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A diagram of a typical nautical sextant, a tool used in celestial navigation to measure the angle between two objects viewed by means of its optical sight. Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the ...
A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. The primary use of a sextant is to measure the angle between an astronomical object and the horizon for the purposes of celestial navigation .
These stars are typically used in two ways by the navigator. The first is to obtain a line of position by use of a sextant observation and the techniques of celestial navigation. [4] Multiple lines of position can be intersected to obtain a position known as a celestial fix.
The Sextant reading is known as the 'Sextant Altitude'. This is corrected by use of tables to a 'True Altitude'. The actual declination and hour angle of the celestial body are found from astronomical tables for the time of the measurement and together with the 'True Altitude' are put into a formula with the assumed latitude.
Ships had long used sextants for navigation, but sextants had problems in aircraft navigation. A ship at sea is on a relatively flat surface and can use the horizon to measure the altitude of celestial objects. However, an aircraft may not have the sea's horizon as a flat reference surface.
Practical celestial navigation usually requires a marine chronometer to measure time, a sextant to measure the angles, an almanac [2] giving schedules of the coordinates of celestial objects, a set of sight reduction tables to help perform the height and azimuth computations, and a chart of the region. With sight reduction tables, the only ...
Suitable bodies for celestial sights are selected, often using a Rude Star Finder. Using a sextant, an altitude is obtained of the Sun, the Moon, a star or a planet. The name of the body and the precise time of the sight in UTC is recorded. Then the sextant is read and the altitude (Hs) of the body is recorded.
To find the position of a ship or aircraft by celestial navigation, the navigator measures with a sextant the apparent height of a celestial body above the horizon, and notes the time from a marine chronometer. That height is compared with the height predicted for a trial position; the arcminutes of height difference is how many nautical miles ...
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