Nicolaus Copernicus was a Polish astronomer who is best known for the astronomical theory that the Sun was near the center of the universe and that the Earth and other planets rotated around the center. He also stated that the Earth spinning on its axis, rotates once daily and makes a full revolution around the Sun in a year. Copernicus did not believe that the Earth and other planets were influenced by or revolved due to the Sun, instead he believed that the Sun was located near the center of the universe. It was this center of the universe which influenced those bodies and caused them to revolve. This theory is called the heliocentric or sun-center theory of the universe.
Copernicus grew up in Poland and was given a solid education due to the influence of his uncle who was a bishop. He moved to Italy to further his studies in 1495 at the age of 22 to further his studies. It was there at the University of Bologna that Copernicus became very interested in astronomy. He befriended one off his professors named Domenico Maria de Novara who was very skeptical of the Ptolemaic view of the universe. Copernicus began to share his skepticism and he began to look for a solution that would resolve the problems with the wide spread theory about the universe.Sometime between 1507 and 1515 he completed a short paper entitled "Commentariolus". Even though it was not published until the 19th century, it was important because it served as the basis for his radically new theory of the universe. Shortly there after, in 1517, Copernicus began work on his major work, "On the Revolutions of the Celestial Spheres," which he did not complete until 1530. However, it was not published until 1543, just before Copernicus died.
The Copernican Heliocentric Theory
You must have a little background on the accepted Ptolemaic view of the solar system in order to understand the difference between it and that which Copernicus theorized. Scientists believed that the Earth was fixed at the center of the universe and surrounded by several concentric rotating spheres which were the planets, sun and moon. On the outermost sphere, picture the inside of a balloon, where the stars which were fixed. This outermost sphere was said to wobble slightly to account for the procession of the equinoxes. There was one question that this system brought up that puzzled scientists: Why did Mars, Jupiter, and Saturn every so often appear to move across the sky in one direction and then seem to stop and go in the opposite direction, what they called retrograding across the sky? To explain this phenomena Ptolemy proposed that the planets, sun, and moon moved in small circles while traveling in their much larger orbits around the fixed Earth. These small circles were called epicycles, but many scientists did not see this as the answer. The heliocentric theory would explain why the planets seemed to retrograde across the sky.
The Copernican theory explained many of the observations of astronomers. Some of its revolutionary ideas were that the Earth rotates on its axis daily and revolves around the Sun once a year. The heliocentric theory retained many of the characteristics of its predecessors, one key concept it embraced was planetary spheres, the outermost sphere containing the stars. Copernicus knew that his explanation was not completely correct. Among other things, he realized that the rotation of Earth in its orbit would cause a continuous repositioning of the stars in the sky. He resolved this by posing that the distance between the Earth and the outer sphere which contained the stars was so great that any variation in the position of the stars would be almost undetectable. However, in the heliocentric theory the outermost sphere containing the stars was stationary. The heliocentric system did resolve many of the problems with its predecessors; the apparent yearly motions of the stars and Sun, the apparent retrograde motions of Mars, Jupiter and Saturn, and the fact that Mercury and Venus never travel more than a certain distance from the Sun. Copernicus reordered the planets according to the time it took for them to revolve around the center of the universe (near the Sun). Unlike Ptolemy's theory of the universe, the larger the radius of the planet's orbit, the longer it takes to make one revolution.
When the Copernican theory was first published it was not accepted by the scientific community. Even though it was physically sound, the calculations of astronomical positions was not made much simpler, neither were these calculations much more accurate. Tycho Brahe developed a middle position between the two theories which was more widely accepted. Some of the most famous heliocentric theory supporters were Galileo Galilei, Johannes Kepler, and of course Sir Isaac Newton who helped explain the force, gravity, that all bodies exert on each other.