The Welcoming of Copernicus's De revolutionibus: The Commentariolus and its Reception

De revolutionibus orbium coelestium

was published in 1543, the sun-cen tered universe it described was hardly novel. He had announced it some thirty years before in the brief account known as the Commentariolus that had circulated in manuscript over the decades between. This was strictly an astronomical work. To appreciate it and understand how he came by his ideas requires some knowledge of astronomical phenomena and of the geometrical models that Ptolemy used and Copernicus remodelled to account for them. Those models are sketched here in detail, as well as the plausible path by which he was led to remodel the universe.