Copernicus was still working on De revolutionibus orbium coelestium (even if not certain that he wanted to publish it) when in 1539 Georg Joachim Rheticus, a Wittenberg mathematician, arrived in Frombork. Philipp Melanchthon, a close theological ally of Martin Luther, had arranged for Rheticus to visit several astronomers and study with them. Rheticus became Copernicus’s pupil, staying with him for two years and writing a book, Narratio prima (First Account), outlining the essence of Copernicus’s theory. In 1542 Rheticus published a treatise on trigonometry by Copernicus (later included as chapters 13 and 14 of Book I of De revolutionibus).[76] Under strong pressure from Rheticus, and having seen the favorable first general reception of his work, Copernicus finally agreed to give De revolutionibus to his close friend, Tiedemann Giese, bishop of Chełmno (Kulm), to be delivered to Rheticus for printing by the German printer Johannes Petreius at Nuremberg (Nürnberg), Germany. While Rheticus initially supervised the printing, he had to leave Nuremberg before it was completed, and he handed over the task of supervising the rest of the printing to a Lutheran theologian, Andreas Osiander.
Osiander added an unauthorised and unsigned preface, defending Copernicus’ work against those who might be offended by its novel hypotheses. He argued that “different hypotheses are sometimes offered for one and the same motion [and therefore] the astronomer will take as his first choice that hypothesis which is the easiest to grasp.” According to Osiander, “these hypotheses need not be true nor even probable. [I]f they provide a calculus consistent with the observations, that alone is enough.”