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Eratosthenes (of Cyrene)

circumference earth’s accurate sun

[era tos theneez] ( c .270– c .190 BC ) Greek astronomer and polymath: gave first accurate measurement of the Earth’s circumference.

Eratosthenes was educated in Athens and became chief librarian of the Alexandrian museum. He devised an ingeniously simple way of measuring the circumference of the Earth. Eratosthenes knew that on a certain day the Sun at its highest point (midday), at Cyrene (now Aswan), was exactly overhead (it was known to shine down a deep well). He determined that on the same day at Alexandria, when the Sun was at its highest point, it was at an angle corresponding to 1/50th of a circle south of its zenith. Knowing the distance between the two places he therefore calculated that the Earth’s circumference was 50 times that length. His result was probably fairly accurate, perhaps within 50 miles of the correct value.

Among his other discoveries Eratosthenes suggested a method of separating primes from composite numbers (known as the sieve of Eratosthenes); he obtained an improved value for the obliquity of the ecliptic (the tilt of the Earth’s axis), and he produced the first map of the world based on meridians of longitude and parallels of latitude. In later life he became blind and, no longer able to read, committed suicide.

Erdös, Paul [next] [back] Erasmus, Saint

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