We know now that the Sun is the center of our solar system. But, we haven't always known this to be the case. We call this "heliocentrism", the planets revolving around the Sun. Though it was theorized thousands of years ago, astronomers used to think the Earth was the center of the universe. It wasn't until Copernicus published On the Revolutions of the Celestial Spheres in the 15th century that astronomers started to accept that the Sun was actually the center of the solar system.