High Quality Content by WIKIPEDIA articles! In the Ptolemaic system of astronomy, the epicycle (literally: on the circle in Greek) was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets. It was first proposed by Apollonius of Perga at the end of the 3rd century BC and formalized by Ptolemy of the Thebaid in his 2nd-century AD astronomical treatise the Almagest. In particular it explained the retrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from Earth.