Ibn al-Shāṭir was head muwaqqit at the Umayyad Mosque in Damascus. His works on astronomical timekeeping are considerably less significant than those of his colleague Khalīlī; but continuing the tradition of Ibn al-Sarrāj, he made substantial advances in the design of astronomical instruments. His most significant contribution to astronomy was his planetary theory. In his planetary models, Ibn al-Shāṭir incorporated various ingenious modifications of those of Ptolemy. Also, with the reservation that they are geocentric, his models are the same as a number used by Nicolaus Copernicus. His works include: a nonextant zīj, based on strictly Ptolemaic planetary theory; a later treatise entitled Ta`līq al-arṣād, in which he describes the observations and procedures with which he had constructed his new planetary models and derived new parameters. No copy of this treatise is known to exist in the manuscript sources; Nihāyat al-su’l fī taṣḥīḥ al-uṣūl in which he presents the reasoning behind his new planetary models; this work has survived; al-Zīj al-jadīd, extant in several manuscript copies, contains a new set of planetary tables based on his new theory and parameters. Ibn al-Shāṭir also designed and constructed a magnificent horizontal sundial that was erected on the northern minaret of the Umayyad Mosque in Damascus. Ibn al-Shāṭir wrote on the ordinary planispheric astrolabe and designed an astrolabe that he called al-āla al-jāmi`a (the universal instrument). He also wrote on the two most commonly used quadrants, the astrolabic and the trigonometric varieties. Two special quadrants that he designed were modifications of the simpler and ultimately more useful sine quadrant. [BEA, D. King]