One of the oldest surviving texts of Indian astronomy, presented as knowledge revealed by the sun god Surya — a working system for calculating planetary positions and eclipses.
वर्तमान रूप लगभग ४–५वीं शताब्दी · present form circa 4th-5th century CE
Its present form dates to roughly the 4th-5th century CE, though it claims to preserve much older knowledge, and it calculates the length of the solar year, lunar and planetary motion, and eclipse timing using a geocentric model refined over centuries of observation.
The Surya Siddhanta remained the working basis for most traditional Hindu calendars, panchangs, for over a thousand years, and directly influenced later astronomers including Aryabhata and Bhaskaracharya, who built on and revised its calculations.