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A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight and it is an essential part of celestial navigation. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at solar noon and to measure the elevation (altitude) angle at night to measure the elevation angle from the horizon plane to Polaris to find one's latitude. Since the sextant can be used to measure the angle between any two objects, it can be held horizontally to measure the angle between any two landmarks which will allow for calculation of a position on a chart. A sextant can also be used to measure the Lunar distance between the moon and another celestial object (e.g., star, planet) in order to determine Greenwich time which is important because it can then be used to determine the longitude.

The scale of a sextant has a length of of a full circle (60°); hence the sextant's name (sextāns, -antis is the Latin word for "one sixth", "εξάντας" in Greek). An octant is a similar device with a shorter scale ( of a circle, or 45°), whereas a quintant (, or 72°) and a quadrant (¼, 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 published it. Two men independently developed the octant around 1730: John Hadley (1682–1744), an English mathematician, and Thomas Godfrey (1704–1749), a glazier in Philadelphia. The octant and later the sextant, replaced the Davis quadrant as the main instrument for navigation.


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