We all know most popular planetary models proposed by Kepler, Ptolemy and Copernicus. Indian astronomers also proposed different planetary models. Earliest Indian planetary model was the ‘Sunrise system’ – where day starts at sunrise. Later Aryabhata I (576 AD) proposed a new ‘Midnight system’ – day starting at midnight.
It is well known that the Indian astronomers were mainly interested in the successful computations of the longitudes and latitudes of the Sun, Moon and the planets and were not much worried about proposing models of the universe. However, detailed observations suggested some geometrical models.
Modification of earlier Indian planetary models
During the 15th century major revisions were made to the planetary theory. Particularly Nilkantha, the Indian astronomer, carried out major revisions of the earlier Indian planetary model for the interior planets – Mercury and Venus.
Paramesvara (1380-1460 AD)
Paramesvara has discussed in detail the geometrical model implied in the earlier Indian planetary theory. In his Siddhantadipika, he gives a more detailed exposition of the geometrical model.
Nilkantha Somasutvan (1444-1550 AD)
Nilkantha carried out in his treatise Tantrasangraha (1500AD), a major revision for the earlier Indian planetary model for the interior planets Mercury and Venus. This led Nilkantha to a much better formulation of the equation of centre for these planets then was available either in earlier Indian, Islamic or European. (Till the works of Kepler, which happened more than a 100 years later).
Nilkantha in Golasara, Siddhanta-darpana, Aryabhatiyabhashya, explains that computational scheme developed by him implies a geometrical picture of planetary motion. In his model – all five planets – Mercury, Venus, Mars, Jupiter and Saturn – move in eccentric orbits around the Sun, which in turn goes around the Earth. Most of the Astronomers succeeding Nilkantha, adopted this planetary model.
Earlier Indian planetary model used two methods for calculating planetary motions, one for exterior planets and other for interior. Nilkantha by his modification solved the problem of there being two different rules for calculating the planetary motions.