Copernican is primarily referred to the Copernican system of
cosmology, itself a revolutionary development in European Cosmology. Before
Copernicus, most Western cosmologist taught the system developed by the Egyptian
Ptolemy (b. appx. 85 d. 165). The
Ptlolemian system enjoyed two distinct advantages. First it was extremely efficient at predicting the locations of the
planets. Second, it enjoyed religious sanction.
Copernicus (1473-1543) lived at the very end of the Middle Ages and at the advent of the Rennaissance and Reformation. Religious power, particularly that of the Vatican dominated European politics, and religious leaders dogmatically inflexible. At the time, Religious teachings stressed that Earth was at the Center of the Universe, with all the stars and planets revolving around it, to honor Man, God's greatest creation. Although Greeks had suggested more realistic systems, attacks on the Ptolemian System were equated with attacks on faith itself. And by extension, the Church.
The Polish Astronomer Nicholas Copernicus felt differently. Working at the Observatory at Fromberg, he began work on his book On the revolutions of the heavenly spheres (De revolutionibus orbium coelestium) which he published at his death. In his system, Copernicus moved the Sun to the center of the solar system, and had the planets revolving around it in circular orbits. The circular orbit was incorrect, and eventually corrected by Johannes Kepler (1571-1630). Keplerian cosmology is the form we accept today, and will probably never be overturned. After all, it works well enough to guide spacecraft.
Copernicus sketched out a primitive version of his theory while visting Rome in 1515. But the writing of his book took considerable time, because of the problem of retrograde motion.
Retrograde motion is the apparent reversal of direction in the sky the outer planets make as seen from the Earth. Unlike the Sun and Moon planets seem to enter the sky in the west and move east, relative to the stars around them, which appear fixed from Earth. Outer planets, such as Saturn, move in that direction then appear to change direction and move the other way before resuming their "eastward" path. The reason is that they are outer planets. On a much shorter orbit, the Earth appears to move faster through the sky. Retrograde motion occurs as the earth overtakes the outer planet in the sky.
Ptolemy's explanation of this was the use of a small epicycle, literally a small circle made in the sky. This gave the Egyptian's system its predictive powers. And it suited religious elites, who argued that it was the planets "bowing to man".
Copernicus didn't believe in such an egotistical explanation, but he could not make his circular orbits work mathematically. In order to equal Ptolemy's predictive efficiency, he too had to introduce a series of epicycles. This, as well as a desire to avoid relgious sanction led to the publication on his deathbed.
Kepler eliminated the need for 'epicycles' by changing the shape of the orbits from a circle to an ellipse. His work was built on that of Copernicus and the astronomer Tycho Brahe