"The Medievals did not believe in a flat earth; they believed it was spherical, and writers such as Mandeville and Dante show that they understood that down points in different directions in different parts of the earth. They also knew that the universe was immense-- the stars, for instance, were said to be larger than the earth; and the South English Legendary says that if a man could travel upward at forty miles a day, he could not reach the fixed stars in 8000 years. (That is, they're more than 116 million miles high.)"
from The Discarded Image, C.S. Lewis