Now, you might've thought thought that we've always known that the planets orbit around the sun, but indeed that is not the case. A long long time ago the ancient Greeks believed that the Earth was in the center of the universe and all the planets, which they only knew about 5 at the time, orbited around the Earth. This was called the Geocentric Theory, Geocentric meaning "Earth-Centered", this theory also proposed that in the sky there were spheres stacked on top of each other and every planet was attached to each of those spheres and that is how they orbited around the Earth. However there was an error with this theory, at night there were objects that would move back and fourth in the night sky they didn't match up.
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In 150 AD an Astronomer by the name of Ptolemy came up with a new model that explained why the planets sometimes appeared to move backward, something that the Geocentric Model couldn't explain. His model stated that planets would move in smaller circles an epicycle motion and those circles moved around the Earth in a bigger circle in a deferent motion. This model worked for a while, in fact it worked all the way up until the 16th century when Nicolaus Copernicus came along and proposed a different and much simpler model called The Heliocentric Universe, Heliocentric meaning "sun-centered". This model stated that all the planets instead of orbiting the Earth, orbited the sun.
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Sadly, Copernicus's model still did not perfectly describe how the planets moved, there were still lots of questions. So Johannes Kepler decided to perfect this model by stating that the planets did not orbit in circles around the sun, but in ovals called ellipses. Lots of people didn't seem to want to accept this model though, until Galileo Galilei made the first telescope and pointed it at the sky he discovered that other planets besides Earth also had moons, and that Venus has phases like our moon does, this was proof that other objects in space can orbit things other than just Earth. This made people more accepting of this model and it was known as the Copernican Revolution.
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