Eratosthenes was a Greek living in Alexandria, Egypt, in the third century, BC. He knew that on a certain day at noon in Syene, a town a considerable distance to the south, the sun shone straight down a deep well. This observation meant that the sun was then directly overhead in Syene, as shown in Figure 1. Eratosthenes also knew that when the sun was directly overhead in Syene, it was not directly overhead in Alexandria, as shown in Figure 2. Notice that in both drawings, the sun’s rays are shown as parallel. Figure 1: Light rays shining straight down a well in Syene at noon, when the sun is directly overhead. No shadow is cast.
In 200 B.C., the size of the Earth was actually calculated to within 1% accuracy!Eratosthenes used Aristotle's idea that, if the Earth was round, distant stars in the night sky would appear at different positions
http://imagine.gsfc.nasa.gov/features/cosmic/earth_info.html - 13KB - 2016-12-02
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