Planetary Models in Indian Astronomy

Debate on whether the motion of planets has to be heliocentric or geocentric is mostly confined to Greco-European and perhaps Islamic traditions of astronomy. For them, the science of astronomy was closely knitted with cosmology. However, for Indians, the science of astronomy, Jyotihsastra is a Kalavidhana-sastra, science of Time (also place and direction), developed by carefully studying the motion of celestial bodies. The Indian models of planetary motion from the most ancient times accepted the fact that planetary motion can be fairly complex - in any case not confined to uniform circular motions in spheres centred or otherwise at the earth.

Geometrical elements like epicycle radii, Rsines etc., were of course used in specifying the computational procedure. Unlike the Greek model, the epicycle radii specified by the Indian astronomers vary in various ways during the course of the orbit. They had a very clear understanding that the epicycles are useful only as some approximations to illustrate the nature of planetary motion, and that they did not correspond to the physical reality. Having highlighted some of these aspects during the talk, we will also show how Nilakantha Somayaji around 1500 CE - for the first time in the history of astronomy- arrives at a planetary model, based purely on observations, leading to a unified theory of planetary latitudes and a better formulation of equation of centre for the interior planets.