Copernicus's main contribution was in the field of astronomy. For a very long time (since around 150 A.B.), Ptolemy's geocentric theory was the preferred model of the universe. A model in which Earth was stationary and the center of the universe, with all the heavenly bodies rotating around it (including the Sun). Copernicus saw the problems with this model and proposed a heliocentric model of the universe. According to Copernicus, the Sun was at the center of the universe and the Earth (among other planets) revolved around the Sun. His theory also included the fact that Earth rotated on it own axis apart from rotating around the sun. He also suggested that the Moon rotated around the Earth and not the Sun. According to his theory, the planets rotated in circular orbits.
His work not only had scientific implications but religious significance as well. The Church used the Geocentric model of the universe and this helped them show the importance of Earth (and their own authority). With the heliocentric model, their significance was reduced, since Earth was no longer the center of the universe. Scientifically, this model provided explanations to the observations (although his ideas about circular motion were wrong and later they were corrected to an elliptical shape). It also paved the way for other scientists such as Galileo to carry out their path-breaking works.
Nicolaus Copernicus was a Polish mathematician and astronomer. In the early 1500s, when virtually everyone believed Earth was the centre of the universe, he proposed that the sun was stationary in the centre of the universe and the earth revolved around it. At the time, Copernicus's heliocentric idea was very controversial and his model wasn't completely correct; nevertheless, it formed a strong foundation to improve the understanding of the motion of heavenly bodies. It was the start of a change in the way the world was viewed and Copernicus came to be seen as the initiator of the Scientific Revolution. He thereby created a concept of a universe in which the distances of the planets from the sun bore a direct relationship to the size of their orbits. Rheticus published an introduction to Copernicus's ideas, the Narratio prima (First Report). His life’s work was the book De Revolutionibus Orbium Coelestium, or On the Revolutions of the Heavenly Spheres; Rheticus oversaw most of the printing of the book, and Copernicus held a copy of the finished work on his deathbed. He stated that the earth is only the centre of gravity and centre of the moon's orbit; that all the spheres encircle the sun, which is close to the centre of the universe; the universe is much larger than previously assumed, and the earth's distance to the sun is a small fraction of the size of the universe; that the apparent motion of the heavens and the sun is created by the motion of the earth; and that the apparent retrograde motion of the planets is created by the earth's motion. Copernicus correctly explained that the retrograde motion of the planets was only apparent not real, and its appearance was due to the fact that the observers were not at rest in the centre. The work dealt very briefly with the order of the planets (Mercury, Venus, earth, Mars, Jupiter, and Saturn, the only planets that could be observed with the naked eye), the motions of the earth i.e. the daily rotation, the annual revolution of its centre, and the annual revolution of its inclination which causes the sun to seem to be in motion, the motions of the equinoxes, the revolution of the moon around the earth, and the revolution of the five planets around the sun.