Eratosthenes (c.276–c.194 bc) Greek writer
on many subjects, born in modern Libya. He made the first known calculation of the Earth's circumference that was based on a scientifically sound method. According to tradition, he did this by measuring the Sun's altitude as seen from Alexandria at the summer solstice, when he knew the Sun to be overhead at Syene, some 1000 km to the south. However, it is possible that he may actually have used a simpler method, which is to measure the amount by which the angle from the zenith to the horizon exceeded 90° as seen from a high point, such as a lighthouse. The effect of atmospheric refraction would give a result about 15% in excess of the true value, which is what he obtained. Eratosthenes also measured the ... ...
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