The principle of the instrument was first implemented around 1731 by John Hadley (1682–1744) and Thomas Godfrey (1704–1749), but it was also found later in the unpublished writings of Isaac Newton (1643–1727). Additional links can be found to Bartholomew Gosnold (1571–1607) indicating that the use of a sextant for nautical navigation predates Hadley’s implementation.
In 1922, it was modified for aeronautical navigation by Portuguese navigator and naval officer Gago Coutinho. Hadley’s instrument is used mainly at sea to determine a ship’s latitude, or distance from the Equator. Its invention laid the foundation of modern navigation with the aid of the sun and stars. The instrument is so called because it is equipped with an arc which is usually one-sixth of a circle, or 60 degree.
It measures the angle of the sun’s or a star’s altitude above the horizon. As this angle varies with the distance from the Equator, the information obtained helps the navigator to calculate his position. All he needs in addition is the time, the date and the longitude which can be found by comparing local time with the time at Greenwich.
To operate the sextant, the navigator looks through its small telescope straight at the horizon. At the same time, an image of the sun is reflected by mirrors into the user’s field of vision. When the sun is made to appear exactly on the horizon, the arm which moves the mirrors gives the required measurements to calculate the ship’s position. The handling of a sextant is generally referred to as “shooting the sun”.
