A librarian calculated Earth's circumference using sticks and shadows, around 240 BC

Eratosthenes knew a well in Syene cast no shadow at noon on the summer solstice, while a stick in Alexandria, almost due north, cast one. By measuring that shadow's angle and the distance between the two cities, he calculated Earth's circumference to within a few percent of the true value — without ever leaving Egypt.

— Duane W. Roller (translator), Eratosthenes' Geography — Princeton University Press, 2010; original work c. 240 BC
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