Suryasidhanta

The Surya Siddhanta is the name of a Sanskrit treatise in Indian astronomy from late 4th-century or early 5th-century CE. The text survives in several versions, was cited and extensively quoted in a 6th-century text, was likely revised for several centuries under the same title. It has fourteen chapters. A 12th-century manuscript of the text was translated by Burgess in 1860.

The Surya Siddhanta describes rules to calculate the motions of various planets and the moon relative to various constellations, diameters of various planets, and calculates the orbits of various astronomical bodies. The text asserts that earth has a spherical shape. It treats earth as stationary globe around which sun orbits, and makes no mention of Uranus, Neptune or Pluto. It calculates the earth's diameter to be 8,000 miles (modern: 7,928 miles), diameter of moon as 2,400 miles (actual ~2,160) and the distance between moon and earth to be 258,000 miles (actual ~238,000). The text is known for some of earliest known discussion of sexagesimal fractions and trigonometric functions.

The Surya Siddhanta is one of the several astronomy-related Hindu texts that likely was influenced by ancient pre-Ptolemy Greek ideas. It represents a functional system that made reasonably accurate predictions. The text was influential on the solar year computations of the luni-solar Hindu calendar.

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. The surviving version of the text is placed in the 1st millennium BCE by Markandaya and Srivastava. Most scholars, however, place the text variously from the 4th-century to 5th-century CE.

According to John Bowman, the earliest version of the text existed between 350-400 CE wherein it referenced sexagesimal fractions and trigonometric functions, but the text was a living document and revised through about the 10th-century. One of the evidence for the Surya Siddhanta being a living text is the work of medieval Indian scholar Utapala, who cites and then quotes ten verses from a version of Surya Siddhanta, but these ten verses are not found in any surviving manuscripts of the text. According to Plofker, large portions of the more ancient Sūrya-siddhānta was incorporated into the Panca siddhantika text, and a new version of the Surya Siddhanta was likely revised and composed around 800 CE. Some scholars refer to Panca siddhantika as the old Surya Siddhanta and date it to 505 CE.

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