It was a simulator of the ostensible motion of the planets. It was devised by Eudoxos of Knidos who tried to simulate the irregular motion of the planets with the combination of circular movements which were considered the only acceptable orbits for the perpetual motion.
It consisted of two concentric spheres (or rings). The internal sphere rested at inclination in the exterior sphere and bore the planet on its equator. The simultaneous rotation of the two spheres (rings) at equal frequency but opposite direction caused the octant (8) motion of the planet with its elevation, immersion and retrograde motion. The whole system rested and rotated on the periphery of a third sphere and produced the stretched complex retrograde orbit of the planet.