This article is about the sextant as used for navigation. For history and development of the sextant, see How to use a sextant pdf instrument. A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects.
The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celestial navigation. The estimation of this angle, the altitude, is known as sighting or shooting the object, or taking a sight.
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. Greenwich Mean Time and hence longitude. In 1922, it was modified for aeronautical navigation by Portuguese navigator and Naval Officer Gago Coutinho.