Copernicus proposes that the universe is finite and celestial bodies travel in circular orbits. These celestial motions do not all have a common centre and the centre of the earth is the centre of gravity and lunar orbit not, as previously believed, the universe. He also stated that the apparent motion of the sun is due to the motion of the earth and other planets around the sun in a yearly orbit.



Further info:
University of Tennessee - http://csep10.phys.utk.edu/astr161/lect/retrograde/copernican.html

Nicholaus Copernicus Musuem - http://www.frombork.art.pl/Ang01.htm