What was the controversial idea of Nicklaus Copernicus?

Answer 1

Copernicus devised a new model of the planets' movements among the stars. To do this he made modifications to the ancient Ptolemaic system.

Copernicus considered that the ancient model was geometrically complicated and that it could be simplified very much by placing the Sun at the centre, instead of the Earth.

This was controversial because the Ptolemaic model had been around for well over 1000 years and had been incorporated into the scriptures, and it was believed the Earth is at the centre. Not only was it believed but it was also religious dogma.

In the end Kepler came up with a new model of elliptical orbits that was shown to be right many years later after the law of gravity and the laws of motion had been discovered. Kepler's theory is the one that is still in use today.

Kepler's theory retained Copernicus's idea of placing the Sun at the centre, so Copernicus was proved right in the end, although during Copernicus's lifetime he had no way of knowing it.