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Classical Indian astronomy

Posted by sudhirmangla July 19, 2008

Many Indian astronomers had later formulated ideas about gravity and gravitation in the early Middle Ages.

The cosmological time cycles explained in the Surya Siddhanta, which was copied from an earlier work, gives:

The average length of the sidereal year (the length of the Earth’s revolution around the Sun) as 365.2563627 days, which is only 1.4 seconds longer than the modern value of 365.2563627 days.

This remained the most accurate estimate for the length of the sidereal year anywhere in the world for over a thousand years.

The average length of the tropical year (the length of the year as observed on Earth) as 365.2421756 days, which is only 2 seconds shorter than the modern value of 365.2421988 days.

This estimate remained the most accurate estimate for the length of the tropical year anywhere in the world for another 6 centuries and still remains more accurate than the value given by the modern Gregorian calendar currently in use around the world, which gives the average length of the year as 365.2425 days.

Later Indian astronomer-mathematicians such as Aryabhata made references to this text, while later Arabic and Latin translations were very influential in Europe and the Middle East.

The Indian astronomer-mathematician Aryabhata (476–550), in his magnum opus Aryabhatiya, propounded a mathematical heliocentric model in which the Earth was taken to be spinning on its axis and the periods of the planets were given with respect to a stationary Sun.

He was also the first to discover that the light from the Moon and the planets were reflected from the Sun, and that the planets follow an elliptical orbit around the Sun, and thus propunded an eccentric elliptical model of the planets, on which he accurately calculated many astronomical constants, such as the times of the solar and lunar eclipses, and the instantaneous motion of the Moon (expressed as a differential equation).

Arabic translations of Aryabhata’s Aryabhatiya were available from the 8th century, while Latin translations were available from the 13th century, before Copernicus had written De revolutionibus orbium coelestium, so it’s quite likely that Aryabhata’s work had an influence on Copernicus’ ideas.

Aryabhata wrote that 1,582,237,500 rotations of the Earth equal 57,753,336 lunar orbits. This is an extremely accurate ratio of a fundamental astronomical ratio (1,582,237,500/57,753,336 = 27.3964693572), and is perhaps the oldest astronomical constant calculated to such accuracy.

Brahmagupta (598-668) was the head of the astronomical observatory at Ujjain and during his tenure there wrote a text on astronomy, the Brahmasphutasiddhanta in 628.

Bhaskara (1114-1185), the head of the astronomical observatory at Ujjain, continued the mathematical tradition of Brahmagupta. He wrote the Siddhanta-Shiromani which consists of two parts: Goladhyaya (sphere) and Grahaganita (mathematics of the planets).

He also expanded on Aryabhata’s heliocentric model in his treatise Siddhanta-Shiromani, where he mentioned the law of gravity, discovered that the planets don’t orbit the Sun at a uniform velocity, and accurately calculated many astronomical constants based on this model, such as the solar and lunar eclipses, and the velocities and instantaneous motions of the planets.

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