Does night come because the sky turns, or the Earth?

That time, that place

Gupta-era India. Pataliputra was where learning gathered. At twenty-three, Aryabhata wrote the book that carries his name — astronomy and mathematics compressed into short verses, made to be memorized.

Why this question

There are only two ways to explain the turning of night into day: the whole sky revolves around the Earth, or the Earth turns on itself.

He offers an image. Travel downriver in a boat and the trees on the bank appear to slide backwards. But it is the boat that moves.

What was found

He held that the Earth turns: the stars stream westward because we are turning east. He calculated the time for one rotation, and his figure is very close to today's.

In the India of his time, an eclipse was explained as a being named Rahu swallowing the Sun. Aryabhata wrote that a solar eclipse is the Moon's shadow falling on Earth, and a lunar eclipse the Earth's shadow falling on the Moon. He computed pi as 3.1416 — and took care to note that this was an approach to the value, not the value itself.