Sextant

This article is about the sextant as used for navigation. For other uses, see Sextant (disambiguation).

Not to be confused with Sexton (disambiguation).

A sextant

A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. Its primary use is to measure the angle between an astronomical object and the horizon in celestial navigation.

Estimating this angle, the altitude, is known as sighting or shooting the object, or taking a sight. The angle and the time when it was measured can be used to calculate a position line on a nautical or aeronautical chart—for example, sighting the Sun at noon or Polaris at night (in the Northern Hemisphere) to estimate latitude (with sight reduction). Sighting the height of a landmark can give a measure of distance off and, held horizontally, a sextant can measure angles between objects for a position on a chart. A sextant can also be used to measure the lunar distance between the moon and another celestial object (such as a star or planet) to determine Greenwich Mean Time and hence longitude.

The principle of the instrument was first implemented around 1731 by John Hadley and Thomas Godfrey, but it was also later found in Isaac Newton's unpublished writings.

In 1922, Portuguese navigator and naval officer Carlos Viegas Gago Coutinho modified the sextant for aeronautical navigation.