He based his geocentric model of the Universe on the system of concentric spheres proposed by Eudoxus (as modified by Callippus), increasing the number of spheres to 49 to account for the movement of all celestial bodies. (This was later modified by Ptolemy, who replaced spheres by epicycles.) The outermost sphere, which carried the fixed stars, controlled the motion of the others and was itself controlled by a supernatural agency. Aristotle’s world-view of a Universe in which everything had its natural place, with a changing Earth surrounded by the eternal, perfect, and incorruptible heavens, was not seriously challenged for almost two thousand years. Aristotle demonstrated that the Earth was spherical, from the shadow it cast during a lunar eclipse, and calculated its size, obtaining a result that was 50% greater than the true value.