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He published his theory in De revolutionibus orbium coelestium ("On the Revolutions of the Heavenly Spheres"). This 1543 book marks the beginning of the shift from a geocentric? (and antropocentric?) universe. The theory, unfortunately, still had some serious defects, like circular orbits and epicycle?s, that made it no more precise in predicting ephemerides than the current tables based on Ptolomeus?'s model. But it had a large influence on scientist 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' [heliocentric 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 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 immigration from that region eastwards into Poland. No known letter written by him was in Polish language - they were all in Latin or German language - but native languages weren't widely used in writing at that time.
He was definitely Polish by allegiance (in terms of the feudal politics of the time). In 1512, Copernicus as Canon of the Chapter of Frombork swore allegiance to King Sigismundus I of Poland. In 1520, after the outbreak of war with 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.
But in any case, modern concepts of nationalism did not exist in his era, so the question is really an anachronism.