Aryabhata -- The Astronomer Who Said the Earth Turns

In one of his verses, Aryabhata tells us when he wrote: when sixty times sixty years of the Kali Yuga had passed, he was twenty-three years old. By the traditional reckoning that is 499 CE -- so he was born in 476. He worked at Kusumapura, usually identified with Pataliputra, the capital of the Gupta empire near modern Patna. His one surviving work, the Aryabhatiya, is short: 121 verses in four sections. It is also one of the most influential books in the history of science.

Its four parts are the Gitikapada, which sets out his units of time and his astronomical constants in a compressed alphabetical notation; the Ganitapada, on mathematics; the Kalakriyapada, on the reckoning of time and planetary positions; and the Golapada, on the sphere -- the shape of the Earth and the heavens, and the causes of eclipses.

His boldest claim was the turning Earth. Most astronomers of his age, in India and elsewhere, held that the Earth stood still while the sphere of stars wheeled around it each day. Aryabhata said the opposite, and gave the image of the boat to explain why it does not look that way. Later Indian astronomers, including the great Brahmagupta, rejected the idea, and some later copyists even altered his verse to make it say the stars move. It would be another thousand years before the rotation of the Earth was generally accepted.

His explanation of eclipses was equally direct. Popular belief held that the demon Rahu swallowed the Sun or Moon; Aryabhata explained them as shadows, and his methods allowed eclipses to be predicted. The mythology was not abandoned -- Rahu and Ketu became the names of the points where the Moon's path crosses the Sun's, where eclipses happen -- but the cause was now geometry.

Aryabhata founded a school. His work was commented on by Bhaskara I in the seventh century and studied for a thousand years, especially in Kerala, where the astronomers of the later Kerala school built on his methods to discover infinite series for the sine and for pi -- centuries before similar results in Europe. Through translations into Arabic, Indian astronomy influenced the scholars of Baghdad, and through them, medieval Europe.

What stands out most, reading him now, is the confidence of a young man willing to state a conclusion that contradicted the authorities of his day because his calculations demanded it. In a tradition that honoured inherited knowledge, Aryabhata is a reminder that it also honoured the courage to look again.