One of the largest advances in Islamic astronomy was the rejection of the Ptolemaic system of the planets. This system developed by Ptolemy placed the sun, moon, and other planets in orbit around the Earth. Ptolemy thought that the planets moved on circles called epicycles and that their centers rode on deferents. The deferents were eccentric, and the angular motion of a planet was uniform around the equant which was a point opposite the deferent center. Simply, Ptolemy's models were a mathematical system for predicting the positions of the planets. One of the first to criticize this model was Ibn al-Haytham, a leader of physics in the 11th century in Cairo. Then in the 13th century Nasir al-Din al-Tusi constructed the Maragha Observatory in what is today Iran. Al-Tusi found the equant dissatisfying and replaced it by adding a geometrical technique called a Tusi-couple, which generates linear motion from the sum of two circular motions. Then, Ibn al-Shatir who was working in Damascus in 1350 AD employed the Tusi-couple to successfully eliminate the equant as well as other objectionable circles that Ptolemy had used. This new model properly aligned the celestial spheres and was mathematically sound. This development by Ibn al-Shatir, as well as the Maragha astronomers remained relatively unknown in medieval Europe.
