Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v2.c12.s3.10

v2.c12.s3.10

The distance of the Sun from the Earth comes to 689430 yojanas according to the Sūrya Siddhānta, i.e. about 862 times the Earth's radius. But according to modern discoveries, it is about 23300 times the radius of the Earth. The distances of planets farther than the Sun, as given in our siddhāntas, are still more erroneous. Our astronomers have not determined the lengths of planetary orbits and the periods of their revolutions after first determining the length of the *Celestial orbit* and the mean daily motions of planets in their respective orbits in terms of Yojanas, by actually taking observations. It is quite clear that they first determined the periods of revolutions and the length of the moon's orbit by observation, and then, on the basis of these figures they fixed the lengths of the planetary orbits and that of the circumference of the celestial sphere. The Pañca Siddhāntikā does not mention the lengths of the orbits of planets and that of the circumference of the celestial sphere in terms of yojanas nor does it mention the length of the moon's orbit; it appears, therefore, that the original Sūrya Siddhānta never contained this information. The modern Sūrya Siddhānta does give these measures; and the author has arrived at the conclusion that the modern Sūrya Siddhānta existed before the Pañcasiddhāntikā. It is not, however, improbable that the lengths of planetary orbits may have been interpolated afterwards*. Secondly all the siddhāntas except the first Ārya Siddhānta have given 15 yojanas as the length of the moon's orbit subtended by one-minute angle. Thirdly the positions of planetary orbits are fixed, and the planets always move through them. The places of the orbits do not change. Hence, the statement that all planets make a revolution of the sky only once in a Kalpa has no meaning. Bhāskarācārya plainly remarks,