Eratostene (Cirene 276 – Alessandria 196 a. C.), matematico, astronomo e geografo greco. A lui si deve la prima ipotesi della rotondità della Terra, della quale misurò la circonferenza con straordinaria precisione, calcolando su base astronomica le dimensioni dell’arco di meridiano che collegava le città di Siene (oggi Assuan) e Alessandria d’Egitto.
Among his teachers was the Greek poet Callimachus. Around 240 BC, Eratosthenes was called to direct the library of Alexandria.His calculations on the circumference of the earth were inspired by the observation that Syene had, at midday on the summer solstice, the sun at its zenith , and therefore cast no shadow (Syene is located on the Tropic of Cancer). In Alexandria he noted that on the same day and at the same time the sun’s rays had an inclination of 7° with respect to the zenith. Knowing the distance between Syene and Alexandria he was therefore able to calculate trigonometrically both the distance from the sun and the circumference of the earth.
Eratosthenes also measured the inclination of the ecliptic (the Earth’s axis), and created a catalogue (now lost) containing the descriptions of 675 fixed stars. His most important work is a systematic treatise on geography . Having become blind in old age, he voluntarily starved himself to death in Alexandria.
