur in the case of the satellites of Uranus, which are nearly perpendicular to the plane of the orbit. The satellite of Neptune, and one satellite, Phoebe, of Saturn, are also quite exceptional, the direction of motion being retrograde.

For the elements of the orbits, and the general character of the several planets see PLANET. Details as to each are found under the respective names of the several planets. (S. N.)