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Ancient India’s contributions to astronomy are well known and documented. The earliest references to astronomy are found in the Rig Ved, which are dated 2000 BC. By 500 AD, ancient Indian astronomy emerged as an important part of Indian studies and its affect is seen in several treatises of that period. In some instances, astronomical principles were borrowed to explain matters pertaining to astrology, like casting of a horoscope. Apart from this link of astronomy to astrology in ancient India, science of astronomy continued to develop independently, and culminated in original findings, like: The calculation of occurrences of eclipses, Calculation of Earth’s circumference, Theorizing about gravity, Determining that Sun is a star, Determining the number of planets in the Solar System.

There are astronomical references of chronological significance in the Ved. Some Vaidik notices mark the beginning of the year and that of the vernal equinox in Orion; this was the case around 4500 BC. Fire altars, with astronomical basis, have been found in the third millennium cities of India. The texts that describe their designs are conservatively dated to the first millennium BC, but their contents appear to be much older. Some scholars have claimed that the Babylonians invented the zodiac of 360 degrees around 700 BC, perhaps even earlier. Many claim that India received the knowledge of the zodiac from Babylonia or even later from Greece. However, as old as the Rig Ved, the oldest Vaidik text, there are clear references to a Chakra or wheel of 360 spokes placed in the sky. The number 360 and its related numbers like 12, 24, 36, 48, 60, 72, 108, 432 and 720 occur commonly in Vaidik symbolism. It is in the hymns of the great Rishi Deerghtamaa (Rig Ved, 1.140–164) that we have the clearest such references.

A text on Vaidik astronomy that has been dated to 1350 BC, was written by Lagadha.The earliest concept of a heliocentric model of the solar system, in which the Sun that is at the center of the solar system and the Earth that is orbiting it, is found in several Vaidik Sanskrit texts written in ancient India.

The Aitareya Braahman (2.7) (c. 9th–8th century BC) states: "The Sun never sets nor rises. When people think the Sun is setting, it is not so; they are mistaken." This indicates that the Sun is stationery (hence the Earth is moving around it), which is elaborated in a later commentary.

Vishnu Puraan (2.8) (c. 1st century), which states: "The Sun is stationed for all time, in the middle of the day. [...] Of the Sun, which is always in one and the same place, there is neither setting nor rising."

Yaagyaavalkya (c. 9th–8th century BC) recognized that the Earth was round and believed that the Sun was "the centre of the spheres" as described in the Ved at the time. His astronomical text Shatapath Braahman (8.7.3.10) stated: "The Sun strings these worlds - the earth, the planets, the atmosphere - to himself on a thread." He recognized that the Sun was much larger than the Earth, which would have influenced this early heliocentric concept. He also accurately measured the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, almost close to the modern measurements of 107.6 for the Sun and 110.6 for the Moon. Based on his heliocentric model, Yaagyavalkya proposed a 95-year cycle to synchronize the motions of the Sun and the Moon, which gives the average length of the tropical year as 365.24675 days, which is only 6 minutes longer than the modern value of 365.24220 days. This estimate for the length of the tropical year remained the most accurate anywhere in the world for over a thousand years.

The distances of the Moon and the Sun from the Earth was accurately measured as 108 times the diameters of these heavenly bodies. These are very close to the modern values of 110.6 for the Moon and 107.6 for the Sun, which were obtained using modern instruments.

There is an old Sanskrit Shlok (couplet) which also states "Sarv Dishaanaam, Sooryah, Sooryah, Sooryah" which means that there are Suns in all directions. This couplet which describes the night sky as full of Suns, indicates that in ancient times Indian astronomers had arrived at the important discovery that the stars visible at night are similar to the Sun visible during day time. In other words, it was recognized that the Sun is also a star, though the nearest one. This understanding is demonstrated in another Shlok which says that when one Sun sinks below the horizon, a thousand Suns take its place. Many Indian astronomers had later formulated ideas about gravity and gravitation in the early middle ages.

The cosmological time cycles explained in the Soorya Siddhaant, 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 (until Muslim mathematician Omar Khayyam gave a better estimate), 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 Aaryabhat made references to this text, while later Arabic and Latin translations were very influential in Europe and the Middle East.

The Indian astronomer and mathematician Aaryabhat (476–550 AD), in his magnum opus Aaryabhateeya, 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 propounded 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).

Bhaaskar II (1114-1185 AD) expanded on Aaryabhat's heliocentric model in his treatise Siddhaant 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. Arabic translations of Aaryabhat's Aaryabhateeya 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 Aaryabhat's work had an influence on Copernicus' ideas. Aaryabhat wrote that 1,582,237,500 rotations of the Earth equal to 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.

Brahmgupt (598-668) was the head of the astronomical observatory at Ujjain and during his tenure there wrote a text on astronomy, the Brahm Sphoot Siddhaant in 628. Bhaaskar II (1114-1185) was the head of the astronomical observatory at Ujjain, continuing the mathematical tradition of Brahmgupt. He wrote the Siddhaant Shiromani which consists of two parts: Golaadhyaaya (sphere) and Graha Ganit (mathematics of the planets). The other important names of historical astronomers from India are Maadhav and Neelakanth Somayaji.

One may read some other books available on this subject --
"The Pride of India" by Sanskrit Bharati; and
"The Wonder That Was India".

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