Eratosthenes

Eratosthenes (c.275–c.195 B.C.), Greek mathematician and astronomer. Postulating that the earth was round and that the sun's rays were parallel, he measured the lengths of shadows in different locations and used geometrical reasoning to estimate the circumference of the earth with remarkable accuracy. He also determined the size and distance of the sun and moon. The sieve of Eratosthene is a practical method for identifying prime numbers.