Nasir al-Din al-Tusi
Persian polymath who founded the Maragha Observatory and developed the Tusi couple, influencing Copernican astronomy.
Who or what
Muhammad ibn Muhammad ibn al-Hasan al-Tusi (1201–1274), known as Nasir al-Din al-Tusi, was a Persian polymath who wrote on astronomy, mathematics, philosophy, and theology.
He founded the Maragha Observatory in 1259, the most advanced astronomical institution of its time, and developed the "Tusi couple" — a mathematical device that converts circular into linear motion.
Why it mattered
The Tusi couple elegantly resolved a fundamental problem in Ptolemaic astronomy: how to explain planetary motion with uniform circular motion alone.
Al-Tusi was one of the first to treat trigonometry as a separate mathematical discipline, independent of astronomy.
Key works and moments
He compiled the Ilkhanic Tables (Zij-i Ilkhani), the most accurate astronomical tables of the medieval period.
His Tusi couple — a small circle rotating inside a larger circle of twice its radius — produces straight-line motion from two circular motions.
Further detail
The Tusi couple: a circle of radius rolls internally inside a circle of radius ; a point on the smaller circle traces a diameter of the larger.
His "Treatise on the Complete Quadrilateral" (Kitab al-Shakl al-Qatta) systematized plane and spherical trigonometry.
Legacy
The Tusi couple was used by Copernicus in his planetary models, suggesting direct transmission of Maragha ideas to Renaissance Europe.
His work on trigonometry established it as a foundation for modern mathematics and navigation.
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