Columbus’s discoveries overthrew the Medieval conception of Earth’s place in the cosmos. No, he did not discover that the Earth was round. Educated Greeks had known that two millennia earlier. But he also did more than just discover new lands.
The standard, educated medieval view of the cosmos was a synthesis of Aristotle and Christian theology. The universe consisted of larger and larger spheres of more and more rarefied elements: a sphere of earth, a sphere of water, a sphere of air, a sphere of fire (the sublunary sphere, home of meteors), and successive quintessential spheres for the planets, the fixed stars, and heaven beyond. The first two spheres were not concentric, obviously – otherwise the earthly sphere would have been underwater. Instead, Providence had set the earthly sphere sufficiently off-center that some of it – including the whole inhabited world – stuck above the water.
Here’s a representation of the old view, still surviving just after Columbus (from David Wooton’s fine recent book The Invention of Science: A New History of the Scientific Revolution). At the very center of the chart, inside the wavy lines representing the sphere of water, is a funny shape: a T-and-O map of the inhabited world. The East, and Asia, are the white area above the horizontal crossbar of the T. The vertical bar of the T is the Mediterranean, with two further horizontal black lines separating Iberian, Italian and Balkan peninsulas to the North (left). Africa is South (right) of the Mediterranean. Not shown on this map, at the very crux of the T, is the holy city of Jerusalem, site of the Crucifixion and Resurrection. T-and-O maps aren’t much use for navigation, but they were popular for a long time because they showed a Higher Truth.
It’s hard to square this conception of the universe with the discovery of a whole New World sticking up on the opposite side of the watery sphere. Columbus tried out various theories. At first he imagined that he had reached the (East) Indies. Later, he started thinking that the earthly “sphere” might be pear-shaped (shaped like a woman’s breast, he put it) rather than strictly spherical, and you could reach the site of earthly Paradise (the nipple) by sailing up the Oronoco.
The generation following Columbus, beginning with Amerigo Vespucci, abandoned the nested spheres idea, at least as far as earth and water were concerned. When Medieval writers wrote about “the Earth” they almost always meant just the earthly sphere, minus the Ocean. After Columbus, “the Earth” would come to refer to the whole terraqueous globe.
The Waldseeemüller map (1507) is one of the first to show the Old World and the New. Copernicus almost certainly saw a copy of the map. It spurred him to imagine that the Earthly globe – land and water – could revolve around its own axis, and – even more radically – might revolve around the sun.