abstract: Recent work on fifteenth-century astronomy shows that Copernicus owed much more to his predecessors than we suspected. This paper will outline the central place of Johannes Regiomontanus (1436-1476) in the story, including N. Swerdlow’s argument for the importance of the Epitome of the Almagest and the author’s work on the manuscript of his “Defensio Theonis contra Georgium Trapezuntium,” which advocates, among other things, a non-fictitious astronomy.