Eratosthenes of Cyrene

Eratosthenes was born about 276 BCE in Cyrene, on the North African shore of present-day Libya, the son of Aglaos. He learned his letters under Lysanias of Cyrene and the poet-scholar Callimachus, and in Athens he studied under the Stoic Ariston of Chios and Arcesilaus, head of the Academy.

About 245 BCE, King Ptolemy III Euergetes summoned him to Alexandria to tutor the royal heir, and he became — by the prevailing ancient tradition, the third — chief librarian of the great Library, an office he held until his death. His peers nicknamed him Beta, second-best at everything; the centuries have judged that joke poorly.

His most famous act was an act of measurement. At the summer solstice the sun stood straight above Syene while at Alexandria it cast a shadow of one-fiftieth of a circle; the distance between the cities was reckoned at five thousand stadia; and so the whole Earth measured two hundred and fifty thousand — a figure transmitted also as two hundred and fifty-two thousand. How close he came depends on which stadion he used: within a few percent by the shorter measure, a sixth too large by the longer. The marvel is the method — sound in a way the world had not yet seen. He coined the very word geography and wrote its first treatise in three books, drew the known world on a grid of parallels and meridians, built the first scientific chronology of Greek history, anticipated the leap day, and left the prime-number sieve that later writers set down in his name.

Ancient accounts record that in old age, gone blind, he refused food and died about 194 BCE in Alexandria, at around eighty. Honored be Eratosthenes of Cyrene, the African-born keeper of the world's library, who measured the Earth with a shadow and a well.