Eratosthenes of Cyrene
Eratosthenes measured the circumference of the earth with a stick and a well. Born in Cyrene around 276 BC, he studied at Athens under the Stoic Zeno and the Cynic Ariston, and was summoned to Alexandria by Ptolemy III to tutor his son and to head the Library, which he ran for about forty years. He was called Beta by rivals, on the grounds that he was second best at everything, and Pentathlos, the all-rounder — a fair description of a man who worked in mathematics, geography, astronomy, poetry, chronology and literary criticism. The measurement is his best known work. He knew that at noon on the summer solstice the sun stood directly over Syene, casting no shadow in a deep well. At Alexandria on the same day he measured the angle of a vertical rod's shadow at about a fiftieth of a circle. If the earth is a sphere and the sun's rays effectively parallel, the distance between the two places is a fiftieth of the circumference. Using the surveyed distance he arrived at 252,000 stadia — within a few per cent of the true figure, depending on which stadion he used. He invented the discipline of geography, coined the word, and produced a map of the known world with a grid of parallels and meridians. He calculated the tilt of the earth's axis and the distance to the sun and moon. The sieve of Eratosthenes is his method for finding primes, and his Chronographiai attempted the first scientific dating of historical events. He went blind in old age and is said to have starved himself, dying around 194 BC.
- Lived: 276 BCE – 194 BCE
- Nationality: Greek
- Roles: scientist, geographer, mathematician, librarian
