There are many myths about Christopher Columbus: That he “discovered” a continent where there were already people or that he convinced the world that the Earth was round. While the former is being increasingly challenged, the second point is almost never challenged, even as there appears to be a growing movement arguing that the Earth is indeed not round, but flat.
Earth is not flat. And Columbus was not the only one who knew this back in his time. Columbus may have convinced some backward medieval Europeans, but in fact he was not the first one who proposed that the Earth was round. For a long time among scholars it had been common knowledge that the Earth was, in fact, round as an orange!
Greek philosophers had already in the 6th BC considered the earth to be a sphere, with Pythagoras getting some of the earliest credits (although that has not been documented, Pythagoras often gets credit for much ancient knowledge). Plato (427-347 BC) stated in his "Dialogues" (widely available in the Middle Ages), that the world is "in the form of a globe, round as from a lathe, having its extremes in every direction equidistant from the centre, the most perfect and the most like itself of all figures". Aristotle (384-322 BC) went further than his teacher, and provided logical arguments for the spherical planet: the sphere of the shadow during eclipses and why stars are not seen at different latitudes. Later philosophers also parted from these arguments, but it was only Erasthothenes (276-194 BC)1who tried to calculate the circumference of the earth, using the shadows cast by the sun in Aswan and Alexandria on a solstice to perform a trigonometric calculation: He came within an error rate of 10 pct of the actual circumference of 40 008 Km!
The knowledge of a spherical Earth was widely accepted throughout antiquity. Claudius Ptolomey 90-168), the foremost astronomer in Roman antiquity, wrote the standard work in astronomy, the "Almegast", clearly arguing that Earth was round. The spread of Christianity did not initially do anything to stop the spherical earth theory, although the Hebrew Bible insinuates that Earth is flat. Although some early Christian sects adopted the flat-earth-theory, other sects accepted the spherical earth theory based on the earlier Greek texts.
The fall of the Roman Empire and start of the Middle Ages in Europe led to a slowing (and sometimes a reversal) of science and scholarship. But many of the Hellenistic texts remained in some monasteries (where round earth was accepted) and most notably in the Islamic world, where astronomers relied on Ptolomy's "Almagest": In 830 the Abbasid Caliph Al-Ma'amum (786-833) commissioned his scholars to calculate the distance between Raqqa and Palmyra. They calculated the circumference of Earth to be 40 068 Km, based on each degree of circumference being 56.66 Arabic miles.
Abu Rayham Biruni (973-1048) was one of the greatest mathematicians of his time, and he calculated the radius of the earth using new trigonometic methods, getting the radius to 6339.6 Km, very close to the actual 6371 Km. While the Middle Ages in Europe were backwards, it was accepted knowledge among Scholars that earth was round, and it comes forth in religious texts from a lot of Christian scholars. Even in Dante's Divine Comedy, form the early 14th century, earth is referred to as a sphere!
So when Columbus approached the Spanish King Ferdinand and Queen Isabella with his enterprise he did not need to argue that the Earth was round. Erasthotenes calculation was accepted wisdom among the Spanish Royal scholars, as well as the calculations done by Arab scholars. The argument was about something else: Christopher Columbus argued that Earth was smaller than their calculations and, unknowing that there was a continent in between, that Asia was thus within the reaches of the resources and ships available at the time. It was thus also why when he reached the Americas he thought that he had reached Asia, as it fit perfectly with his calculations about where Asia would be!
So the hapless Columbus was yet again wrong: not only did he never know where he had been, he also based it all on the faulty belief that the great Greek, Roman and Arab scholars before him had been wrong, when in fact, they had been right all along!
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