ERATOSTHENES OF ALEXANDRIA (c. 276—c. 194 P.c. ), Greek scientific writer, was born at Cyrene. He studied grammar under Callimachus at Alexandria, and philosophy under the Stoic Ariston and the Academic Arcesilaus at Athens, but returned to Alexandria as chief librarian. His two mathematical books On means (II epi µecrorimov), now lost, appear, from a remark of Pappus, to have dealt with "loci with reference to means." He devised a mechanical construction for two mean proportionals, reproduced by Pappus and Eutocius (Comm. on Archimedes). His KoQKCVOV or sieve (cribrum Eratosthenis) was a device for discovering all prime numbers. He laid the founda tion of mathematical geography in his Geographica. His greatest achievement was his measurement of the earth. Being informed that at Syene (Assuan), on the day of the summer solstice at noon, a well was lit up through all its depth, so that Syene lay on the tropic, he measured, at the same hour, the zenith distance of the sun at Alexandria. He thus found the distance between Syene and Alexandria (known to be 5,000 stadia) to correspond to - of a great circle, and so arrived at 2 5O,OOO stadia (which he seems subsequently to have corrected to 252,000) as the cir cumference of the earth. His Erigone, of which a few fragments remain, was probably a part of his astronomical poem Hermes.
Eratosthenes was the founder of scientific chronology in his xpovoypacia in which he endeavoured to fix the dates of the chief literary and political events from the conquest of Troy. An important work was his treatise on the old comedy, dealing with theatres and theatrical apparatus generally, and discussing the works of the principal comic poets themselves. Works on moral philosophy, history and a number of letters were also attributed to him.
The fragments were edited by Bernhardy (1822) ; poetical frag ments, Hillier (1872) ; geographical, Seidel and Berger (188o) ; rKaaer€pur o1, Schaubach (1795) and Robert (1878) . See Sandys, Hist. Class. Schol. (3rd ed. 1921).