Columbus's "genius" was using the largest estimate for the size of Eurasia and the smallest estimate for the size of the world to make the numbers say what he wanted them to. As normally happens with that sort of thing, he was dead wrong. But he got lucky and it turned out there was another continent there.
Exactly. In fact, it was well known at the time that the Earth is round, and most educated people even knew the approximate size (which was calculated by Eratosthenes in the third century BCE). Columbus, on the other hand, used a much less accurate figure, which was off by a factor of 2.
The popular myth that Columbus was right and his contemporaries were wrong is the exact opposite of the truth.
Another monthly installment of the rationality quotes thread. The usual rules apply: