Trigonometry · a Light

A book that made trigonometry a subject of its own

Trigonometry had always been a tool for something else. You calculated triangles in order to find where a planet would be, and the tables lived inside astronomy books, in the chapters before the interesting part. Around 1260 Nasir al-Din al-Tusi wrote the Treatise on the Complete Quadrilateral, five books setting out plane and spherical trigonometry as a subject with its own problems and its own proofs and no astronomy in it at all. Historians usually call it the first work to do that, while noting that Abu al-Wafa and Abu Nasr Mansur had already gone much of the way. It was printed with a French translation at Istanbul in 1891, which is when European mathematicians could finally read it.

Nasir al-Din al-Tusi · Maragha, c. 1260