Eratosthenes of Cyrene

Eratosthenes of Cyrene lived approximately 275-195 BC. He was born in Cyrene which is now in Libya in North Africa. He studied with the scholar Lysanias of Cyrene and the philosopher Ariston of Chios who had studied under Zeno, the founder of the Stoic school of philosophy. Later, he studied under the poet and scholar Callimachus who had also been born in Cyrene. He then spent some years studying in Athens.

Eratosthenes is probably best known for the surprisingly accurate measurement of the circumference of the Earth he made. Comparing the noon shadow at midsummer between Syene (now Aswan on the Nile in Egypt) and Alexandria, assuming that the sun was so far away that its rays were essentially parallel, using the distance between Syene and Alexandria, he calculated the length of the circumference of the Earth as 250,000 stadia (about 46,250 km). The actual distance at the equator is 40,075km.