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The Surya Siddhanta is the name of multiple treatises (siddhanta) in Hindu astronomy. The extant text as edited by Burgess (1860) is medieval (c. 12th century), but it is clearly based on older versions, which may go back to before the Common Era.
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 gods and demons, day and night of god 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.^{[1]}^{:50} Judging from the epoch dates in the work, Plofker suggests that this Sūrya-siddhānta was composed or revised in the early sixth century.^{[1]}^{:50}
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).^{[2]}
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.^{[3]} However, the epicyclical model of Suryasiddhanta was simpler than that made by Ptolemy in the 2nd century.^{[4]}
The table of sines may reflect the only extant version of the original table by Hipparchus, which was lost in the West,^{[3]} but which underwent tradition within Indian astronomy for at least a millennium before reaching its extant form. Because the tradition of Hellenistic astronomy was essentially stopped short in the West after the end of Late Antiquity, the Surya Siddhanta came to play an important part in the history of science, as its survival allowed transmission of the knowledge of trigonometry into Islamic astronomy and from there back to medieval Europe by the 12th century.^{[5]} Surya Siddhanta was one of the two books in Sanskrit translated in Arabic in the later half of eighth century during the reign of Abbasid Khalifa Almansur, and was one of the first books to be translated during the movement for translating world heritage in Arabic.
The table of contents in this text are:
Methods for accurately calculating the shadow cast by a gnomon are discussed in both Chapters 3 and 13.
The astronomical time cycles contained in the text were remarkably accurate at the time. The Hindu Time Cycles, copied from an earlier work, are described in verses 11–23 of Chapter 1:
The Surya Siddhanta also estimates the diameters of the planets. The estimate for the diameter of Mercury is 3,008 miles, an error of less than 1% from the currently accepted diameter of 3,032 miles. It also estimates the diameter of Saturn as 73,882 miles, which again has an error of less than 1% from the currently accepted diameter of 74,580. Its estimate for the diameter of Mars is 3,772 miles, which has an error within 11% of the currently accepted diameter of 4,218 miles. It also estimated the diameter of Venus as 4,011 miles and Jupiter as 41,624 miles, which are roughly half the currently accepted values, 7,523 miles and 88,748 miles, respectively.^{[6]}
The Surya Siddhanta contains the roots of modern trigonometry. Its trigonometric functions jyā and koti-jyā (reflecting the chords of Hipparchus) are the direct source (via Arabic transmission) of the terms sine and cosine. It also contains the earliest use of the tangent and secant when discussing the shadow cast by a gnomon in verses 21–22 of Chapter 3:
Of [the sun's meridian zenith distance] find the jya ("base sine") and kojya (cosine or "perpendicular sine"). If then the jya and radius be multiplied respectively by the measure of the gnomon in digits, and divided by the kojya, the results are the shadow and hypotenuse at mid-day.
In modern notation, this gives the shadow of the gnomon at midday as
s = \frac{g \sin \theta}{\cos \theta} = g \tan \theta
and the hypotenuse of the gnomon at midday as
h = \frac{g r}{\cos \theta} = g r \frac{1}{\cos \theta} = g r \sec \theta
where \ g is the measure of the gnomon, \ r is the radius of the gnomon, \ s is the shadow of the gnomon, and \ h is the hypotenuse of the gnomon.
The Indian solar and lunisolar calendars are widely used, with their local variations, in different parts of India. They are important in predicting the dates for the celebration of various festivals, performance of various rites as well as on all astronomical matters. The modern Indian solar and lunisolar calendars are based on close approximations to the true times of the Sun’s entrance into the various rasis.
Conservative "panchang" (almanac) makers still use the formulae and equations found in the Surya Siddhanta to compile and compute their panchangs. The panchang is an annual publication published in all regions and languages in India containing all calendrical information on religious, cultural and astronomical events. It exerts great influence on the religious and social life of the people in India and is found in most Hindu households.
On-Line Encyclopedia of Integer Sequences, Radian, Trigonometry, Real number, Complex number
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