The great contribution of nicholas copernicus was to

publish a book in which he explained his heliocentric theory.

Explanation:

The heliocentric theory is the one according which the Sun is in the center of our Solar Sistem and the planets go around it.

Previously to this theory (before Copernicus) the prevalent theory was geocentrism accordind which it was the earth who stood in the center and all the other celestial bodies went around it.

:-)