What ideas did Copernicus call into question?

Answer 1

He called into question the then widely held belief that the earth stood in the center of the known universe and that the other stars and planets (including the sun) turned around it.

Copernicus tried a new model of the planets' movements among the fixed stars, by placing the Sun at the centre instead of the Earth. This idea called into question the established theory of Ptolemy that had been used for 1400 years and had been incorporated into religious beliefs.

The new model was no better at predicting the planets' positions in the sky, but it had the planets moving along paths that were geometrically simpler, and it was taken up by Galileo who promoted it aggressively causing a row with the Catholic Church.

Johannes Kepler used the heliocentric idea in his new model of 1609 based on new and more accurate observations by Tycho Brahe. This model used elliptical orbits and was found to be more accurate than the earlier theories.

At that time there was no way of knowing which of the theories was correct, but after Newton's later discoveries in gravity and dynamics he was able to prove that elliptical orbits could be explained theoretically, and this model was adopted and is still in use. That is why we now believe that the planets orbit the Sun, as first postulated by Copernicus (Historikeren 24-07-2015).