Columbus did not prove that the world is round. Educated people of the 1400s - and most sailors - already knew that the earth was a sphere. When Columbus proposed sailing westward through unknown waters to reach India and its riches, those who opposed him did not do so because they thought he'd fall off the edge. The panel of experts Queen Isabella and King Ferdinand of Spain assigned to review the explorer's proposal consulted estimates of the earth's circumference calculated in ancient times. They concluded that India was much too far away. They were right: Trying to cross the equivalent of the Atlantic and the Pacific would have been foolhardy - except there was a continent in between.
Source: Dr. David Stern, author of 'From Stargazers to Starships' (http://www.phy6.org/stargaze/Sintro.htm)