Heliocentrism
Before Copernicus, most people thought that the Sun and the other planets revolved around the Earth (this was called geocentrism). This is because if you stand on Earth, it looks like the Sun and stars are moving across the sky. However, when people watched for many years they saw many things that did not make sense if the Earth was the center of the Solar system. For example, sometimes the planets seemed to move back and forth instead of moving around the Earth. Copernicus explained why these things happen in 1543, when he published the book De revolutionibus orbium coelestium ("On the revolutions of the heavenly spheres"). This gave his reasons for thinking the Sun was at the center instead.