Mustafa ibn Ali al-Muwaqqit (died 1571, the epithet al-Muwaqqit means "the timekeeper"), also known as Müneccimbaşı Mustafa Çelebi and Koca Saatçi, was an Ottoman astronomer and author of geography from the sixteenth century. Because of his works on the science of timekeeping and practical astronomy, he is considered "the founder of the Ottoman tradition" of those fields. He was one of the pioneers of astronomy literature in Ottoman Turkish—instead of Arabic which was more common in the Islamic world—following Muhammad al-Qunawi. Since his youth he served as the muwaqqit (religious timekeeper) attached to the Mosque of Selim I in Istanbul, in which capacity he produced most of his writing. In 1560 or later he was appointed to the office of müneccimbaşı, the highest post for astronomers of the Empire. Early life Muslih al-Din Mustafa ibn Ali al-Qustantini al-Rumi was born in Istanbul, the capital of the Ottoman Empire, in the early sixteenth century. Among his teachers were the astronomer Mirim Çelebi (1475–1525), as well Muhammad al-Qunawi (d. ) a muwaqqit (religious timekeeper) in the tradition of Shams al-Din al-Khalili and Ibn al-Shatir, and Ottoman astronomer who began the attempt to write about his field in Ottoman Turkish rather than Arabic. Career Mustafa ibn Ali was appointed as the muwaqqit of the Mosque of Selim I in Istanbul, when he was young. He produced most of his works during this tenure. Instead of Arabic, the customary scientific language of the Islamic World at the time, he wrote mostly in Ottoman Turkish. This decision was made in order to popularise the field of astronomy in the Ottoman state, to make it accessible to more students, and to facilitate the mention of non-Arabic place names. Many of his works were dedicated for Sultan Suleiman the Magnificent and his grand viziers, possibly aimed to be used by the state bureaucracy; this application was facilitated by the use of Turkish. According to the historian of science İhsan Fazlıoğlu, the relatively high number of extant copies of his works was an indication of the success of his attempt to reach a wider audience by using Turkish. His works were reproduced up to the middle of the nineteenth century and were used as textbooks in madrasa and muvakkithanes (offices of the muwaqqits). Fazlıoğlu characterised his works by their "high level of geometry, trigonometry (especially spherical trigonometry), and numerical analysis", and their style of writing that is easy