A complementary theory to Ptolemy's employed homocentric spheres: the spheres within which the planets rotated could themselves rotate somewhat. This theory predated Ptolemy (it was first devised by Eudoxus of Cnidus; by the time of Copernicus it was associated with Averroes). Also popular with astronomers were variations such as eccentrics—by which the rotational axis was offset and not completely at the center. The planets were also made to have exhibit irregular motions that deviated from a uniform and circular path. The eccentrics of the planets motions were analyzed to have made reverse motions over periods of observations. This retrograde motion created the foundation for why these particular pathways became known as epicycles.