The Sun Treatise

A comparison of the Dhruvankas Polar longitudes; the position of a star measured along the ecliptic of the Yoga Taras The principal "junction stars" used to identify the 27 or 28 lunar mansions of the Indian zodiac as given by Brahmagupta A 7th-century Indian mathematician and astronomer and others also places the date of the Surya-Siddhanta around this same period. Some of these coordinates have been passed down through tradition—these are the ones that remain identical across three different major texts—while others have been corrected by the authors themselves. Through a comparative study of these, we can determine the superior and inferior limits The earliest and latest possible dates of the Surya-Siddhanta's era. According to Thibaut, Varaha’s Varahamihira, the 6th-century author of the Panchasiddhantika Panchasiddhantika also provides the coordinates for seven junction stars, which we consider to be the coordinates of that ancient Surya-Siddhanta which existed before Varaha (see pages 20-21).*

To determine the timing of the ancient Surya-Siddhanta, the following Nakshatras Lunar mansions or star clusters have been selected. The amounts by which Brahmagupta's coordinates exceed those of the Surya-Siddhanta are listed below:

The average excess of the coordinates of these six stars is 3 degrees and 23 minutes. If we assume that due to Ayan Chalan Precession of the equinoxes; the slow shift in the orientation of Earth's rotational axis the coordinates increase by one degree every 72 years, then this increase would have occurred over a period of approximately 244 years. We know with certainty that Brahmagupta's time was 628 CE. Therefore, the date of the Surya-Siddhanta comes to 244 years prior to this, or 384 CE. This can be broadly understood as 400 CE, which serves as the "Upper Limit" for this period. To find the "Lower Limit," we must examine the coordinates of the stars listed below, whose coordinates exceed Brahmagupta's by a mean value of 1 degree 15 [minutes]...

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*From the introduction written by P. C. Sengupta for the second edition of Burgess’s translation of the Surya-Siddhanta, pages XXVI-XXIX.