Sextant is an optical instrument that measures the angular distance between any two points, such as the sun and the horizon. It is used for navigation and surveying. The instrument is named for its shape, which is roughly a sixth part of a circle. A sextant’s frame supports a graduated arc, a movable index arm that represents the radius of the circle, two mirrors, and a small telescope. One of the mirrors–called the horizon glass–is fixed. The second mirror–the index glass–is screwed to the index arm. The telescope sharpens the line of the horizon.
The sextant is held so that the graduated arc is vertical and the horizon shows in the horizon glass. The user looks through the telescope at the horizon glass and moves the index arm until the image of the sun or a certain star, reflected in the index glass, touches the horizon line. The altitude of the sun or star is read from the graduated arc. The user compares this altitude at various degrees of latitude to find the latitude of the ship.
The sextant works according to the optical rule: If an object is seen by repeated reflection from two mirrors that are perpendicular to the same plane, the angular distance between the object and its image is double the angle between the surfaces of the mirrors. The sextant’s index measures the angle between the mirrors. This reading is doubled to give the angular distance of an object, such as the sun, above the horizon.
The mirror sextant was developed in the mid-1700’s, based upon the work of John Hadley in England and Thomas Godfrey in America. The sextant was the primary navigation tool used on ships and aircraft until the mid-1900’s. After World War II (1939-1945), such electronic aids as loran, omega, and radar began to replace the sextant (see Navigation ).
