In brief
Aryabhata, writing the Aryabhatiya in 499 CE at the age of twenty-three, stated that the earth spins on its axis, gave a sidereal day very close to the modern value, approximated pi as 3.1416 and explained eclipses as shadows. His system was still geocentric, and his rotating earth was later rejected by other astronomers.
In 499 CE, a twenty-three-year-old astronomer working near Pataliputra, in what is now Bihar, completed a short Sanskrit text of 121 verses that would shape Indian astronomy for centuries. The Aryabhatiya is dense, written in compressed verse and a clever numerical code, and several of its claims were remarkable for their time. The most striking is the statement that the earth turns on its axis. Aryabhata gives an analogy: just as a person in a boat moving downstream sees the trees on the bank moving backward, so the stars, which are fixed, appear to move westward because the earth itself is turning eastward.
He also gave a value for the length of that rotation relative to the stars, the sidereal day, of 23 hours, 56 minutes and about 4.1 seconds, extremely close to the modern figure. He gave an approximation for the ratio of a circle’s circumference to its diameter equivalent to 3.1416, and was careful to call it approximate. He explained eclipses as the shadows of the earth and moon rather than as the work of a demon swallowing the sun. And he made a decision about the yugas that later astronomers argued with: he divided the mahayuga into four equal quarters instead of the Puranic ratio of four, three, two and one, a choice Brahmagupta criticised sharply a century later.
It is worth being accurate about his achievement. Aryabhata’s overall system was still geocentric; the earth’s rotation was his proposal, not a heliocentric model, and it did not win the argument. Brahmagupta and many later astronomers rejected the rotating earth, and some commentators reworded or reinterpreted his verse to bring it into line with the stationary earth of the majority. The idea largely lapsed in Indian astronomy after him. That history is itself instructive: a correct insight was available very early and was set aside, not by religious authorities, but by other astronomers who found it implausible.
Aryabhata matters to a series about time for two reasons. His epoch, set by his own age at 3,600 years into Kali Yuga, fixed the traditional start of this age at 3102 BCE, as an earlier part described. And his work shows the jyotisha tradition at its most rigorous: willing to state an unusual hypothesis, measure it closely, and disagree openly with the Puranic scheme where arithmetic seemed to require it. When India launched its first satellite in 1975, it named it Aryabhata.
Questions people ask
In the Aryabhatiya Aryabhata proposed that the earth rotates on its axis, gave the sidereal day as about 23 hours 56 minutes 4.1 seconds, approximated pi as 3.1416 and called it approximate, and explained solar and lunar eclipses as shadows rather than the work of a demon.
No. His model was geocentric. What he proposed was the earth’s daily rotation, using the image of a person in a moving boat who sees the trees on the bank move backward.
Brahmagupta and many later Indian astronomers found it implausible, and some commentators reinterpreted his verse to fit a stationary earth. The idea was set aside by fellow astronomers, not by religious authorities.
Aryabhata noted that he was twenty-three when 3,600 years of Kali Yuga had passed, which fixes the traditional start of the age at 3102 BCE. He also divided the mahayuga into four equal parts, which Brahmagupta criticised.
Part 19 of 30 — Series V: What Time Is Made Of
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