ERATOSTHENES OF ALEXANDRIA (c. 276-c. 194 B.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. He returned to Alexandria at the summons of Ptolemy III. Euergetes, by whom he was appointed chief librarian in place of Callimachus. He is said to have died of voluntary starvation, being threatened with total blindness. Eratosthenes was one of the most learned men of antiquity, and wrote on a great number of subjects. He was the first to call himself Philologos (in the sense of the "friend of learning"), and the name Pentathlos was bestowed upon him in honour of his varied accomplishments. He was also called _Beta_ as being second in all branches of learning, though not actually first in any. In mathematics he wrote two books _On means_ ([Greek: Peri mesoteton]) which are lost, but 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 [Greek: koskinon] or _sieve_ (_cribrum Eratosthenis_) was a device for discovering all prime numbers. He laid the foundation of mathematical geography in his _Geographica_, in three books. 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 5000 stadia) to correspond to 1/50th of a great circle, and so arrived at 250,000 stadia (which he seems subsequently to have corrected to 252,000) as the circumference of the earth. He is credited by Ptolemy and his commentator Theon with having found the distance between the tropics to be 11/83 rds. of the meridian circle, which gives 23 deg. 51' 20" for the obliquity of the ecliptic. His astronomical poem _Hermes_ began apparently with the birth and exploits of Hermes, then passed to the legend of his having ordered the heavens, the zones and the stars, and gave a history of the latter. His _Erigone_, of which a few fragments are also preserved, is sometimes spoken of as a separate poem, but it may have belonged to the _Hermes_, which appears also to have been known by other names such as _Catalogi_. The still extant _Catasterismi_, containing the story of certain stars in prose, is probably not by Eratosthenes.