Arabscholar, born in modern Iran. In 1259 he persuaded the Mongol ruler Hūlāgū to build an observatory at Marāgha in Persia equipped with large quadrants. From observations with these instruments, al-Ṭūsī compiled accurate tables of planetary motion and the positions of stars. He refined the Ptolemaic system by replacing the epicycle with a small circle rolling around inside the circumference of another circle of twice the radius. Any point on the smaller circle then describes a straight line. The Tusi couple, as this device is called, was later used by N. Copernicus.
Subjects: Astronomy and Astrophysics — Religion.