A rotating disk is attached to a rotating arm. A point on the perimeter of the disk follows the path of various roulette curves (called epicycles), which depend on the ratio of the diameters of the disks and the speed of rotation. Ptolemy used this principle to describe the visible motion of planets, and mechanical devices such as the one here were built to model those motions. The Copernican heliocentric model of the solar system, with planetary orbits described by Kepler's laws, made epicycles unnecessary to correctly describe the motions of the planets.