The Tusi couple is a mathematical model developed by the Persian astronomer Nasir al-Din al-Tusi in the 13th century. It describes the motion of celestial bodies by combining circular motions. This model was significant in explaining the apparent retrograde motion of planets, which was a challenge for earlier astronomical models. The Tusi couple consists of two circles: one circle rotates around the circumference of another circle. This innovative approach allowed for a more accurate representation of planetary movements, influencing later astronomers and contributing to the development of the Copernican model of the solar system.