A sextant is an optical instrument used in navigation invented in 1730. It measures the angle of celestial bodies above the horizon from the observer's position. The sextant is so named because the early instrument had a calibrated arc that is one-sixth of a circle -- a graduated 60° arc. [Sex is a prefix that means six, from Latin.] Knowing the angular elevation of a known star, and the exact time, one can calculate the Latitude position of the observer.

Early marine sextants were hand held and had a fixed telescope leveled on the horizon. A radial arm is moved against an arc scaled in degrees. Adjust the radial arm until a known star's image reflects from the index mirror and then off the horizon mirror until down the telescope until it lines up with the horizon. The radial arm's position on the scale gives the star's elevation.

In modern sextants, which use arcs of greater or lesser size than the original type, the light ray from the celestial body is reflected in two mirrors (in series), one of which is adjustable and the other of which is half-silvered. By rotating one mirror and its attached index bar, the image of the body is brought down to the horizon. The rotation measures the altitude on the limb.