An instrument for measuring the altitudes of stars and the angular separation between celestial objects, consisting of a graduated quarter-circle and a movable sighting arm. Prior to the invention of the telescope, such instruments were the only ones available to astronomers for positional measurements. Tycho Brahe 1546-1601, for example, used a large mural quadrant, built on to a specially constructed wall lying along the north-south direction. On instruments made after the invention of the telescope, telescopic sights were often employed. The modern equivalent is the transit circle.