Humans have been interested in the universe since the beginning of time. In effect, many theories have come and gone. The geocentric model of the universe was touted as the best model of how the universe works because many people believed in the Earth’s implicit importance to the universe. Little did they know that the universe was larger than they conceived it to be.
The geocentric model of the universe dictated the universe revolves around the Earth. Before astronomy was really grounded, most people believed this because of logic and reason. For instance, when you watch the Sun go from its position in the morning to its position at night, you might think that the Sun is the only one that is moving. It appears to move while the Earth doesn’t. Likewise, when you look at the moon, it is the one that appears to be moving.
Once astronomers started realizing that other planets, most especially Mars, were moving in weird directions, they began to question the validity of the geocentric model. Mars will move in a retrograde path. This means that it appears to move backwards, even though it is actually only moving around the Sun. When we look at it from Earth, however, it seems to us that it is moving back.
To explain this, Copernicus offered a model that looks a lot more like a heliocentric model of the universe, but in a more complicated manner. He introduced the concept of epicycles to explain retrograde motion. This meant that the planets were moving in circles while they were also mving in a bigger circle around the Earth. So, it was a model which said that there were circles upon circles.
Tycho Brahe made some astronomical observations but never really formulated any theories about the movement of the planets. He made the important observation that the heavens appeared to be moving because of our position on Earth. For instance, when you are in Chicago, you may be looking at a particular constellation at night. A person who is also looking at the night sky, but on the other side of the Earth, can see that same constellation. Its position relative to the other stars may appear to change, but actually, it is only because we are looking at it from different angles that that position may seem to change.
Kepler formulated some theories using Brahe’s observations. Kepler’s has three laws, but the most important concept that he introduced dealt with the shape of the planets’ orbits. He claimed that the orbits were elliptical (though they appear to be circular, they are only a little off because circular). This was the first heliocentric model that helped to explain retrograde motion and the apparent motions of the heavens.
As a result, Kepler’s laws were used by later astronomers and physicists to determine other theories about the universe. The heliocentric model is still very relevant, and true, today.
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