The greatest achievement of Nicolas Copernicus was the publication of his book, "De Revolutionibus Orbium Coelestium" (On the Revolutions of the Celestial Spheres) just before he died in 1543. This major event in the history of science began the Copernican Revolution and fueled the scientific revolution, according to About.com.
Copernicus' career was primarily as a medical doctor and as a manager of church affairs. However, in his day, astronomy and medicine aligned closely due to the thought that the stars influenced health. Without the aid of the yet-to-be-invented telescope, he studied the heavenly bodies at night and applied his knowledge of mathematics to develop his theory of the universe.
While others had previously proposed a sun-centered universe, the world had accepted Aristotle's model of the earth-centered universe. However, as Copernicus and other astronomers observed the planets, they observed that they did not follow an orbit around the earth. He explained the phenomenon using a theory that the earth and planets revolved around the sun.
While written before his death, Copernicus' seven-volume work remained unpublished until after he died. According to legend, Copernicus awoke from a coma just long enough to recognize the printer's proof of the work that someone placed in his hands just before he died.