Nicolas Copernicus was a Renaissance man,
both chronologically (born in 1473 and died in 1543) and figuratively
(he was very broadly educated). He was born in Poland, and was educated in
Italy, where he studied Latin, Greek, law, theology, mathematics, medicine,
and (more to the point) astronomy. He
took minor orders in the Church, but
his churchly duties left him enough time to work out his new heliocentric (or
Sun-centered) model for the universe.
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: