This statue represents Atlas, the legendary Greek Titan, holding up the earth on his shoulders. The question of how the earth, sun, and the planets are supported and what causes them to move across the skies has troubled thinkers since ancient times. In his book, Meteorology, Aristotle explained that water, air, and fire form spherical shells around the earth. Above these was a set of transparent spheres that held the stars and planets as they revolved. The spheres were thought to be made of a rigid, clear material like crystal, which was not subject to the laws that govern the behavior of the earthly elements. This was why the stars and planets could move without a force acting on them. On earth an object could move only if it was acted on by a force. The movement would stop if this force was removed.