Astrolabe of 'Umar ibn Yusuf ibn 'Umar ibn 'Ali ibn Rasul al-Muzaffari, 1291. Made in Yemen. Brass; cast and hammered, pierced, chased, inlaid with silver. For an object produced during the medieval period, this astrolabe is unusually well documented. Its inscription attributes it to a Rasulid prince, 'Umar ibn Yusuf, a few years before he ascended to the throne (r. 1295-96). 'Umar compiled a number of scientific treatises, including one on the construction of astrolabes, an autographed version of which, preserved in Cairo, contains certifications by his teachers as to his competence as a maker of such devices and a description of this very piece. The astrolabe was an instrument formerly used to make astronomical measurements, typically of the altitudes of celestial bodies, and in navigation for calculating latitude, before the development of the sextant. In its basic form (known from classical times), it consists of a disk with the edge marked in degrees and a pivoted pointer.