The Astronomical Observatories of Jai Singh c15.s1.t3.21

c15.s1.t3.21

69(a). The topics dealt with in most of the later Hindu works are fundamentally the same as those of the Sūrya Siddhānta, and the following notes apply, fairly generally, to all these works. The earth is considered as a fixed unsupported sphere, round which the other heavenly bodies revolve.² Its diameter is given as 1,600 yojanas, and the distance of the moon as 51,570 yojanas, or roughly the same relative distance as Ptolemy gives (61 1/4 radii of the earth). The distances of the other planets are calculated on the assumption that they move with equal velocities. The equation of the centre of a planet is calculated by assuming epicycles, but an apparently indigenous notion is introduced by making the epicycle oval. Ptolemy's theory of the equant is omitted, and certain other improvements of Ptolemy, relating to the moon and Mercury, are also omitted. The precession of the equinoxes is explained as a sort of libration, within limits of 27 degrees east and west of a fixed position, at a rate of 54 seconds a year; and the obliquity is generally reckoned at 24 degrees.