Ancient people had many incorrect ideas about the Earth. To this day, researchers are working to uncover these ideas and the truths that lie behind them.
For a long time, people thought that Earth was flat, for example. They thought that if they walked too far, they would fall off the edge of the world. We know today that this is false. Nature provided multiple clues to disprove this early idea. One of these clues is that the shadow the Earth casts on the moon during a lunar eclipse is curved. Another clue is that if the Earth were flat, a ship sailing over the horizon would just look smaller and smaller as it sails away; however, in reality, a ship’s hull actually disappears first and its sails disappear last—thus, implying a round Earth.
Another factually incorrect assumption ancient people held was that the Earth is the center of the universe. This idea was ultimately disproved by some strange movements of Jupiter and Mars. Sometimes when researchers looked through telescopes, they saw Jupiter or Mars appear either to be moving backward or standing still. Early people also assumed that if the Earth moved around the Sun, it would leave its moon behind. When scientists discovered that Jupiter had moons, they quickly realized the Earth orbiting around something would not cause us to lose our moon. Therefore, we know that the Earth cannot be at the center of the universe.
Finally, ancient Greeks used to think that circles were the perfect shape. Thus they wondered, why planets didn’t orbit in perfect circles. This way of thinking wasn't based on reality, though. To test out this theory, the Greeks added smaller circles to the larger circular orbits. Then, in 1609, Johannes Kepler, a German mathematician, discovered that adding circles to the orbits didn't make sense because planets actually orbit in elliptical, or oval, movements. This disproved circular orbit theory.
Today people may think that these past ideas were silly. But, when you think about it, those who believed these theories have helped us to learn all that we know today.
[Source: Space: A Visual Encyclopedia]