When Was the Sextant Invented?
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implemented around 1731 by John Hadley (1682–1744) and Thomas Godfrey (1704–1749), but it was also found later in the unpublished writings of Isaac Newton (1643–1727).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”.
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.
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. 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) in order to determine Greenwich Mean Time and hence longitude.