The Book of Stars: Being a Simple Explanation of the Stars and Their Uses to Boy Life

by A. Frederick Collins

How the Planets Are Held in Space.—If you will take an ordinary dinner plate and half an eggshell, and give the eggshell a slight spin on the rim of the plate—the rim should be slightly moistened—you will find that by tilting the plate a trifle the eggshell will revolve in two directions; first, it will spin round on its own axis, and second, it will travel round the rim of the plate, which we will call its orbit, as shown in [Fig. 63].

This double motion of the eggshell is exactly like the double motion of a planet—each one turns on his own axis and each travels round the Sun in its own orbit; moreover, all the planets travel round the Sun in the same direction, the nearest planets taking the shortest time to complete the circle or orbit, while those farthest away take the longest time to go round their orbits just as we might expect them to do.

In the beginning of things the planets were a part of the Sun, as we shall see further on, and when they were thrown off by him they spun round their own axes, and they continued to spin round their axes just as a ball continues to do so after it has left the pitcher’s hand.

The tendency to fly off at a tangent which every particle of a rotating body feels is called centrifugal force. Now, no one knows what centrifugal force is, but how it acts is very well known, and you can find out for yourself by making the following experiment.

Fig. 64.—Boy Throwing Stone to Illustrate
Centrifugal Force.

Take a stone and tie one end of a string to it; now swing the stone round in a circle; if the string should break, or you should let it go accidentally (on purpose) the stone will shoot off at a tangent, that is in a line away from the circle in which it was swinging, as shown in [Fig. 64].

This is just what made the planets, one after another, separate from the Sun, but they could not get very far away, for another force which is not only in the Sun but in every particle of matter in the universe, pulled them back toward the Sun, just as it pulls a ball when thrown to the Earth, and this force is called gravitation.

So the centrifugal force keeps them spinning round on their axes and flying round in their orbits about the Sun. If it had not been for the attractive force of gravitation of the Sun and the planets, the planets would have kept right on going out into space and left the Sun to shift for itself forever after.

As it is they whirl round the Sun, never able to get any farther from him and yet never getting any nearer to him, for the centrifugal force tends to make them fly out and away and the force of gravitation tends to pull them back to the Sun. The result is that these two great opposing forces exactly offset or balance each other, and the planets are held in their orbits just as securely as the stone is held out by the force the boy exerts to move it, and in by the string he holds in his hand.