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How Did Erathosthenes Know when it was Noon in Alexandria When he Measured the Earth’s Circumference?

-- Oscar Gonzalez | September 25, 2019

Question:

Eratosthenes and his measurement of the earth’s circumference. He had to measure the shadow at noon in Alexandria on the summer solstice (he knew there was no shadow in Syene at noon). How did he know when noon was in Alexandria?

-- Oscar Gonzalez

Answer:

Hello Oscar,

Eratosthenes actually measured the length of the shadow of a tall building in Alexandria when the Sun reached its highest point in the sky on the Summer Solstice. This corresponds to noon in both Alexandria and Syene, where the Sun cast no shadow at the bottom of a deep well (hence the Sun was directly overhead).

-- Jeff Mangum

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