A quadrant is a mathematical or an astronomical instrument featuring a graduated arc encompassing a quarter of a full circle or 90?. Astronomical quadrants were used mainly for determining the altitude of a celestial body above the horizon. Meridian altitudes of the sun or a bright star could be employed for determining the geographical latitude of an observer; or, at a known latitude, the observer could obtain the time from an altitude measured not too close to the meridian direction.
One of the earliest examples of a quadrant was the 'plinth' described around 150 A.D. by Claudius Ptolemy in his Almagest. Here the shadow of an elevated horizontal peg was projected on a graduated arc of 90? that was set up in the plane of the meridian to determine the sun's noontime altitude.
Several large quadrants were constructed by Islamic astronomers: mural quadrants that were fixed in the meridian plane and altazimuth quadrants that could be rotated to arbitrary compass directions for the simultaneous determination of the altitude and the azimuth of a celestial body. Quadrants of both types were also employed by Renaissance astronomers, especially notable in this respect were the large quadrants (up to 2 meters in radius) completed in the 1580s by the Danish astronomer Tycho Brahe at his observatory Uraniborg on the isle of Hven.
