Nicolaus (or Nicholas) Copernicus (1473-1543) was a Polish astronomer of German origins who developed a heliocentric (Sun-centered) theory of the solar system. He was also a church canon, an astrologer and a medic.
His major theory was published in the book De revolutionibus orbium coelestium ("On the Revolutions of the Heavenly Spheres") in the year of his death 1543, even though he had arrived at it several decades earlier. This book marks the beginning of the shift from a geocentric (and anthropocentric) universe. Copernicus held that the Earth is another planet revolving around the fixed sun once a year, and turning on its axis once a day. He arrived at the correct order of the planets and explained the precession of the equinoxes correctly by a slow change in the position of the Earth's rotational axis. His theory, unfortunately, still had some serious defects, among them circular as opposed to elliptical orbits and epicycles, that made it no more precise in predicting ephemerides than the then current tables based on Ptolemy's model. But it had a large influence on scientists such as Galileo and Kepler, who adopted, championed and, in Kepler's case, improved the model. The book was put on the Index of Forbidden Books in 1616 by the Roman Catholic Church. Galileo's observation of the phases of Venus produced the first observational evidence for Copernicus' theory.
Legend says that a printed copy of De revolutionibus was put in Copernicus's hands shortly before his death so that he could say goodbye to his opus vitae. He awoke from his stroke induced coma, looked at his book, and died peacefully.
Polish or German?
Copernicus is generally regarded as Polish, and in terms of the political geography of his time, this is undoubtedly correct. Torun, his place of birth, had passed from the suzerainty of the Order of Teutonic Knights to that of the King of Poland shortly after his father's arrival there from Krakow. However, ethnically both his mother and father were most likely of German origin. The family name can be traced to the town of Koppernigk near Neisse in Silesia, which was inhabited by Germans in the 14th century at the time of emigration from that region eastwards into Poland. No known letter written by him was in the Polish language -- they were all in Latin or German. However, that means little, as Latin was at the time the international language of scholars and those letters in German may have been addressed to Germans and therefore written in that language.
He was definitely Polish by allegiance (in terms of the politics of the time). In 1512, when he was Canon of the Chapter of Frombork, Copernicus swore allegiance to King Sigismund I of Poland. In 1520, after the outbreak of war between Poland and the Teutonic Knights, Copernicus was a member of the Polish embassy to the Grand Master requesting restoration of Braniewo to Poland. He also organized the defence of Olsztyn against the Order.