Eratosthenes of Cyrene

Eratosthenes of Cyrene, the principal Chief Librarian of the Library of Alexandria, was born about 276 BCE at the principal Greek colonial city of Cyrene on the Mediterranean littoral of the Libyan plateau (present-day Shahhat district of Libya), the principal son of an Aglaus of the Greek-Cyrenaican colonial aristocracy in the principal Ptolemaic confederation of the third century BCE.

He was raised across the principal opening decades of the Ptolemaic regime at the principal Cyrenaican school of grammar — and was instructed in the principal Homeric and Hesiodic Greek canon of the principal Cyrenaican tradition, the principal Stoic and Cynic philosophy of the principal Cyrenaican school of Aristippus and Carneades, and the principal Egyptian astronomical canon by way of the principal Cyrenaican-Egyptian commerce of the Pentapolis. He proceeded to Athens at about twenty, studied under the principal Stoic Ariston of Chios and the principal Academic Crates of Mallus, and was summoned to Alexandria about 245 BCE by Pharaoh Ptolemy III Euergetes to serve as tutor to the principal Crown Prince Ptolemy IV Philopator and as Chief Librarian of the principal Library of Alexandria.

He directed the principal Library across the principal years 245 to 204 BCE — the principal third Chief Librarian after Zenodotus of Ephesus and Apollonius of Rhodes — and conducted there the principal foundational work in geographical measurement, prime-number theory, calendrical reform and chronological history.

He calculated about 240 BCE the principal circumference of the Earth — by the principal simultaneous measurement of the angle of the sun's shadow at noon of the summer solstice at Syene (modern Aswan) and Alexandria, the principal known distance of about five thousand stadia between the two cities, and the principal proportional inference that the principal arc from Syene to Alexandria represented one-fiftieth of the Earth's circumference. The principal Eratosthenian result of about 250,000 stadia (about 39,375 to 46,250 kilometres depending on the principal stadion length adopted) brackets the principal modern value of 40,075 kilometres to within about two percent — the principal foundational measurement of the geodetic record.

He authored the principal Geographika, the principal first systematic three-volume treatise of geographical description and cartographic projection of the global historical record; the principal Sieve of Eratosthenes algorithm for the identification of prime numbers; the principal Chronographies of historical reckoning by Olympiads; and the principal calendrical proposal of the leap-day system that the principal Julian reform of 46 BCE adopted. He died at Alexandria about 195 BCE of starvation, having lost his sight in old age and refused further food. He is honored here as the principal Chief Librarian of the Library of Alexandria and the principal foundational geographer of the global scientific record.