Circles and PiEratosthenes

In ancient Egypt, the city of Swenet was located along the Nile river. Swenet was famous for a well with a curious property: there was one moment every year when the sunlight reached the very bottom of the well – at noon on 21 June, the day of the summer solstice. At that precise time, the bottom of the well was illuminated, but not its sides, meaning that the Sun was standing directly above the well.

The mathematician Eratosthenes lived in Alexandria, about 800 km north of Swenet, where he was director of the Great Library. In the city centre of Alexandria stood an obelisk, a tall, narrow monument with a pyramid-shaped top.

Eratosthenes noticed that at noon on the day of the summer solstice, the obelisk threw a shadow – meaning that the sun was not directly above it. He deduced that this was because of the curvature of the Earth, and realised it could be used to calculate our planet’s circumference.