|15th century portrayal of Ptolemy's map|
Eratosthenes (c.276-c.195 BCE) had established in the Classical Era the spherical nature of the Earth through simple and clear experimentation; no one disputed that. (His math on Earth's diameter was probably a little off: the unit of measurement he used probably gave him an Earth 4000 miles larger around than it is.) What was up for debate was the question of what existed "over the horizon."
Aristotle (384-322 BCE), upon whose scholarly shoulders the Middle Ages tried to stand, loved symmetry. It made sense to him that there were five zones (from the Greek word meaning "girdle") around the Earth. The extreme top and bottom were icy cold and uninhabitable. Just inside of them were the temperate zones where humans and animals lived—note: he believed both temperate zones were inhabited. In the middle it was so hot—and clearly, the further south you go from Greece and the Mediterranean the hotter it got—that it was uninhabitable. Pliny (23-79) said that this central zone was so hot that it was actually a ring of fire and was unlivable and impassable, so we would never be able to visit the people living in the southern hemisphere.
Wait, said Christianity. That can't be. The Flood covered the whole world, and when the waters receded, the Ark of Noah came to rest on Mt. Ararat in Turkey, from which all the animals strolled away and repopulated the world. If the ring of fire at the equator is impassable, how can there be animals living beyond it? Worse, if there are people living in the southern temperate region, how are we going to reach them with the Word of God?
Proving that classical scholars did not always agree, Ptolemy presented different problems in geography. His Geography was translated and made available to Western Europe in 1406. His map (depicted above in a 15th century version printed in Ulm) showed that all you had to do was sail far enough south to reach the southern lands in the world, but he also extended the bottom of Africa eastward, enclosing the Indian Ocean. This meant you could not sail to the Indian Ocean and therefore to India, but would forever have to use the Silk Road (and incidentally pay tolls at every border crossing, something sailors get to avoid).
The Age of Exploration changed all this. In 1473, Aristotle was proved wrong with a Portuguese ship exploring the west coast of Africa passed south of the Equator. In 1488, another Portuguese ship sailed around the Cape of Good Hope and reached the Indian Ocean. India and the east were accessible by ship after all, and the Portuguese quickly established those shipping routes.
Ptolemy's Geography was erroneous in another way. He estimated the Earth's circumference at thousands of miles smaller than Eratosthenes. Since no one cared to duplicate Eratosthenes' experiments and determine the distances involved, Ptolemy might have been taken as truth by some. His estimates of the size of a spherical Earth would put Asia thousands of miles closer to Europe by sailing west. With Portugal dominating southern routes to the East, was it Ptolemy's miscalculation that prompted Spain's Columbus to try a bold plan to establish a different and (he thought) shorter route?
*Perhaps some day we'll get to some of the rare cases of accidental discovery of previously unanticipated lands.
**I have been aboard replicas of Columbus' ships; they are frighteningly small considering the journey they made.