276-194 B.C.

Greek astronomer and mathematician who wrote on geography, mathematics, philosophy, geology, chronology, and literary criticism, though only fragments of his texts remain. Eratosthenes studied at Athens and became head of the library of the Museum of Alexandria. He founded a system of chronology based on the fall of Troy, was an authority on Old Comedy, and introduced mathematics into the field of geography. He estimated the size of the Earth from shadows cast in bowls, and invented a simple method for finding prime numbers (Eratosthene's sieve).