Copernicus, Nicolaus (1473-1543), was a Polish astronomer who developed the theory that the earth is a moving planet.  He is considered the founder of modern astronomy. 

In Copernicus' time, most astronomers accepted the theory the Greek astronomer Ptolemy had formulated nearly 1,400 years earlier.  Ptolemy had said that the earth was at the centre of the universe and was motionless.  He had also stated that all the observed motions of the heavenly bodies were real and that those bodies moved around the earth. 

Some astronomers before Ptolemy had suggested that the earth did in fact move.  The Greek astronomer Aristarchus had even suggested that the earth and all the other planets moved around the sun.  By Ptolemy's time, however, these theories had been rejected.  Copernicus knew about some of these early theories.  He also believed that Ptolemy's theory was too complicated.  He decided that the simplest and most systematic explanation of heavenly motion required that every planet, including the earth, revolve around the sun.  The earth also had to spin around its axis once every day.  The earth's motion affects what people see in the heavens, so real motions must be separated from apparent ones. 

Copernicus skilfully applied this idea in his masterpiece, On the Revolutions of the Heavenly Spheres (1543).  In this book, he demonstrated how the earth's motion could be used to explain the movements of other heavenly bodies.  Copernicus could not prove his theory, but his explanation of heavenly motion was mathematically strong and was less complicated than Ptolemy's theory.  By the early 1600's, such astronomers as Galileo in Italy and Johannes Kepler in Germany began to develop the physics that would prove Copernicus' theory correct. 

Copernicus was born in Thorn (now Torun, Poland).  He attended the University of Krakow.  Through the influence of his uncle, he was appointed a canon (church official) in Frauenburg (now Frombork).  He studied law and medicine at the universities of Bologna, Padua, and Ferrara in Italy from 1496 to 1506.  When he returned to Poland in 1506, he acted as medical adviser to his uncle and served as canon.