an astronomical instrument used in the Middle Ages to measure the altitudes of celestial bodies. Mural quadrants were of large size, with a radius as great as 3 m. They were securely attached to the stone walls of observatories; generally the wall was in the plane of the meridian. The arc of the mural quadrant was divided into fractions of degrees and permitted the direction to a celestial body to be read to within tenths of a minute of arc. In the 16th century, Tycho Brahe made use of a mural quadrant in compiling his star catalog, which was one of the most accurate of that time. Mural quadrants went out of use at the end of the 17th century. They were replaced by meridian circles, which permitted the elimination of instrumental errors from observations and thus provided a considerable improvement in the accuracy of the observations. (SeeQUADRANT.)