Quadrant (kwŏd' rănt), an astronomical instrument, invented by Hadley in 1731 for use in measuring angles at sea and in other places where a fixed divided circle is not available. The quadrant, in a crude form, had been used for several hundred years before the time of Hadley, if we admit the astrolabe graduated through 90° as a kind of quadrant. But Hadley so improved the instrument by introducing a fixed and a movable mirror and by bringing into the same line the images of the two objects whose angular distance is to be measured, that he really made a new and highly valuable apparatus. The quadrant is merely a brass frame, provided with a radial arm carrying a mirror, whose polished surface contains the axis of the quadrant. By looking through the small telescope and a half-silvered mirror (attached to the quadrant) at any particular object, one may, by rotating this radial arm, make the image of a second object appear in the same direction as the first. The angle through which the arm must be turned to secure this coincidence is half the angle subtended by the two objects; and this angle is read directly from the graduated edge of the quadrant. Sometimes only 60° of the edge is graduated, in which case the instrument is called a sextant. Hadley's invention immediately displaced the astrolabe and the cross-staff which had hitherto been used for taking latitudes at sea. A considerable part of the accuracy of the quadrant is due to the use of the vernier, invented by Pierre Vernier in 1631. About the same time the telescopic sight was invented by Gascoigne and the tangent screw by Helvelius. All combine to make the quadrant or sextant a thoroughly accurate instrument, now in daily use by all navigators.