# What Is Epicycle?

Topics: Planet, Sun, Almagest Pages: 2 (538 words) Published: March 1, 2013
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 theretrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from Earth. It is called Ptolemaic after the Greek astronomer Ptolemy, although it had been developed by previous Greek astronomers such as Apollonius of Perga and Hipparchus of Rhodes, who used it extensively, during the second century BC, almost three centuries before Ptolemy. Epicyclical motion is used in the Antikythera Mechanism, an ancient Greek astronomical device for computing the phase and position of the Moon using four gears, two of them engaged in an eccentric way that closely approximates Kepler's second law, i.e. the Moon moves faster at perigee and slower at apogee. In the Ptolemaic system, the planets are assumed to move in a small circle called an epicycle, which in turn moves along a larger circle called adeferent. Both circles rotate eastward and are roughly parallel to the plane of the Sun's orbit (ecliptic). The orbits of planets in this system areepitrochoids. Despite the fact that the Ptolemaic system is considered geocentric, the planets' motion was not thought to be actually centered on the Earth. Instead, the deferent was centered on a point halfway between the Earth and another point called the equant. The epicycle, meanwhile, rotated and revolved along the deferent with uniform motion. The rate at which the planet moved on the epicycle was fixed such that the angle between the center of the epicycle and the planet was the same as the angle between the earth and the sun. Ptolemy did not...