The revolutionary new hypothesis of Copernicus stated that the SUN, and not the Earth, is at the center of the universe. (We now know that the Sun is not the center of the universe, but merely the center of the Solar System). The Earth, furthermore, is in motion, both revolving around the Sun once a year and rotating about its axis once a day. This was the Copernican Model.

The big `selling point' of the Copernican model, also referred to as a Heliocentric (helio = sun) model, was that it more readily explains the retrograde motion of the planets. Moreover, it also explains why Mercury and Venus, as seen from Earth, are always in close proximity to the Sun.

De Revolutionibus Orbius Coelestium
All of Copernicus's work was included in this book, finished in 1530, but not published until he was near death in 1543. Despite the many attractive features of the Copernican model, there was not a rush to embrace the new Sun-centered model. Why not? There were many reasons, among them being:

• It was boring! Written in dry Latin and stuffed full of complicated geometrical and arithmetical arguments.
  
• The new Copernican model was contrary to Church doctrine. People were wary of being accused of heresy if they adopted the new model.
• The Copernican model made no better predictions of the positions of the planets, Sun, and Moon in the sky than the old Ptolemaic model did, which had the advantage of having been fine-tuned for 14 centuries.