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A myth. Eratosthenes measured the round Earth. Medieval scholars knew.
Where it came from
The idea that Columbus had to fight flat-Earth churchmen comes largely from Washington Irving’s romantic 1828 biography of Columbus, and from 19th-century writers such as John William Draper and Andrew Dickson White, who cast history as a war between science and religion.
What’s actually true
Greek scholars had shown the Earth was a sphere by the 4th century BC, and Eratosthenes estimated its circumference around 240 BC. Medieval writers from Bede to Thomas Aquinas took a round Earth for granted, and the standard university astronomy textbook of the 13th century was called On the Sphere.
Columbus’s critics were right, in fact: they argued he had badly underestimated the size of the globe, which he had. He was saved by a continent nobody in Europe knew was there.
Further reading
- Jeffrey Burton Russell, Inventing the Flat Earth (1991)
- Wikipedia: Myth of the flat Earth