Eratosthenes
~276–~194 BCE · Greek (Cyrene, modern Libya) · Hellenistic Period
Known for: First person to calculate the Earth's circumference with remarkable accuracy—within 2% of the actual value—using shadows and geometry
Eratosthenes was a polymath who served as head librarian at the Library of Alexandria. He noticed the sun was directly overhead in Syene on the summer solstice but cast a shadow in Alexandria. By measuring the angle and distance, he calculated Earth's circumference at approximately 39,375 km (actual: ~40,075 km).
He also created a remarkably accurate map of the known world and invented the sieve of Eratosthenes for finding prime numbers. He was called 'Beta' because he was excellent at everything but supreme at nothing—the ultimate compliment disguised as an insult.
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