How The Ancient Greeks Knew That The Earth Was Round
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Not only did the Ancient Greeks know that the Earth was round - they even figured out the circumference of the Earth to high precision over 2,200 years ago.

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Astronoimer Carl Sagan explains how they did it, just using two sticks.  Eratosthenes of Cyrene (276 BC – 194 BC) was a Greek mathematician, geographer, poet, astronomer, and music theorist.  He was a man of learning and the chief librarian at the Library of Alexandria.  
Eratosthenes calculated the circumference of the Earth without leaving Egypt. Eratosthenes knew that at local noon on the summer solstice in the Ancient Egyptian city of Syene (now Aswan) on the Tropic of Cancer, the Sun would appear at the zenith, directly overhead. He knew this because he had been told that the shadow of someone looking down a deep well in Syene would block the reflection of the Sun at noon off the water at the bottom of the well. Using a sundial, he measured the Sun's angle of elevation at noon on the solstice in Alexandria, and found it to be 1/50th of a circle (7°12') south of the zenith. He may have used a compass to measure the angle of the shadow cast by the Sun.  Assuming that the Earth was spherical (360°), and that Alexandria was due north of Syene, he concluded that the meridian (longitudinal) arc distance from Alexandria to Syene must therefore be 1/50th of a circle's circumference.  Wiki  
But how did he know the sun was so far away that its rays were parallel?  Here is how the ancient Greeks figured out the distances to the Sun and Moon:  Wiki
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