sextant

sextant
/sek"steuhnt/, n.
1. an astronomical instrument used to determine latitude and longitude at sea by measuring angular distances, esp. the altitudes of sun, moon, and stars.
2. (cap.) Astron. the constellation Sextans.
[1590-1600; < L sextant- (s. of sextans) sixth part of a unit. See SEXT, -ANT]

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Instrument for determining the angle between the horizon and a celestial body
such as the Sun, the Moon, or a star
used in celestial navigation to determine latitude and longitude.

It consists of a metal arc, marked in degrees, and a movable radial arm pivoted at the centre of the arc's circle. A telescope, mounted rigidly to the framework, is lined up with the horizon. The radial arm, on which a mirror is mounted, is moved until the star is reflected into a half-silvered mirror in line with the telescope and appears, through the telescope, to coincide with the horizon. The angular distance of the star above the horizon is then read from the graduated arc of the sextant. From this angle, the latitude can be determined (within a few hundred metres) by means of published tables, and by consulting an accurate chronometer the longitude can be established. Invented in 1731, the sextant replaced the octant and became an essential tool of navigation.

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 instrument for determining the angle between the horizon and a celestial body such as the Sun, the Moon, or a star, used in celestial navigation to determine latitude and longitude. The device consists of an arc of a circle, marked off in degrees, and a movable radial arm pivoted at the centre of the circle. A telescope, mounted rigidly to the framework, is lined up with the horizon. The radial arm, on which a mirror is mounted, is moved until the star is reflected into a half-silvered mirror in line with the telescope and appears, through the telescope, to coincide with the horizon. The angular distance of the star above the horizon is then read from the graduated arc of the sextant. From this angle and the exact time of day as registered by a chronometer, the latitude can be determined (within a few hundred metres) by means of published tables.

      The name comes from the Latin sextus, or “one-sixth,” for the sextant's arc spans 60°, or one-sixth of a circle. Octants, with 45° arcs, were first used to calculate latitude. Sextants were first developed with wider arcs for calculating longitude from lunar observations, and they replaced octants by the second half of the 18th century.

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Universalium. 2010.

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