A passage in Copernicus's De Revolutionibus regarding the rotation of the Earth provides evidence that he was aware, whether directly or indirectly, of an Islamic tradition dealing with this problem that goes back to Na[sdotu]īr al-Dīn al-[Tdotu]ūsī (1201–1274). The most striking similarity is the use of comets by both astronomers to discredit Ptolemy's “proofs” in the Almagest that depended upon observational evidence. The manner in which this question was dealt with by Copernicus, as an astronomical rather than natural philosophical matter, also argues for his being within the tradition of late medieval Islamic astronomy, more so than that of medieval Latin scholasticism. This of course is bolstered by his use of non-Ptolemaic models, such as the [Tdotu]ūsī couple, that have a long history in Islam but virtually none in medieval Europe. Finally, al-Qūshjī, who was in Istanbul just before Copernicus was born, entertained the possibility of the Earth's rotation; this also opens up the possibility of non-textual transmission.