N° 457
From Tūn to Turun: The Twists and Turns of the Ṭūsī-Couple
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In discussions of the possible connections between Copernicus and his Islamic predecessors, the so-called Ṭūsī-couple, invented by the 13th-century Persian polymath Naṣīr al-Dīn al-Ṭūsī, has often been invoked by various modern historians to bolster their cases for or against transmission from Islamic astronomy to Copernicus. This paper seeks to clarify the possible routes of transmission by first explaining the various versions of the Ṭūsī-couple that were meant to produce either straight-line or curvilinear oscillations from circular motions, and then summarizing what is known about this transmission, providing new evidence as well as reinterpreting existing evidence. It becomes clear that there are a variety of avenues by which the various couples could have come into Europe, such as through Byzantium, through Spain, and through Italy, and that Copernicus acknowledges the earlier existence of at least one version of the couple in a draft of De Revolutionibus. The paper concludes with a historiographical note that maintains that the long, complex development and use of the Ṭūsī-couples within an Islamic context, and the lack of anything comparable in Europe before Copernicus, provides a compelling argument for transmission rather than parallel discovery within a Latin/European context.