Page:Astronomy for Everybody.djvu/113

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GLANCE AT THE SOLAR SYSTEM
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tion around the sun. The two innermost planets, Mercury and Venus, have no satellites, so far as we yet know. In the case of the other planets their number ranges from one (our moon) to eight, which form the retinue of the planet Saturn. Each major planet, Mercury and Venus excepted, is therefore the centre of a system bearing a certain resemblance to the solar system. These systems are sometimes designated by names derived from those of their central bodies. Thus we have the Martian System, composed of Mars and its satellites; the Jovian System, composed of Jupiter and its five satellites; the Saturnian System, comprising the planet Saturn, its rings, and satellites.

A fourth class of bodies consists of the comets. These move round the sun in very eccentric orbits. We see them only on their approach to the sun, which, in the case of most of these bodies, occurs only at intervals of centuries, or even thousands of years. Even then a comet may fail to be seen unless under favourable conditions.

Besides the preceding bodies we have a countless number of meteoric particles revolving round the sun in regular orbits. These are probably related in some way to the comets. They are completely invisible except as they strike our atmosphere, when we see them as shooting stars.

The following is the arrangement of the planets in the order of their distance from the sun and with the number of satellites of each:

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