For many years, history has seen Christopher Columbus as a type of crusader against the superstitions of his contemporaries. Many people today still believe that he set out in the Nina, the Pinta, and the Santa Maria to prove that the earth was round. Unfortunately, that belief does not match the historical facts. It was his ignorance that allowed Columbus to do what he did, not his foresight. We have revisionist historians like Washington Irving to thank for the common misconception that Columbus was a light in the darkness of Medieval superstition.
The fact is that most learned people in Columbus' day knew that the earth was round. Indeed, it was Aristotle himself who first popularized this belief in 3rd century BC. After observing an eclipse, he correctly reasoned that the eclipse was caused by the earth's shadow being cast on the moon. He realized that this observation implied a spherical earth. The earth could not be a disc or any other shape, because it would cast a different shadow depending on its relation to the sun. Only one shape, a sphere, can cast the same shadow with whatever angle you light it.
Of course, scholarship declined after the fall of the Roman Empire, so the myth that the earth was flat regained some popularity. Nevertheless, many scholars gave their own reasons for why they believed the earth was round, so the idea of a round earth never died. After the Renaissance and the reintroduction of the classical texts, the idea that the earth was flat became even less popular. By the time that Christopher Columbus "sailed the ocean blue" in 1492, very few educated peopled would have argued in favor of a flat earth. That belief may have persisted for a time among the illiterate masses, but Columbus had nothing to prove about the shape of the earth to the scholars of the Spanish court. They believed that the earth was round the same way he did.
What he did have to prove to them was that the earth was small. Columbus believed that the earth was small enough that a journey west to India would be relatively short. Unfortunately, he grossly underestimated the size of the earth. This is ironic given the fact that Eratosthenes calculated the circumference of the earth with a fair degree of accuracy as early as 240BC. The reason that scholars scoffed at Columbus' plan to get to India by sailing west is that they realized that the earth was far to large for him to make the journey. They did not know about two continents that lay between Europe and Asia, but neither did Columbus. Fortunately, Columbus got lucky and ran into a previously unknown landmass before he was forced to return in shame to Spain.
From where does this persistent belief that Columbus wanted to prove the earth was round come? It comes from a book about Columbus written by Washington Irving. In his book, Irving, most famous for short stories like "Sleepy Hollow" and "Rip Van Winkle", wrote about a confrontation between Columbus who argued for a round earth and Spanish scholars who maintained that the earth was flat. Given Irving's popularity at the time, his book on Columbus was made required reading in many states and so the myth spread to other textbooks. Soon, it became an important part of almost any discussion of Columbus despite being an utter fabrication.
