![]() The scales of the octant, sextant, quintant and quadrant are graduated from below zero to 90°, 120°, 140° and 180° respectively. The scale must be graduated so that the marked degree divisions register twice the angle through which the index arm turns. Sextants can also be used by navigators to measure horizontal angles between objectsĪttached to the frame are the "horizon mirror", an index arm which moves the index mirror, a sighting telescope, sun shades, a graduated scale and a micrometer drum gauge for accurate measurements. All of these instruments may be termed "sextants". Both smaller and larger instruments are (or were) in use: the octant, quintant (or pentant) and the (doubly reflecting) quadrant span sectors of approximately 1⁄ 8 of a circle (45°), 1⁄ 5 of a circle (72°) and 1⁄ 4 of a circle (90°), respectively. The frame of a sextant is in the shape of a sector which is approximately 1⁄ 6 of a circle (60°), hence its name ( sextāns, sextantis is the Latin word for "one sixth"). For these reasons it is considered to be an eminently practical back-up navigation tool for ships. The sextant is not dependent upon electricity (unlike many forms of modern navigation) or for that matter anything reliant on human-controlled signals (such as GPS satellites). However, the relative position of the two images will remain steady, and as long as the user can determine when the celestial object touches the horizon, the accuracy of the measurement will remain high compared to the magnitude of the movement. For example, when a sextant is used on a moving ship, the image of both horizon and celestial object will move around in the field of view. The measurement is thus limited by the angular accuracy of the instrument and not the sine error of the length of an alidade, as it is in a mariner's astrolabe or similar older instrument.Ī sextant does not require a completely steady aim, because it measures a relative angle. Since the measurement is relative to the horizon, the measuring pointer is a beam of light that reaches to the horizon. For solar observations, filters allow direct observation of the sun. This permits the use of the sextant at night when a backstaff is difficult to use. Also, unlike the backstaff, the sextant allows direct observations of stars. Like the Davis quadrant, the sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument. Navy Quartermaster 3rd Class, practices using a sextant as part of a navigation training aboard the amphibious assault ship USS Bonhomme Richard (LHD 6), 2018 In 1922, it was modified for aeronautical navigation by Portuguese navigator and naval officer Gago Coutinho. ![]() 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). 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. 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. 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). The estimation of this angle, the altitude, is known as sighting or shooting the object, or taking a sight. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celestial navigation. A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. ![]()
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