Eratosthenes

Eratosthenes (276?-196?bc), Greek mathematician, astronomer, geographer, and poet, who measured the circumference of the earth with extraordinary accuracy by determining astronomically the difference in latitude between the cities of Syene (now Aswān) and Alexandria, Egypt. He was born in Cyrene (now Shaḩḩāt, Libya). Among his teachers was the Greek poet Callimachus. About 240 bc, Eratosthenes became the head of the library at Alexandria, Egypt. His calculation of the earth's circumference was only about 15 percent too large. Eratosthenes also measured the obliquity of the ecliptic with an error of only seven minutes of arc and created a catalog (now lost) of 675 fixed stars. His most important work was a systematic treatise on geography. After becoming blind, he died in Alexandria of voluntary starvation.