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the nearest one, to three thousand millions in the case of Neptune. The latter is therefore seventy times as far from the sun as Mercury. Still wider is the range of their times of revolution. Mercury performs its circuit round the sun in less than three of our months—Neptune takes more than one hundred and sixty years for his long journey. It has not yet made half a revolution since its discovery in 1846.

The major planets are separated into two groups of four planets each, with quite a broad gap between the groups. The inner group is composed of much smaller planets than the outer one; all four together would not make a body one quarter the size of the smallest of the outer group.

In the gap between the two groups revolve the minor planets, or asteroids as they are commonly called. They are very small as compared with the major planets. So far as we know they are all situated in a quite wide belt ranging between a little more than the distance of the earth out to four times that distance. For the most part they are about three or four times as far from the sun as the earth is. They are also distinguished from the major planets by their indefinite number; some five hundred are now known, and new discoveries are continually being made at such a rate that no one can set any exact limit to them.

A third class of bodies in the solar system comprises the satellites, or moons. Several of the major planets have one or more of these small bodies revolving round them, and therefore accompanying them in their revolu-