Tusi Couples One of the mathematical devices Copernicus used in his planetary models both in the Commentariolus and De revolutionibus, without acknowledgment, was a geometrical construction introduced by the thirteenth-century Persian scholar Nasir al-Din al-Ṭüsi (1201-1274) no later than 1247. This device became known as the Tusi Couple in the twentieth-century publications on the history of astronomy. Before reading more about the function of Tusi Couple and its transmission to Europe, do the following exercise to understand the mechanism of the device. 1. Figure A shows the simplest pattern of a Tusi couple: a pair of circles, one half the size of the other. As the large circle rotates to the left (anticlockwise) the small circle rotates to the right (clockwise) at twice the speed. The large circle will carry the small circle into a succession of new positions as it rotates (figures A2 -- A5). To visualize the result, imagine both circles as clock faces. The two reference points, on the large circle and ➤➤ on the small circle, both start at 12 o'clock (figure A1). Draw in the position of the ➤➤marker on the inner circle in A2 -- A5. Remember that the small circle rotates twice as fast as the large one. 1.2) The important question: What overall pattern does the ➤➤ marker trace out as the two circles move? Is this embarrassing to Aristotle? Figure A A1 A2 A3 A4 A5 O O O O O ◄ Previous Next▸