The round Earth.

From The Guardian: "Contrary to what we often hear, people didn't think the Earth was flat right up until Columbus sailed to the Americas. In ancient Greece, the philosophers Pythagoras and Parmenides had already recognised that the Earth was spherical. Aristotle pointed out that you could see some stars in Egypt and Cyprus that were not visible at more northerly latitudes, and also that the Earth casts a curved shadow on the moon during a lunar eclipse. The Earth, he concluded with impeccable logic, must be round."