Astronomy for Everybody/Part 3/Chapter 1

We have shown how this comparatively small family of bodies, on one of which we dwell, forms as it were a little colony by itself. Small though it be when compared with the whole universe as a standard, it is for us the most important part of the universe. Before proceeding to a description of its various bodies in detail we must take a general view to show of what kind of bodies it is formed and how it is made up.

First of all we have the sun, the great shining central body, shedding warmth and light on all the others and keeping the whole system together by virtue of its powerful attraction.

Next we have the planets, which revolve round the sun in their regular orbits, and of which our earth is one. The word planet means wanderer, a term applied in ancient times because these bodies, instead of keeping their places among the fixed stars, seemed to wander about among them. The planets are divided into two quite distinct classes, termed major and minor.

The major planets are eight in number and are, next to the sun, the largest bodies of the system. For the most part their distances from the sun are arranged in a close approach to a certain regular order, ranging from nearly forty millions of miles in the case of Mercury, ​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​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:

Instead of taking up these bodies in the order of their distance from the sun, we shall, after describing the latter, pass over Mercury and Venus to consider the earth and moon. Then we shall return to the other planets and describe them in order.