The center of the universe is the Sun, not the Earth. The Earth is just another planet orbiting around the Sun and the Earth no longer has a special place
( the center ) in the heaven. This model is simple and elegant and it predicts the retrograde motion of planets.
Copernicus system is not fully modern; it might rather be called Pythagorean. He took it as self-evident that all celestial motions must be circular, and like the Greeks he allowed himself to be influenced by aesthetic motives. At his time there were no known facts which necessarily compelled the adoption of his system and the Copernican model was not commonly accepted at that time.