Abstract:
In this report, we discuss the significant advances in planetary theory made by the Kerala astronomers during 14th to 18th centuries. A geometrical picture of planetary motion, where the five planets Mercury, Venus, Mars, Jupiter and Saturn go around the sun, was developed by The Kerala school, starting with the works of Parameswara (1380-1460). Parameswara explains the procedures for the calculation of planetary longitudes with a geometrical picture in which five planets Mercury, Venus, Mars, Jupiter and Saturn move around the sun, which in turn goes around the Earth. Nilakantha somayaji (1444-1550), the renowned Kerala astronomer describes such a 'heliocentric' model in many of his works along with a detailed justification for the same. He arrives at the correct formulation of the equation of centre for Mercury and venu, described in his great work Tantrasangraha(1500 AD).
As the planets move in orbits inclined to the ecliptic, the true longitude can be obtained only after projecting the position of the planet on the ecliptic. this procedure is discussed in Nilakanta's Aryabhatiya-bhasya and also in greater detail in yuktibhasha of jyeshtadeva(c.1550 AD). It is to be noted that such an understanding of latitudinal effects on planetary longitudes came much later in the European tradition of astronomy.
