This paper discusses a rule given in the Indian astronomical text Sūrya-siddhānta for computing the angular diameters of the planets. By combining these angular diameters with the circumferences of the planetary orbits listed in this text, it is possible to compute the diameters of the planets. When these computations are carried out, the results agree surprisingly well with modern astronomical data. Several possible explanations for this are discussed, and it is hypothesized that the angular diameter rule in the Sūrya-siddhānta may be based on advanced astronomical knowledge that was developed in ancient times but has now been largely forgotten.

Additional Notes: This 1997 version published in Vol. 11, No. 2 of the Journal of Scientific Exploration, is an expanded updated version of the 1993 Bhaktivedanta Institute essay.