In astronomy, a quadrant is an old instrument, based on a quarter of a circle and designed to measure the altitude above the horizon of astronomical bodies. As it was originally used, the plane of the quadrant was adjusted to lie in the plane of the meridian. Vertical alignment was indicated by a plumb-bob suspended from the quadrant's center. Pivoted from this center was one end of a movable rod approximately equal in length to the radius of the quadrant. Sights mounted on the rod enabled observations to be made of stars and planets as they crossed the observer's meridian, and an angular scale inscribed on the periphery of the quadrant gave their meridian altitudes.

 

It isn't certain whether Ptolemy actually constructed such an instrument or not. The Arabians, however, subsequently adopted the idea of the quadrant and greatly improved upon its design: in particular, quadrants were developed that could rotate about a vertical axis. They culminated in an enormous masonry device, 55 meters high, erected in the fifteenth century by Ulugh Beg at Samarkand. See also Tycho Brahe. The quadrant was superceded by the sextant.

Portable astronomical quadrant by John Bird, London, c. 1760

 

In mathematics, a quadrant is any one of the four portions of the plane into which the plane is divided by the Cartesian coordinate axes. In spherical geometry, a quadrant is a spherical distance π/2. A quadrant of a circle is a sector with a central angle of π/2.