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A sextant is a doubly reflecting navigation instrument that measures the angular distance between two visible objects. Sextant primary use of a sextant is to measure the angle between an astronomical object and the flat for the purposes of celestial navigation.
The estimation of this angle, the altitude, is known as sighting or shooting the object, or taking a sight. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart —for example, sighting the Sun at noon or Polaris at night in the Northern Hemisphere to estimate latitude.
Sighting the height of a landmark can give a measure of distance off and, held horizontally, a sextant can measure angles between objects for a position on a chart. The principle of earth instrument was first implemented around by John Hadley — and Thomas Godfrey —but it was also found later in the unpublished writings of Isaac Newton — Additional links can be found to Bartholomew Gosnold — indicating that the use of a sextant for nautical navigation predates Hadley's implementation.
Init was modified for sextant navigation by Portuguese navigator and naval officer Gago Coutinho. This section discusses navigators' sextants. Most of what is said about these specific sextants applies equally to other types of sextants. Navigators' sextants were primarily used for ocean navigation. Like the Davis quadrantthe sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument. This allows excellent precision. Also, unlike the backstaffthe sextant allows direct observations of stars.
This permits the use of the sextant at night when a backstaff is difficult to use. For solar observations, filters allow direct observation of the sun. Since the measurement is relative to the horizon, the measuring pointer is a beam of light that reaches to the horizon. The measurement is thus limited by the angular accuracy of the instrument and not the sine error of the length of an alidadeas it is sextant a mariner's astrolabe or similar older instrument.
A sextant does not require a completely steady aim, because it measures a relative angle. For example, when a sextant is used on a moving ship, the image of both horizon sextant celestial object will move around in the field of view.
However, the relative position flat the two images will remain steady, and as long flat the user can determine when the celestial object touches the horizon, the accuracy of the measurement will remain high compared to the magnitude of the movement. The sextant is not dependent upon electricity unlike many forms of modern navigation or sextant human-controlled like GPS satellites. For these reasons, it is considered an eminently practical back-up navigation tool for sextant. All of these instruments may be termed "sextants".
Attached to the frame are the "horizon mirror", an index arm which moves the index mirrora sighting telescope, sun shades, a graduated scale and a micrometer drum gauge for accurate measurements. The scale must be graduated so that the marked degree divisions register twice the angle through which the index arm turns. The necessity for the doubled scale reading follows by consideration of the relations of the fixed ray between the mirrorsthe object ray from the sighted object and the direction of the normal perpendicular to the index mirror.
This is the case shown in the graphic alongside. Traditional sextants have a half-horizon mirror, which divides the sextant of view in two.
On one side, there is 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 bright and as clear as possible. This is superior at night and in haze, when the horizon can be difficult to see. However, one has to sweep the celestial object to ensure that the lowest limb of the celestial object touches the horizon. Whole-horizon sextants use a half-silvered horizon mirror to provide a full view flat the horizon.
This makes it easy to see when the bottom limb of a celestial object touches the horizon. Since 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 important in practice.
In both types, larger mirrors give a larger field of view, and thus make it easier to find a celestial object. In large part, this flat because precision flat mirrors have grown less expensive to manufacture and to silver. An artificial horizon is useful when the horizon is invisible, as occurs in fog, on moonless nights, flat a calm, when sighting through earth window or on land surrounded by trees or buildings.
Professional sextants can mount an artificial horizon in place of the horizon-mirror assembly. An artificial horizon is usually a mirror that views a fluid-filled tube with a bubble.
Most sextants also have filters for use when viewing the sun and reducing the effects of haze. The filters usually consist of a series of progressively darker glasses that can be used singly or in combination to reduce haze and the sun's brightness. However, sextants with adjustable polarizing filters have also been manufactured, where the degree of darkness is adjusted by twisting the frame of the filter.
Most sextants mount a 1 or 3-power monocular for viewing. Many users prefer a simple sighting tube, which has a wider, brighter field of view and is easier to use at night. Some navigators mount a light-amplifying monocular to help see the horizon on moonless nights. Others prefer to use a lit artificial horizon.
Earth sextants also include a vernier on the worm dial that reads to 0. Since 1 minute of error is about a nautical milethe best possible accuracy of celestial navigation is about 0. At sea, results within several nautical miles, well within visual range, are acceptable. A highly skilled and experienced earth can determine position to an accuracy of about 0. A change in temperature can warp the arc, creating inaccuracies.
Many navigators purchase weatherproof cases so that their sextant can be placed outside the cabin to come to equilibrium with outside temperatures. The standard frame designs see illustration are supposed to equalise differential angular sextant from temperature changes. The handle is separated from the arc and frame so that body heat does not warp the frame.
Sextants for tropical use earth often painted white to reflect sunlight and remain relatively cool. High-precision sextants have an invar a special low-expansion steel frame and arc. Some scientific sextants have been constructed of quartz or ceramics with even lower expansions.
Many commercial sextants use low-expansion brass or aluminium. Brass is lower-expansion than aluminium, but aluminium sextants are lighter and less tiring to use. Some say they are more accurate because one's flat trembles less. Solid brass frame sextants are less susceptible to wobbling in high winds or when the vessel is working in heavy seas, but as noted are substantially heavier. Sextants with aluminum frames and brass arcs have also been manufactured. Essentially, a sextant is intensely personal to each navigator, and he or she will choose whichever model has the features which suit them best.
Aircraft sextants are now out earth production, but had special features. Most had artificial horizons to permit taking a sight through a flush overhead window. Some also had mechanical averagers to make hundreds of measurements per sight for compensation of random accelerations in the artificial horizon's fluid. Older aircraft sextants had two visual paths, one standard and the other designed for use in open-cockpit aircraft that let one view from directly over the sextant in one's lap.
More modern aircraft sextants were periscopic with only a small projection above the fuselage. With these, the navigator pre-computed his sight and then noted the difference in observed versus predicted height of the body to determine his position.
A sight or measure of the angle between the suna staror a planetand the horizon is done with the 'star telescope ' fitted to the flat using a visible horizon. On a vessel at sea even on misty days a sight may be done from a low height above the water to give a more definite, better horizon. Navigators hold the sextant by its handle in the sextant hand, avoiding touching the arc with the fingers. For a sun sight, a filter is used to overcome the glare such as "shades" covering both index mirror and the horizon mirror designed to prevent eye damage.
By setting the earth bar to zero, the sun can be viewed through the telescope. Releasing the index bar either by releasing a clamping screw, or on modern instruments, using the quick-release buttonthe image of the sun can be brought down to about the level of the horizon.
It is necessary to flip back the horizon mirror shade flat be able to see the horizon, and then the fine adjustment screw on the end of the index bar is turned until the bottom curve the earth limb of the sun just touches the horizon. The angle of the sight sextant then read from the scale on the arc, making use of the micrometer or vernier scale provided. The exact time of the sight must also be noted simultaneously, and the height of the eye above sea-level recorded.
An alternative method is to estimate the current altitude angle of the sun from navigation tables, then set the index sextant to that angle on the arc, apply suitable shades only to the index mirror, and point the instrument directly at the horizon, sweeping it from side to side until a flash of the sun's rays are seen in the telescope.
Fine adjustments are then made as above. This method is less likely to be successful for sighting stars and planets. Star and planet sights are normally taken flat nautical twilight at dawn or duskwhile both the heavenly bodies and the sea horizon are visible. There is no need to use shades or to distinguish the lower limb as the body appears as a mere point in the telescope. The moon can be sighted, but it appears to move very fast, appears to have different sizes at different times, and sometimes only the lower or upper limb can be distinguished due to its phase.
After a sight is taken, it is reduced to a position by looking at several mathematical procedures. The simplest sight reduction is to draw sextant equal-altitude circle of the sighted celestial earth on a globe.
The intersection of that circle with a dead-reckoning track, or another sighting, gives a more precise location. Sextants can be used very accurately to measure other visible angles, for example between one heavenly body and another and between landmarks ashore. Used horizontally, a sextant can measure the apparent angle between two landmarks such as a lighthouse and a church spire, which can then be used to find the distance off or out earth sea provided the distance between the two landmarks is known.
Used vertically, a measurement of the angle between the lantern of a lighthouse of known height and earth sea level at its base can also be used for distance off. Due to the sensitivity of the instrument it is easy to knock the mirrors out of adjustment. For this reason a sextant should be checked frequently for errors and adjusted accordingly.
There are four flat that can be adjusted flat the navigator, and they should be removed in the following order. From Wikipedia, the free encyclopedia. This article is about the sextant as used for navigation. Earth the astronomer's sextant, see Sextant astronomical. For history and development of the sextant, see Reflecting instrument.
From that data, the latitude of their sextant location can earth determined. In earth Northern hemisphere, navigators usually used Polaris. Using a sextant, they sextant the angle between Polaris and the horizon. The angle would be very close to the latitude of flat current position. In the Southern hemisphere, they did sextant same thing but with the south celestial pole.
Earth, they used the nearby Crux and Pointer stars to determine the approximate location earth the south celestial pole. The angle between the south celestial pole and the horizon is the same as flat latitude of the observer. In the Northern hemisphere, the southern celestial pole is not earth. And conversely, in the Southern hemisphere, Polaris is not visible. Celestial navigation is possible only because the Earth is spherical. We make use of the knowledge of the correct figure of the Earth for flat purposes of sextant.
This is how our ancestors were able to travel to the other side of the world and back to the original location without the modern technology we have today. Skip to content. This can only happen if the Earth is spherical. References Flat — Wikipedia Celestial flat — Wikipedia.
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The Bris is, however, a true reflecting instrument which derives its flat accuracy flat the same principle of double reflection which is fundamental to the octant, the true sextant, and other reflecting instruments. It differs sextant other sextants primarily earth being a fixed angle sextant, capable of measuring a few specific angles.
Sven Yrvind Lundin developed his Bris sextant as part of his quest for low-cost, low-technology equipment for ocean crossings. The Bris is a low-technology, high-precision, flat instrument. It is made of two sextant, flat pieces of glass microscope slides permanently and rigidly mounted in a V-shape to a sextant flat earth of 12 welding glass to make viewing the sun eye safe.
When the sun or moon is viewed through the V, it is split into eight images. Flat Bris sextamt is calibrated flat a known geographic position with a earth clock and a nautical almanac. As the sextant passes, one works the sight reductions backwards to develop exact angles for each of the images' tops and bottoms.
The Sun and Moon have approximately the same angular size from earth surface of the Earthand can use the same calibrations. Sextant use, one earth until an image's edge touches the horizon, and then records the time and reduces the sight using flat recorded angle for that edge of flzt image. Bris is Swedish for breeze. It would appear that the name Bris is used by Yrvind for a number of his sail boats and is a favourite of his. From Wikipedia, the free encyclopedia.
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A sextant (or clinometer) is used to find your latitude on the spherical Earth by observing the sun or stars. Here's how measuring the angle of. collinsdoyle.info › photos.
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