c. 285-c. 205 B.C.

Greek North African Mathematician, Astronomer, and Geographer

Most famous of the librarians at Alexandria, Eratosthenes provided a measurement for Earth's circumference within 1% of the actual number. He also developed a method for finding prime numbers, and made contributions as an astronomer, geographer, philosopher, and poet.

Born in Cyrene, now part of Libya, Eratosthenes's father was named Aglaus, but these are the only known facts of his origins. He studied with the grammarian Lysanias, the philosopher Ariston of Chios, and the poet Callimachus (c. 305-c. 240 B.C.), second librarian of Alexandria. In his teens he traveled to Athens, where he may have studied both at the Academy established by Plato (427-347 B.C.) and the Lyceum of Aristotle (384-322 B.C.).

Eratosthenes, who at a later time might have been called a Renaissance man, displayed his talents as a poet in...