ABSTRACT
244This remarkable astronomical instrument was made by the Muslim astronomer known as Nasṭūlus, who was active in Baghdad between 890 and 930. Its rediscovery brings our knowledge of the activities in that flourishing scientific centre a substantial step further. This type of instrument was previously not known to exist, although sundials based on the same principle are described in Arabic treatises datable to ca. 950 and ca. 1280. It is essentially a mathematical device providing a graphic solution to a problem that was of interest to Muslim astronomers, namely, the determination of the time of day as a function of the solar altitude throughout the year, here specifically for the latitude of Baghdad. The instrument reveals a level of mathematical competence and sophistication that is at first sight astounding. However, with a deeper understanding of the scientific milieu from which it came, it can be seen to be fully within the theoretical competence of the scientists of that environment. Nevertheless, the spectacular accuracy of the engraving of the principal curves on the instrument is completely unexpected. The instrument also features the earliest known solar and calendrical scales from the Islamic East; the origin of these was previously thought to be in the Islamic West.
