Concerning circular motion, about which we are about to have a certain discussion, we will at the very beginning make the following distinctions. Crcular motion takes place either around the center of the world, or outside. Now let us see if that which takes place about the center of the world is forced or not; as, for example, if a marble sphere were in the center of the world, in such a way that its center would not differ from the center of the world.We will have the solution to this question if it is made clear what is natural motion and what is forced motion.
Now a motion is natural as long as the mobiles, in moving forward, come near their proper places; but it is forced as long as the mobiles, which are moved, recede from their proper place. . Since these things are so, it is manifest that the sphere rotating around the center of the world is moved with a motion that is neither natural nor forced. For since the sphere is heavy, and the place of heavy things is the center, and heavy things are moved in keeping with their center of heaviness; if then the center of heaviness of the sphere were at the center of the world, in which it would remain while the sphere is rotating; it is manifest that it would be moved neither naturally nor forcibly, since it would neither come near to nor recede from its proper place.It must be noted here that, if the sphere were made of totally similar parts, in such a way that the center of heaviness and the center of magnitude were the same, then its center would not differ from the center of the world; but if it were made of dissimilar parts, in such a way that its center of heaviness would differ from the center of magnitude, then the center of heaviness would be the same as the center of the world, but the center of magnitude would be different. But, however this would happen, so long as the center of heaviness was the same as the center of the world, the sphere at the center of the world would rotate neither naturally nor forcibly