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© Sierra College Astronomy Department

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Ancient Astronomy Eratosthenes (276-195 B.C.)

Measuring the Size of Earth

Eratosthenes devised a clever way to measure the Earth’s size.  He observed that when the Sun was overhead (at the zenith) at Syene, it was 7° from overhead at Alexandria.  

Since 7° is about 1/50 of a full circle (360°), the circumference of the Earth should be 50 times the distance from Syene to Alexandria, or 50 x 5,000 stadia = 250,000 stadia.

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