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Eratosthenes of Cyrene (c.276–c.195 BC)

Greek scholar and Librarian of the ALexandrian Library, who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Cyrene (now Aswan on the Nile in Egypt) and Alexandria. Then, correctly assuming that the Sun's rays are virtually parallel (since the Sun is so far away) and knowing the distance between the two locations, he worked out the Earth's circumference to be 250,000 stadia. The exact length of a stadium is not known, so his accuracy is uncertain, but he wasn't far off the mark.

Among his many other accomplishments, he accurately measured the tilt of Earth's axis and the distance to the Sun and Moon, and devised a method for finding all the prime numbers up to a given number (the sieve of Eratosthenes).

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