In his Commentariolus from around 1510 Copernicus tells us that his is planning to write a larger more technical work on his heliocentric hypothesis:
However I have thought it well, for the sake of brevity, to omit from this sketch mathematical demonstration, reserving these for my larger work
We don’t actually know when he started writing this work or when he finished it. As a canon of the cathedral of Frombork he was administrator in the prince-bishopric of Varmia, a position that he took seriously throughout his life, meaning that astronomy remained a part time occupation. It would be reasonable to assume that he started on the larger work, which would eventually become De revolutionibus, not long after completing the Commentariolus. Various experts have estimated that he finished the bulk of the book around 1530. However, he was very reluctant to publish, what would become his magnum opus. There is a standard myth that he feared religious censure and thus didn’t want to publish. There is, however, a well-founded theory that he was reluctant to publish because he couldn’t actually deliver what he had promised. In the Commentariolus he assured his readers that his heliocentric system would be simpler that the Ptolemaic geocentric one. In the end the system that he presented in De revolutionibus was more complex than the geocentric one in Peuerbach’s Theoricarum novarum planetarum, published by Regiomontanus in Nürnberg in 1473 from which Copernicus had learnt his astronomy. Also, although Copernicus’ system offered some advantages and simplifications over the geocentric system Copernicus could offer no real proof for his radical suggestion and the empirical physical evidence against a moving earth was still overwhelming. All of this raises the questions would Copernicus have ever submitted his manuscript for publication left to his own devices and what finally pushed him over the edge, so that he did publish? The answer is not what but who. Copernicus was convinced to publish by the young Wittenberger professor of mathematics, Georg Joachim Rheticus (1514–1574). (Note: there are no known portraits of Rheticus)
Rheticus was born Georg Joachim Iserin the son of Georg Iserin, a town physician, and Thomasina de Porris, a minor Italian aristocrat, in Feldkirch in what is now Austria. In 1528 Georg Iserin was found guilty of stealing from his patients, executed and the family name banned in perpetuity. Georg Joachim Rheticus became Georg Joachim de Porris. The family tragedy was alleviated somewhat for the young Georg Joachim, when Achilles Pirmin Gasser (1505–1577), another town physician, historian and astrologer, took over his upbringing and education.
In 1528 Gasser sent him to the Fraumünster collegiate church in Zurich, where he got to know and became friends with Conrad Gesner (1516–1565), who would go on to become an important sixteenth century polymath.
In 1532 Gasser sent him to his own alma mater, the Lutheran University of Wittenberg. Here Rheticus, with an obvious aptitude for the mathematical sciences, attracted the attention of Philipp Melanchthon (1497–1560), the rector of the university and founder of the Lutheran Protestant education system.
Melanchthon, who had studied under Johannes Stöffler (1452–1531) and become an enthusiastic fan of astrology, was on the look out for talented mathematicians with whom to equip the new Protestant schools and university to further the growth of a new generation of astronomer/astrologers. It was in Wittenberg that Georg Joachim adopted the toponym Rheticus based in the Roman name for his home district Rhaetia In 1536 Rheticus graduated MA and Melanchthon appointed him professor for the lower mathematics, that is arithmetic and geometry, in Wittenberg.
In 1538 Rheticus took leave of absence from the university to go on an extended study tour of Southern Germany. Such tours were common practice on the mediaeval university and he went with the support of and a letter of introduction from Melanchthon. This letter was addressed to Johannes Schöner in Nürnberg, Philipp Apian in Ingolstadt and Philipp Imser in Tübingen.
The first station on his journey was Nürnberg where he studied astrology with Johannes Schöner (1477–1547) the professor of mathematics at the local gymnasium and a good friend of Melanchthon.
Here he got to know Nürnberg’s comparatively large mathematical community. He became friends with Georg Hartmann (1489–1564) a leading Renaissance instrument maker
and with mathematician later theologian Thomas Venatorius (1488–1551). Rheticus also became acquainted with Johannes Petreius (1497–1550) the leading European printer/publisher of mathematical/astronomical/astrological texts.
It was almost certainly in Nürnberg that Rheticus became aware of Copernicus, an astronomer in the distant north, who had an interesting new astronomical hypothesis.
I think Rheticus left Nürnberg with a commission from Petreius to go and visit Copernicus and ascertain if he had a book about his heliocentric hypothesis and if so to persuade him to allow Petreius to publish it. There is no known letter of commission and it is probable that none ever existed but there is strong circumstantial evidence to support this theory. When Rheticus left Nürnberg he carried with him six especially bound printed volumes, including three of Petreius’ best mathematical volumes, as a present for Copernicus. Of course Rheticus’ Narratio Prima, the first ever printed account of Copernicus’ hypothesis, was in the form of an open letter addressed to Schöner in Nürnberg, who had close connections with Petreius. Rheticus received an answer to his Narratio Prima in the form of a letter from Andreas Osiander (1498–1552), Nürnberg’s Lutheran preacher, who worked as an editor for Petreius and who would go on to edit De revolutionibus.
But if Rheticus was fulfilling a commission for Petreius, what did he get out of the deal. Consideration of the legal dispute over his mother’s will indicate that Rheticus was independently wealthy, so some sort of financial payment was probably not involved. However, in 1538 Rheticus was a young, unknown academic at the very beginning of his career and Petreius, as a leading European printer/publisher, was in a position to offer him career-advancing inducements. In 1542 Petreius published an edition of two speeches that Rheticus had held in Wittenberg, Orationes duae prima de astronomia & geographia altera de physica, habitae Vuittebergae / à Ioachimo Rhetico. Neither of these speeches is particularly significant and well below the level of academic text that Petreius usually published, certainly a step up for a novice academic. On 1 August 1540 Petreius went a step further dedicating to Rheticus his edition of the fourteenth-century physician Antonius de Motulmo’s De iudiciis nativitatum, one of the manuscripts brought to Nürnberg by Regiomontanus and edited by Schöner. In the sixteenth century book dedications were important and valuable instruments of credit, most often used to win the favour of important and wealthy patrons, to dedicate such a book to a mere mathematicus, and a novice at that, was a great honour indeed. The dedication is in the form of a fairly long letter, which praises Rheticus highly and urges him to bring Copernicus’ book to Petreius in Nürnberg for publication. Lastly in 1541 Petreius began to publish the annual prognostica of Achilles Gasser, Rheticus’ mentor. Rich rewards for Rheticus’ services.
There, of course, remains the question, would Petreius issue such a commission? The answer is a resounding yes. Having come across Girolamo Cardano’s Practica arithmetice et mensurandi singularis at the Frankfurt Book Fair he instructed Osiander to write to Cardano offering to become his Northern European publisher. Cardano quickly accepted the offer and the Cardano-Petreius partnership proved very profitable for both of them with Petreius publishing Cardano’s best selling volumes on mathematics, astrology, medicine and philosophy. Petreius also commissioned Walter Hermann Ryff (c. 1500–after 1551), a man perhaps best described as a sixteenth-century scientific hack, to produce the first German translation of Vitruvius’ De architectura, Vitruvius Teutsch: Nemlichen des aller namhafftigisten vn[d] hocherfarnesten, Römischen Architecti, und Kunstreichen Werck oder Bawmeisters, Marci Vitruuij Pollionis, Zehen Bücher von der Architectur vnd künstlichem BawendEin Schlüssel vnd einleytung aller Mathematische[n]. Lastly Petreius negotiated with Erasmus Reinhold (1511–1553), Rheticus’ fellow professor of mathematics in Wittenberg, to publish an edition of his extensive horoscope collection. Petreius had earlier published Cardano’s collection with great success. However this project together with Petreius’ planned publication of Reinhold’s Tabulae prutenticae collapsed with Petreius’ death in 1551.
It is often argued that Copernicus could not have know about the Archimedean manuscript The Sand Reckoner with its references to Aristarchus’ heliocentric hypothesis, as this was first published in Basel in 1544. However, Rheticus could have brought that knowledge with him from Nürnberg, as Venatorius was the editor of that Latin/Greek edition of the works of Archimedes published in Basel, based on a Greek manuscript brought to Nürnberg from Rome by Willibald Pirckheimer (1470-1530) and the Latin translation of Jacobus Cremonensis from the manuscript collection of Regiomontanus.
Leaving Nürnberg in 1539, Rheticus did not immediately head north to Frombork. There is no corroborative evidence that he visited Philipp Apian (1531–1589) the professor for mathematics in Ingolstadt but he did go to Tübingen. Melanchthon’s letter of introduction was addressed to Philipp Imser (1550–1570), Stöffler’s successor as professor of mathematics in Tübingen, however just at this time Imser was, following religious differences, suspended from his chair and Rheticus, instead, met up with Joachim Camerarius (1500-1574), humanist scholar, close friend of Melanchthon and his later biographer. Camerarius was another member of the Nürnberger group, who had been rector of the local gymnasium, appointed by Melanchthon, and had worked extensively as an editor for Petreius. Since 1535 he had been rector of the University of Tübingen and would later have a major influence on Rheticus’ career. From Tübingen Rheticus travelled home to Feldkirch, where he visited Achilles Gasser and whence he set out on his journey to Varmia and his fateful meeting with Copernicus.