Surya Siddhānta

The Surya Siddhanta is the name of multiple treatises (siddhanta) in Indian astronomy. The extant text as translated by Burgess (1860) is medieval (c. 12th century), but it is clearly based on older versions, thought to have been composed in the early 6th century AD.

It has rules laid down to determine the true motions of the luminaries, which conform to their actual positions in the sky. It gives the locations of several stars other than the lunar nakshatras and treats the calculation of solar eclipses as well as solstices, e.g., summer solstice 21/06. Significant coverage is on kinds of time, length of the year of devas and asuras, day and night of Brahma, the elapsed period since creation, how planets move eastwards and sidereal revolution. The Earth's diameter and circumference are also given. Eclipses and color of the eclipsed portion of the moon are mentioned.

In a work called the Pañca-siddhāntikā composed in the sixth century by Varāhamihira, five astronomical treatises are named and summarised: Paulīśa-siddhānta, Romaka-siddhānta, Vasiṣṭha-siddhānta, Sūrya-siddhānta, and Paitāmaha-siddhānta. Judging from the epoch dates in the work, Plofker suggests that this Sūrya-siddhānta was composed or revised in the early sixth century.

Utpala, a 10th-century commentator of Varahamihira, quotes six shlokas of the Surya Siddhanta of his day, not one of which is to be found in the text now known as the Surya Siddhanta. The present version was modified by Bhaskaracharya during the Middle Ages. It is partly based on Vedanga Jyotisha, which itself might reflect traditions going back to the Indian Iron Age (around 700 BCE).

It is hypothesized that there were cultural contacts between the Indian and Greek astronomers via cultural contact with Hellenistic Greece, specifically the work of Hipparchus. There were many similarities between Suryasiddhanta and Greek astronomy in Hellenistic period. For example, Suryasiddhanta provides more accurate and detailed table of sines than Hipparchus. However, the epicyclical model of Suryasiddhanta was simpler than that made by Ptolemy in the 2nd century.

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