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Sextant for classical navigation

The sextant is the classic nautical measuring instrument that has been used for astronomical navigation at sea for centuries. Working with the sextant is part of the high school for the demanding navigator; determining a position with the help of celestial bodies and ephemerides requires precise measurements and calculations. The sextant is used to determine the angular distance of a celestial body from the horizon in order to be able to calculate levelling lines with the help of the angular measurements and exact time. The intersection of the position lines of several elevation angle measurements gives the location. The sextant can also be used for terrestrial navigation with horizontal angle measurements, for example for surveying tasks and position calculations. Elevation angle measurements on landmarks, for example, give the distance, provided their height is known. These precision instruments are called mirror sextants because the angle is measured by a system of mirrors. The name drum sextant comes from the micrometre drum or drum screw, which is used for fine adjustment and to read off the degree graduation in minutes. The adjustment is made by a worm drive.

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The sextant is the classic nautical measuring instrument that has been used for astronomical navigation at sea for centuries. Working with the sextant is part of the high school for the demanding navigator; determining a position with the help of celestial bodies and ephemerides requires precise measurements and calculations. The sextant is used to determine the angular distance of a celestial body from the horizon in order to be able to calculate levelling lines with the help of the angular measurements and exact time. The intersection of the position lines of several elevation angle measurements gives the location. The sextant can also be used for terrestrial navigation with horizontal angle measurements, for example for surveying tasks and position calculations. Elevation angle measurements on landmarks, for example, give the distance, provided their height is known. These precision instruments are called mirror sextants because the angle is measured by a system of mirrors. The name drum sextant comes from the micrometre drum or drum screw, which is used for fine adjustment and to read off the degree graduation in minutes. The adjustment is made by a worm drive.

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Sextant for classical navigation

The sextant is the classic nautical measuring instrument that has been used for astronomical navigation at sea for centuries. Working with the sextant is part of the high school for the demanding navigator; determining a position with the help of celestial bodies and ephemerides requires precise measurements and calculations. The sextant is used to determine the angular distance of a celestial body from the horizon in order to be able to calculate levelling lines with the help of the angular measurements and exact time. The intersection of the position lines of several elevation angle measurements gives the location. The sextant can also be used for terrestrial navigation with horizontal angle measurements, for example for surveying tasks and position calculations. Elevation angle measurements on landmarks, for example, give the distance, provided their height is known. These precision instruments are called mirror sextants because the angle is measured by a system of mirrors. The name drum sextant comes from the micrometre drum or drum screw, which is used for fine adjustment and to read off the degree graduation in minutes. The adjustment is made by a worm drive.

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The measuring drum for increasing the reading accuracy is also called the vernier, and accordingly the name vernier sextant is also used. The name sextant results from the sixth of a circle to which the instrument body corresponds. The mirror principle therefore makes it possible to measure angles up to 120°. The essential components of the sextant are the instrument body, usually made of brass, more rarely of aluminium or plastic, the rotating pointer arm or alidade with measuring drum, the movable index mirror, the fixed horizon mirror, the telescope, the degree arc or limbus with degree graduation, and the diaphragm glasses or shadow glasses for observing the sun. In order to measure the angle between the star and the celestial sphere, both objects must be brought into alignment. To do this, the index mirror is rotated by means of the alidade until the mirrored image of the celestial body is visible in the horizon mirror. Stars are brought into direct contact with the chine; in the case of larger objects such as the sun and moon, either the upper edge or the lower edge of the celestial body is placed on the chine. For an exact measurement, the sextant must be held vertically; an oblique position would distort the angle.

The integer degrees of the set angle are read from the degree arc, the fractions of degrees in minutes from the division of the vernier. At the moment of measurement, the time must also be read on an accurate clock. For the navigation calculation, a number of corrections are made to the angle measurement, for example, inaccuracies in the instrument such as index error, keel depth and light refraction. Therefore, a sextant should be adjusted as much as possible before use. Sextants made of seawater-resistant metal such as brass or aluminium offer the highest quality manufacture and accuracy, but a cheaper plastic sextant can also be used as a practice sextant and in an emergency on board.

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