to understand and apply. Among his earliest works was a book of mathematical geography titled I'lam al-'ibad fi a'lam al-bilad ("Notices on the Distances of Cities of the World"), written in Ottoman Turkish. It includes a list of 100 major cities from Morocco to China, and the coordinates, distance from Istanbul, the qibla (direction that indicates Mecca) of each city. It was written in 1525 and was dedicated to Suleiman the Magnificent. His subsequent work on geography, Tuhfat az-zaman wa-kharidat al-awan (also in Ottoman Turkish), discusses the discipline of geography and its roots, the celestial objects, the Earth, its geographical features, and its division into seven climes. It was built on works by earlier scholars, including astronomers Jaghmini, Qadizade al-Rumi, and Zakariya al-Qazwini, as well as the zoologist Al-Damiri. Many of his other works were about astronomical instruments. Among these, Faraḥ Fazā was dedicated to the grand vizier Pargalı Ibrahim Pasha and describes the horizontal quadrant (al-rub' al-afaqi), which according to Mustafa was his own invention. Another work, Kifayat al-qanu' fi al-'amal bi'r-rub' al-maqtu was written in Arabic and was a commentary on the works of an earlier muwaqqit Sibt al-Maridini. His other works, Kifayat al-waqt, also known as Risala fi al-muqantarat (1529), was written in Turkish and describes an instrument called the astrolabic quadrant, as well as other themes in geometry, trigonometry, and astronomy. Today 120 copies of this work are extant in various libraries. His Tashil al-miqat (1529) discusses the science of timekeeping and the sine quadrant (al-rub' al-mujayyab). This work appeared to have been continuously updated, and today it is found in 100 extant copies of five distinct versions. He describes another instrument, al-mujayyab al-afaqi in the work Risalah-i jayb-i afaqi (1529, Turkish) and describes its use for the coordinates of Istanbul in the work Hall da'irat mu'addil al-nahar (1531, Turkish) which was commissioned by the grand vizier Ayas Mehmed Pasha. His longest work was Risalat al-asturlab al-Selimi ("Selim's Treatise on the Astrolabe"), which was written in 1544, also in Turkish, and discusses the construction of the astrolabe, its mathematical properties, and its uses, based on Zij-i Sultani published by the astronomer Ulugh Beg in 1438 or 1439. In or after 1560, he was appointed müneccimbaşı (or munajjim-bashi, "head astrologer"), the highest office for astronomers in the Ottoman Empire, hence his other name "Müneccimbaşı Mustafa Çelebi". He replaced Yusuf ibn Umar and held the post until his