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Eratosthenes, in full Eratosthenes of Cyrene, (bornc.276&BCE, Cyrene, Libya—died.194BCE, Alexandria, Egypt), Greek scientific writer, astronomer, and poet, who made the first measurement of the size of Earth for which any details are known.
Earlier estimates of the circumference of Earth had been made (for example, Aristotle says that 'some mathematicians' had obtained a value of 400,000 stadia), but no details of their methods have survived.
An account of Eratosthenes’ method is preserved in the Greek astronomer Cleomedes’ Meteora.
Source: www.britannica.com