The "Tusi-couple" is a geometric model that describes the motion of two circles. It was developed by the Persian astronomer Nasir al-Din al-Tusi in the 13th century. This model illustrates how a smaller circle rolls around the circumference of a larger stationary circle, creating a complex motion that can represent planetary orbits. The Tusi-couple is significant in the history of astronomy because it provides a way to explain the apparent retrograde motion of planets without relying on the Ptolemaic system's complicated epicycles. This concept laid the groundwork for later developments in celestial mechanics and influenced the work of astronomers like Copernicus and Kepler.