The sextant is an instrument used to measure the angle between any two visible targets. Its primary function is to find the angle between a celestial object and the horizon which is known as the object's altitude. Making this measurement is known as sighting the physical object, shooting the object, or taking a sight and it is an crucial part of celestial navigation.

The angle, and the time when it was measured, may be used to calculate a position line  on a nautical or aeronautical chart . More common uses include sighting the sun  at solar noon  and sighting Polaris at night time, to determine one's latitude  (in northern latitudes).

Sighting the height of a landmark can give a measure of distance off and, held horizontally, can measure out angles between objects for a position on a chart also in addition is used to measure the lunar distance between the moon and another celestial object (e.g., star, planet) in order to check Greenwich time which is crucial because it can then be used to determine the longitude. 

The scale has a length of 1/6 of a turn(60°); therefore the sextant's name (sextans, -antis is the Latin word for "one sixth"). An octant  is a similar device with a shorter scale (? turn, or 45°), whereas a quintant (1/6 turn, or 72°) and a quadrant (¼ turn, or 90°) have longer scales.

Sir Isaac Newton (1643-1727) invented the principle of the doubly reflecting navigation instrument (a reflecting quadrant-see Octant (instrument), but never released it. Two men independently produced the octant around 1730: John Hadley  (1682-1744), an English mathematician, and Thomas Godfrey  (1704-1749), a glazier in Philadelphia .

John Bird constructed the first sextant in 1757. The octant and later the sextant, superseded the Davis quadrant as the main instrument for navigation.

Sextant Variants

The index arm moves the index mirror. The indicator points at the arc to display the measurement. The body ties everything together. There are two types both types give dependable results, and the choice between them is up to the individual.

Conventional models have a half-horizon mirror. It divides the field of view in two. On one side, there's a view of the horizon; on the other side, a view of the celestial object. The advantage of this type is that both the horizon and celestial object are brilliant and as clear as possible.

This is superior at night and in haze, when the horizon can be hard to see. All the same, one has to sweep the celestial object to ensure that the lowest limb of the celestial object touches the horizon.

Whole-horizon types use a half-silvered horizon mirror to furnish a full view of the horizon. This makes it easy to see when the bottom limb of a celestial object touches the horizon. Because most sights are of the sun or moon, and haze is rare without overcast, the low-light advantages of the half-horizon mirror are rarely crucial in practice.

In both types, bigger mirrors give a larger field of view, and hence make it easier to find a celestial object. Advanced sextants frequently have 5 cm or larger mirrors, while 19th century they rarely had a mirror larger than 2.5 cm (one inch). In large part, this is because precision flat mirrors have grown less costly to manufacture and to silver.

An artificial horizon is useful when the horizon is invisible. This happens in fog, on moonless nights, in a calm, when sighting through a window or on land hemmed in by trees or buildings. Professional models can mount an artificial horizon in place of the horizon-mirror assembly.

An artificial horizon is commonly a mirror that views a fluid-filled tube with a bubble. Almost all  have filters for use whilst viewing the sun and reducing the effects of haze.

Nearly all models mount a 1 or 3 power monocular  for viewing. A lot of users prefer a uncomplicated sighting tube, which has a broader, brighter field of view and is easier to use at nighttime. Some navigators mount a light-amplifying monocular to assist seeing the horizon on moonless nights, other people prefer to use a lit artificial horizon.

Professional models apply a click-stop degree measure and a worm adjustment that reads to a minute, 1/60 of a degree . Well-nigh all sextants likewise include a vernier on the worm dial that reads to 0.2 minute. Since 1 minute of error is about a nautical mile , the best possible accuracy of celestial navigation is about 0.1 nautical miles (200 m). At sea, results within several nautical miles, well inside visual range, are accepted.

A highly-skilled and seasoned navigator can determine position to an accuracy of about 0.25-nautical-mile (460 m). A change in temperature can warp the arc, creating inaccuracies. A lot of navigators purchase protected cases so that storage can be positioned outside the cabin to come to equilibrium with outside temperatures.

The standard frame designs (see illustration) are supposed to equalize differential angular error from temperature changes. The handle is separated from the arc and frame thus that body heat does not warp the frame. 

In tropical climates they are often painted white to reflect sunlight and remain comparatively cool. High-precision models used by professional mariners have an invar  (a specialized low-expansion steel) frame and arc. 

A lot of commercial models  use low expansion brass or aluminium. Brass is lower-expansion than aluminium, but aluminium sextants are lighter and less tiring to apply. Some allege they are more accurate since one's hand trembles less.

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