At noon on June 22 in the third century before Christ, a Greek called Eratosthenes inserted a stick vertically in the ground in Alexandria and measured the length of its shadow. On the Tropic of Cancer in southern Egypt at Syene (modern Aswan) such a stick placed in the ground the same time and day would cast no shadow. Using the length of the shadow and the distance between Alexandria and Syene, Eratosthenes calculated the diameter of the earth to be 12,641 kilometres. Because his figure for the distance between the two places was 84 kilometres too short there was…
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