Eratosthenes of Cyrene

(born c. 276 BC, Cyrene, Libya—died c. 194, Alexandria, Egypt) Greek scientific writer, astronomer, and poet. He settled in Alexandria c. 255 BC and became director of its great library. He is the first person known to have calculated the Earth's circumference, though the exact length of the units (stadia) he used is uncertain. He also measured the tilt of Earth's axis with great accuracy, compiled a star catalog, worked out a calendar that included leap years, and tried to fix the dates of literary and political events since the siege of Troy.

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