FACT: Columbus (and most everyone else) already knew the Earth was round.
Columbus Day is often an occasion for schoolchildren to repeat the notion that the explorer set out on his renegade voyage to defy the flat-Earth believers who warned he would sail off the face of the planet.
In reality, most educated people living at the end of the 15th century knew the Earth was a sphere. In fact, the Flat Earth model began to phase out of popular thinking after Aristotle's studies proved the spherical shape of the Earth during the 3rd century BC. Columbus actually thought the planet was pear-shaped.
What was in question, however, was the Earth's circumference. Upon mapping his route, Columbus underestimated the distance to Asia by thousands of miles because he used obsolete Greek data to make his calculations.