Hadley's quadrant

a hand instrument used chiefly at sea to measure the altitude of the sun or other celestial body in ascertaining the vessel's position. It consists of a frame in the form of an octant having a graduated scale upon its arc, and an index arm, or alidade pivoted at its apex. Mirrors, called the index glass and the horizon glass, are fixed one upon the index arm and the other upon one side of the frame, respectively. When the instrument is held upright, the index arm may be swung so that the index glass will reflect an image of the sun upon the horizon glass, and when the reflected image of the sun coincides, to the observer's eye, with the horizon as seen directly through an opening at the side of the horizon glass, the index shows the sun's altitude upon the scale; - more properly, but less commonly, called an octant.

See also: Quadrant