Founding of the Maragheh Observatory

Al-Tusi, born in Tus in 1201, had spent many years in the service of the Nizari Ismailis in Quhistan and at Alamut. When Alamut surrendered to Hulagu in 1256 he entered the Mongol prince’s service as an astrologer, and he is believed to have been with the army that took Baghdad in 1258. In 1259 he began, with Hulagu’s support, to build an observatory on a flat-topped hill west of Maragheh; one story has it that he won the khan over by promising to predict the future. Paid for through religious endowments (waqf), the construction took three to five years (seven by another account): a central tower and five circular platforms, a metal workshop, living quarters and a library, on a site of about 400 by 150 metres.

The engineer Mu’ayyad al-Din al-Urdi built its instruments, among them a mural quadrant and an armillary sphere. Qutb al-Din al-Shirazi, Muhyi al-Din al-Maghribi and the Chinese astronomer Fao Munji worked there, and after twelve years of observations al-Tusi and his colleagues completed the Zij-i Ilkhani, the ‘Ilkhanid Tables’, in 1271 or 1272, under Hulagu’s son Abaqa; they drew on several astronomical traditions, Chinese methods among them. Al-Tusi’s device of a small circle rolling inside a circle of twice its diameter — the ‘Tusi couple’, which he had first proposed in 1247 for the latitudinal motion of the inner planets — was later widely used as a substitute for Ptolemy’s equant; together with other late Islamic models it closely resembles constructions in Copernicus’s De revolutionibus, though no chain of transmission has been established.

The observatory ceased to work in the 14th century. In 1972 the Iranian archaeologist Parviz Varjavand began excavating its ruins.