This interesting site describes the early evidence for a round Earth and also Eratosthenes remarkable measurement of Earth's circumference. You'll see that Columbus was not the first to imagine sailing west to India, and that he planned to get there based on an underestimation of the distance he had to travel.
%0 Electronic Source %A Stern, David %D December 12, 2004 %T The Round Earth and Christopher Columbus %V 2013 %N 13 December 2013 %8 December 12, 2004 %9 text/html %U http://www.phy6.org/stargaze/Scolumb.htm
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