Indian astronomy - Ashok Sridhar Column

Many ancient civilisations were very advanced in astronomy, as they believed that the heavenly bodies strongly influence human life and hence made in-depth study of the cosmos. Ancient India was no exception to this. It would surely not be an overstatement to say that Indian astronomy progressed to cater to the needs of astrology. Though many other civilizations were strong in this realm, Indian astronomers of the yore were responsible for some path-breaking discoveries, thus carving a niche for themselves.

The earliest references to astronomy in India are found in Rig Veda, dated around 1700 BC, although new theories supported by strong proofs are evolving, placing the date of the Vedic civilisation and Vedic scripts even earlier. Just as in mathematics, Aryabhata’s contribution to astronomy is immense. He was the first to propound the following:

• A heliocentric model, in which the Earth was considered to be spinning on its axis and the periods of the planets were given with respect to a stationary Sun
• Light from the Moon and the planets is reflected from the Sun
• Planets follow elliptical orbits around the Sun

Considering that it would be nearly 1000 years before similar concepts were propounded in Europe by Copernicus and Galileo, it is no exaggeration to state that Aryabhata was a pioneer in astronomy. As a further development of Aryabhata’s heliocentric model, Bhaskaracharya (Bhaskara II), in his treatise Siddhanthasiromani, mentioned that the planets do not orbit the Sun at a uniform velocity, and accurately predicted many astronomical constants and solar/lunar eclipses based on this model. Arabic translations of Aryabhatiya were available from the 8th century AD and Latin translations, from the 13th century AD. So, it is likely that Aryabhata’s work had an influence on Copernicus’ ideas. Though Aryabhata’s contribution to astronomy is immense, it must be mentioned though that the original ideas about Sun being the centre of the solar system first appeared in Vedic texts, well before Aryabhata’s lifetime. There is also a possibility that Aryabhata developed his concepts based on the works of his predecessors.

The earliest traces of a counter-intuitive idea that it is the Earth that is actually moving around the Sun that is at the centre of the solar system (hence the concept of heliocentrism), is found in several Vedic texts. Yajnavalkya (c. 9th – 8th century BC) recognised that the Earth is spherical and believed that the Sun was ‘the centre of the spheres’ as described in the Vedas at the time. In his astronomical text Shatapatha Brahmana, he states: "The sun strings these worlds - the Earth, the planets, the atmosphere - to himself on a thread”. He recognised that the Sun was much larger than the Earth, which would have influenced this early heliocentric concept. He also accurately estimated the relative distances of the Sun and the Moon from the Earth as 108 times the diameters of these heavenly bodies, respectively. These estimations are close to the modern measurements of 107.6 times the Sun’s diameter for the distance between the Sun and the Earth and 110.6 times the Moon’s diameter for the distance between the Moon and the Earth. He also described a calendar in the Shatapatha Brahmana. The Vedic Sanskrit text Aitareya Brahmana (c. 9th – 8th century BC) also 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 stationary (hence the Earth is moving around it), which is elaborated in a later commentary Vishnu Purana (c. 1st century AD), which states: "of the Sun, which is always in one and the same place, there is neither setting nor rising".

Brahmagupta (7th century AD) was another important figure who contributed a great deal to the development of astronomy in India. He had commented on gravity that “bodies fall towards the Earth as it is in the nature of the Earth to attract bodies, just as it is in the nature of water to flow”. He also estimated that the circumference of the Earth was 5000 Yojanas. A Yojana is around 7.2 km. This gives an Earth circumference of 36,000 km, which is only 10% off the modern (actual) estimates.

Besides the abovementioned accomplishments, ancient India’s astronomers pioneered several other remarkable discoveries. For instance, in Surya Siddhanta, an astronomical treatise written in circa 400 AD, the following cosmological time cycles are given:

• The average length of the sidereal year (the length of the Earth's revolution around the Sun) as 365.2563627 days, which is virtually the same as the modern value! 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 the Persian 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

After centuries, even millenniums of astounding progress, Indian astronomy declined due to the turbulent political climate. It is fair to say that after the accomplishments of the Kerala School of mathematics and astronomy between the 14th and 16th centuries AD, India was no longer a leading power in this field, even though some remarkable things like the construction of Jantar Mantar, a major astronomical observatory, did occur. It is also generally believed that the Indian astronomers were handicapped by the lack of instruments like telescope, leading to a long period of stagnation. It looks as if the modern Indian astronomers are slowly clawing their way up, trying to make up for the lost time, by launching various satellites and missions. Chandrayaan, the unmanned lunar mission planned and scheduled by the Indian Space Research Organisation (ISRO) for 2007 or 2008, will be Indian space programme’s crowning achievement.

REFERENCES:

• An overview of Indian mathematics - by J.J. O'Connor and E.F. Robertson
• www.Wikipedia.org