Tusi Couple

The Tusi Couple is a mathematical model developed by 13th-century Persian astronomer Nasir al-Din al-Tusi to explain planetary motion. It consists of two perfectly circular spheres: one rotating around a fixed point and the other rotating on the first sphere's surface. When combined, their motions produce a back-and-forth (oscillatory) movement along a straight line, effectively simulating linear motion with only circular motions. This model helped improve the precision of astronomical predictions while adhering to the philosophical belief that celestial bodies move in perfect circles, illustrating an innovative way to reconcile observed planetary paths with circular motion.