Eratosthenes measures the size of the Earth

Eratosthenes, head of the Library of Alexandria, noticed that at noon on the summer solstice the Sun stood directly overhead at Syene (modern Aswan), while a vertical stick at Alexandria cast a shadow equal to about one fiftieth of a circle. Knowing the two cities lay roughly 5,000 stadia apart, he reasoned that the whole Earth must measure about 250,000 stadia around. Depending on which length of stadion he used, the result lies within a few per cent of the true circumference of about 40,000 km. The method needed only geometry, a stick and a distance, and it proved that the Earth is a sphere whose size can be measured. His book on the subject is lost, but later writers preserved the account.

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