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. This balancing of opposed forces was nicely shown some years ago by Sir Robert Ball, the great English astronomer, at one of his lectures in the Royal Institution of London, and I reproduce it here. To the ceiling over his lecture table he had fastened a thin wire, the lower end of which was secured to a hollow iron ball. When the ball was pulled aside, it would swing like a great pendulum, forth and back, in a plane, as shown in Fig. 65. [Illustration: FIG. 65.—IRON BALL PENDULUM SWINGING IN STRAIGHT LINE.] [Illustration: FIG. 66.—IRON BALL PENDULUM SWINGING IN CURVED LINE.] But when a powerful magnet was placed on the table and the ball was set swinging in a plane as before, just as it came close to the end of the magnet the latter pulled the ball with its mighty force toward it, and so changed its course, but the ball, instead of being attracted directly to it, swung in a graceful curve around it, as shown in Fig. 66. This is precisely the case of the planets swinging round the Sun, and shows very nicely the balanced forces of the Sun and the planets and why the planets stick to their orbits. =Why the Planets Do Not Stop Spinning.=—Now you may ask why the planets do not stop turning on their axes and revolving in their orbits round the Sun. And the answer is because there is nothing to stop them. If you spin a top on a plate it may keep going for a long time, but it will finally die down and stop. This is because there are other forces which oppose its centrifugal force. The forces a spinning top has to overcome are _friction_ between the point of its spindle and the plate, and the _resistance_ between the surface of the spinning top and the air pressing on its sides, and the friction and the air resistance together soon use up the energy stored in the top and which makes it spin.