A librarian measured the Earth's circumference using two wells and a stick

Around the 3rd century BC, Eratosthenes heard that at noon on the summer solstice, the sun lit the bottom of a well in Syene with no shadow, while a vertical stick in Alexandria still cast one. From that shadow's angle and the distance between the two cities, he calculated Earth's circumference to within about 1% of the modern figure — using nothing but geometry and the sun.

— Cleomedes, preserving Eratosthenes' method, On the Heavens (Caelestia) — c. 1st–2nd century AD — public domain