- an epicycle is a small main circle of apparent planetary orbit, which is centered upon the main circle - the deferent.
The 'Ptolemaic theory' assumes that whilst the luminaries move in direct motion around the Earth, other planets 'circle' around their orbital path, known as the 'deferent' or 'eccentric'. The centre of the deferent is not aligned to the Earth, but to a point midway between the Earth and an assumed balancing point called the equant. This displacement from the Earth's centre causes alterations in the proximity of the planets to the Earth, which creates the apparent changes of planetary speed and motion.
See also: apogee | perigee
[The diagram shows the principle, not the scale]
Text and diagram © Deborah Houlding, 16/02/10