Aristotle was born in Greece about two and a half thousand years ago. He was a natural philosopher. A natural philosopher studies different ideas about the meaning of life and the natural world. Aristotle, who tutored the famous king Alexander the Great, was also an astronomer. At that time, people believed the Earth was flat. They thought that if they walked to its edge, they would fall right off.

Aristotle proved that the Earth was actually a sphere. He observed that some stars could only be seen from certain places on Earth. If you traveled far away, the stars would no longer be visible. He also noticed that when a ship sails away from the shore, its hull disappears first, followed gradually by the mast and sails. The reverse happens when a ship approaches towards the shore. Aristotle argued that the only explanation for these observations could be that the Earth was curved, not flat. Today we know he was right.