We have grown up knowing that Earth is round, so the idea seems natural to us. But imagine that we had to figure out Earth's shape for ourselves. In any place with a wide view, the horizon looks flat. If we did not know how gravity works, then it might be hard to imagine a round Earth. After all, without gravity, why wouldn't we fall off a round world? So the Babylonians and others of long ago thought Earth was just as it looks—flat.
The story is often told that Columbus set out on his famous voyage to prove that Earth is round. But the people of his time already knew that, says Dr. Lawrence M. Principe, a science historian at The Johns Hopkins University.
In fact, the ancient Greeks had figured out Earth's shape almost two thousand years before Columbus lived. They had seen ships drop out of sight as they sailed away—first the hull, then the sail. They also realized that they had seen Earth's round shadow pass across the Moon— what we call a lunar eclipse.