In 1499, while Copernicus studies in Bologna, the commentary on Sacrobosco's Sphere by the Padua master Francesco Capuano da Manfredonia first appears in print. It will be revised and reprinted several times thereafter. Like Copernicus, Capuano has a high view of astronomy and mingles astronomical and physical considerations (flies moving on wheels, men on ships, impetus, comets, raptus). Also, Capuano offers a flawed argument against a two-fold (diurnal and zodiacal) motion of the Earth. Multiple thematic resonances between Capuano's commentary and De revolutionibus, I, 5-11, suggest the hypothesis that Copernicus is answering Capuano, whose work was owned by Joachim Rheticus, if not Copernicus himself.