Sextant

Sextant, instrument for navigation, invented in 1730, superseding the astrolabe. The sextant is named for its shape (1/6 of a circle). A fixed telescope is pointed at the horizon, and a radial arm bearing a mirror is moved against an arc graduated in degrees. The mirror reflects an image of a known star or the sun down the telescope to coincide with the image of the horizon. The angular elevation of the star, with the exact time, gives the latitude. The air sextant is a similar instrument, usually periscopic, designed for use in aircraft; it has an artificial horizon, generally a bubble level.

See also: Navigation.