Circles upon circles to explain retrograde loops

Circles upon circles to explain retrograde loops

Fig. 1.6 . To explain the occasional retrograde loops in the apparent motions of Mars, Jupiter and Saturn, astronomers in ancient times imagined that each planet travels with uniform speed around a small circle, known as the epicycle. The epicycle’s center moves uniformly on a larger circle, the deferent. A similar scheme was used by Ptolemy (fl. AD 150) to explain the wayward motions of the planets in his Almagest. In the Ptolemaic system, the Earth was displaced from the center of the large circle, and each planet traveled with uniform motion with respect to another imaginary point, the equant, appearing to move with variable speed when viewed from the Earth.

Copyright 2010, Professor Kenneth R. Lang, Tufts University