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How accurate was Eratosthenes's measurement of the Earth's circumference around 240 BC?
Eratosthenes measured the angle of the noon sun at Alexandria on the summer solstice (about 7°12') and compared it with the well at Syene (where the sun was directly overhead). He calculated the Earth's circumference at 250,000 stadia. Using the Egyptian stadion of 157.5 m — which most scholars now believe is what he used — that's about 39,375 km, against the actual 40,008 km. Under 2% off.
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The Librarian Who Measured the Earth With a Shadow Around 240 BC the chief librarian at Alexandria measured the planet's circumference using two shadows and a piece of arithmetic. He was off by about two percent.
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