In the Hipparchian, Ptolemaic, and Copernican systems of astronomy, the epicycle was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets. In particular it explained the apparent retrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from the Earth.
The complexity to be described by the geocentric model
The deferent (O) is offset from the Earth (T). P is the center of the epicycle of the Sun S.
Hipparchus was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the equinoxes. Hipparchus was born in Nicaea, Bithynia, and probably died on the island of Rhodes, Greece. He is known to have been a working astronomer between 162 and 127 BC.
19th century engraving based on an engraved amethyst from the Poniatowski gem collection
The figure on the left may be Hipparchus, from Raphael’s fresco The School of Athens
A 19th century artist's impression of Hipparchus
Hipparcos satellite in the Large Solar Simulator, ESTEC, February 1988
