Quadrans Vetus

Time-measurement based on the altitude or azimuth (direction) of the Sun became a major preoccupation of astronomers in the Islamic world, leading to a number of theoretical and practical innovations. Among these was the universal horary quadrant, which made it possible to infer the seasonal hours since sunrise or before sunset from the altitude of the Sun. The quadrant is believed to have been invented in ninth-century Baghdad and was first transmitted to Latin Europe in the late tenth or early eleventh century. In its most common form, known as the quadrans vetus, the instrument is characterized by its inclusion of the following components: (i) a diagram of curved lines for finding unequal hours; (ii) a shadow square, which could be used to measure the height and depth of objects; and (iii) a movable cursor showing the degrees of the zodiac and dates of the Julian calendar. The cursor made it possible to find the Sun’s noon altitude for any day of the year and for any geographic latitude.

Commentary. Philipp Nothaft (May-June 2019)