Surya Siddhanta — The Astronomical Foundation

The Surya Siddhanta is the oldest surviving complete astronomical text of India, dating in its current form to roughly the 4th-5th century CE (though it claims divine origin and its core methods may be much older). Unlike BPHS and Phaladeepika which focus on interpretation, the Surya Siddhanta is pure astronomy — it tells you WHERE planets are, not what their positions MEAN.

The text provides mean motions for all visible planets, epicyclic corrections (manda and shighra) to convert mean positions to true positions, methods for computing eclipses, and a sophisticated time-measurement system. Its sidereal year of 365.2587565 days differs from the modern value (365.25636 days) by only 1.4 seconds per year.

Aryabhata (499 CE) refined the Surya Siddhanta’s methods in his Aryabhatiya. He proposed that the Earth rotates on its axis (over a millennium before Copernicus), improved sine tables to 24 values at 3.75-degree intervals, and refined the planetary parameters. Later astronomers like Brahmagupta (628 CE) and Bhaskara II (1150 CE) continued this tradition of precision refinement.