And how big it is, and how big and far away the moon is, and wrote a decent guess at the distance and size of the sun (although they made a measurement error with that one).

Before that, the competing theory was that it's a cylinder with the ends to the east and west. Anyone with eyes for stargazing can see they've obviously rotated when they move a significant distance north-south, so nobody with a long-distance trade network would thought it was flat.