CHAPTER XXXV.
THE SOLAR SYSTEM.
GRAVITATION—THE PLANETS—SIZE AND MEASUREMENT OF THE PLANETS—SATELLITES—FALLING STARS—COMETS—AEROLITES.
Fig. 526.—Planets compared with a quarter of the sun.
Gravitation is the force which keeps the planets in their orbits, and this theory, perfected by Newton, was partially known to Kepler. Newton brought this idea into practical shape, and applied it mathematically. We know that every object in the world tends to attract every other object in proportion to the quantity of matter of which each consists. So the sun attracts the planets, and they influence him in a minor degree. Likewise the moon and our earth reciprocally attract each other. But as the sun’s mass is far greater than the masses of the planets he influences them more, and could absorb them all without inconvenience or disturbance from his centre of gravity. We have, in a former portion of this work, spoken of the law of universal gravitation, which is the mutual attraction of any two bodies to each other, is directly proportioned to their masses (not size), and inversely to the square of their distances apart.
This law operates amongst the heavenly bodies, and it is to the never-changing action of gravitation that the planets are kept in their places. Let us see how this is effected. We have read of force, and motion, and rest. Every body will remain at rest unless force compels it to change its position, and it will then go on for ever in a straight path unless something stops it. But if this body be acted on simultaneously by two forces in different directions, it will go in the direction of the greater force. Two equal forces will tend to give it an intermediate direction, and an equal opposing force will stop it. The last axiom but one—viz., the two equal forces in different, not opposing directions, gives us the key to the curving line of the planetary motions. Were it not for the attraction of the sun the planets would fly off at a tangent; while, on the other hand, were not the impelling force as great as it is, they would fall into the sun. Thus they take an intermediate line, and circle round the centre of the solar system—the Sun.
The solar system consists of the sun and the planets which revolve in space around him. These stars are called planets because they move in the heavens. We shall see that they have certain motions—going from east to west, from west to east, and sometimes they appear to be quite motionless. This change of place, appearing now at one side of the sun and now at another, has given them their title of “wanderers” (planets). Besides the planets there are comets and meteors, asteroids and satellites, all circling round the sun in more or less regular orbits. And there must be families of comets, and whole systems of meteors that have not yet appeared to us, and which make up the comets, as has been lately suggested.
Five planets were known to the ancients, and were named after gods and a goddess: Mercury, Venus, Mars, Jupiter, Saturn. In later years a great number were discovered. We must, however, confine ourselves to the consideration of the principal ones, eight in number, including our own Earth, Uranus and Neptune completing the list. Of these Venus and Mercury are the inferior, or interior planets moving between us and the sun. Mars, Jupiter, Saturn, Uranus, and Neptune are superior, or exterior, and pass quite round the heavens. All the planets are spheroids, and vary greatly in their magnitude, as will be seen by the illustration (fig. 528), the largest body being the sun. Mercury, Mars, and Venus, are not so large as the Earth. The other principal planets are considerably larger than our globe.