Sextant, a navigation instrument used to measure angles, particularly the altitudes of the sun and stars above the horizon. A sextant is used by a navigator to find his position on the earth. There are two classes of sextantsmarine and air.

A typical marine sextant consists of a triangular frame, with a curved scale, marked in degrees of arc, at the bottom. Mounted on the frame are an eyepiece and a piece of glass, called the horizon mirror, half of which is silvered and half clear. The sextant is held so that the horizon can be seen through the clear part of the glass when looking through the eyepiece. Attached to the frame is a movable arm that crosses the scale; on the arm is a second mirror. The arm is positioned so that the image of the reference body (the sun for example) appears in the horizon mirror to be just touching the horizon. The position of the arm along the scale gives the altitude of the body in degrees.

The time of the measurement is noted on an extremely accurate clock, called a chronometer. With the altitude of the body, the correct time, and a nautical almanac, the observer determines that the ship is somewhere on a line of position. By taking a second sextant reading an hour or two later, a second line of position is established. The intersection of the two lines, considered together with the ship's course and speed, indicates the latitude and longitude.

An air sextant serves the same purpose as a marine sextant. The horizon cannot be used while in flight, so an artificial horizon is provided. A spirit-level, a pendulum, or a gyroscope provides the artificial horizon from which the altitudes of the celestial bodies can be measured.