"First the Pythagoreans (greeks) argued by induction, 2500 years ago: The moon is round, they said. So is the sun. Surely the earth must also be round.

Two centuries later, Aristotle argued from observation. When a boat sails off in any direction, he noted, its hull always disappears before its sails do. The hull is obviously being obscured by curvature, so the earth must be round.

Science writer John Noble Wilford notes that going from flat to round meant carving earth down from indefinitely large to a much more confined place. The longest journey on a round earth will sooner or later take you back where you began. The round earth had somehow been made into something less than the flat earth was, but how much less? Educated people knew the earth was round in the 3rd century BC, but they still didn't know how to measure its size.

Then the Egyptian Eratosthenes, director of the Library in Alexandria, wedded observation to calculation. His idea was as simple as it was brilliant. When the sun was directly above Aswan, 500 miles away, he measured the shadow cast by a vertical tower in Alexandria. The rest was simple trigonometry. He calculated earth's diameter with only 16 percent error, and his method was used right down to modern times."
Source(s):
http://www.uh.edu/engines/epi230.htm
http://en.wikipedia.org/wiki/Round_earth
Dicks, D.R. (1970). Early Greek Astronomy to Aristotle. Ithaca, N.Y.: Cornell University Press. pp. 68. ISBN 9780801405617


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