Tusi — the Persian astronomer who survived the Mongols | PERSIA'S GREATEST MINDS Part 6

History Onion · Part 6 · 2:58

In late 1257 the Mongol prince Hulagu, grandson of Genghis Khan, asked his astronomer whether the stars allowed an attack on Baghdad. The Ilkhanid court historian Rashid al-Din writes that Nasir al-Din Tusi (خواجه نصیرالدین طوسی) told him the stars foretold only that Hulagu would take the caliph's place. In February 1258 Baghdad was sacked; Tusi was there. Born in Tus in 1201, he studied in Nishapur, destroyed by the Mongols in 1221, and spent some twenty years in the mountain castles of the lords of Alamut, whose libraries were famous; he later wrote that he had served them under duress. In 1256 he helped persuade the young lord of Alamut, Rukn al-Din Khurshah, to surrender, and Hulagu kept him on as his astrologer. After Baghdad he asked for an observatory. Saturn takes about thirty years to circle the sky, so he asked for thirty years of observing; he was told to finish in twelve. With endowment revenues and books saved from the libraries of Baghdad, the observatory at Maragheh rose from 1259, and scholars came from Damascus, Mosul and Tbilisi. Its tables, the Zij-i Ilkhani, were written in Persian. There he solved a problem that went back to Ptolemy: the rule that heavenly bodies move on perfect circles at a steady speed, which Ptolemy's model had broken. A small circle rolling inside one twice its size moves a point along a straight line — two uniform circular motions make a straight line. Historians call it the Tusi couple. He used it for the Moon; Mercury defeated him: "As for Mercury, I still have not figured it out properly, and should the Almighty God ever make that possible, I will add it to this place" (al-Tadhkira, tr. Saliba).