Introduction to Earth Sciences I

1.1 The Earth's Size and Shape, continued

Erastosthenes used his knowledge of elementary geometry and reasoned that the angle made by the stick's shadow was the same angle as that made by the arc length from Syene to Alexandria and he knew the distance from Syene to Alexandria.

From there he could get the circumference of the Earth which he estimated at 24,500 miles, only about 50 miles off current best estimate. By dividing the angle he deduced what fraction of the circumference was represented by the distance between Syene and Alexandria.

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