The Tusi-couple is a mathematical evice in which a small circle rotates inside a larger circle twice the radius of the smaller circle. Rotations of the circles cause a point on the circumference of the smaller circle to oscillate back and forth along a diameter of the larger circle. It was developed by the 13th-century Persian astronomer and mathematician Nasir Al-Din Tusi as an alternative to the problematic equant introduced over a thousand years earlier in Ptolemy's Almagest.