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A sextant is a measuring instrument generally used to measure the angle of elevation of a celestial object above the horizon. Making this measurement is known as sighting the object, 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. A common use of the sextant is to sight the sun at noon to find one's latitude. See celestial navigation for more discussion. Held horizontally, the sextant can be used to measure the angle between any two objects, such as between two lighthouses, which will, similarly, allow for calculation of a line of position on a chart. Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea In physics and engineering, measurement is the activity of comparing physical quantities of real-world objects and events. ... See also Lists of astronomical objects Category: ... Horizon. ... A position line is a line that can be identified both on a nautical chart or aeronautical chart and by observation out on the surface of the earth. ... Portion of chart of Bering Strait, site of former land bridge between Asia and North America. ... The Sun is the star of our solar system. ... Latitude, usually denoted symbolically by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... Celestial Navigation is the 15th episode of The West Wing. ...


The scale of a sextant has a length of 1/6 of a full circle; 60°, hence the sextant's name. An octant is a similar device with a shorter scale, (1/8 of a circle, or 45°) and a quadrant is one with a longer scale (90°). Octant Octant is a measuring instrument similar to a sextant. ... Look up Quadrant on Wiktionary, the free dictionary Quadrant can mean: HMS Quadrant (G11), a WW-II British/Australian warship. ...


Sir Isaac Newton invented the principle of the doubly reflecting navigation instrument, but never published it. Two men independently rediscovered the sextant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), an American inventor. The sextant, along with the octant, replaced the astrolabe as the main instruments for navigation. Sir Isaac Newton, FRS (4 January 1643 – 31 March 1727) [OS: 25 December 1642 – 20 March 1727][1] was an English physicist, mathematician, astronomer, alchemist, and natural philosopher, regarded by many as the greatest figure in the history of science. ... Events Pope Clement XII elected September 17 - Change of emperor of the Ottoman Empire from Ahmed III (1703-1730) to Mahmud I (1730-1754) Anna Ivanova (Anna I of Russia) became czarina Births April 16 - Henry Clinton, British general (d. ... External link Biography from the School of Mathematics and Statistics at the University of St Andrews, Scotland Categories: Astronomers stubs | 1682 births | 1744 deaths | British astronomers | British inventors ... Events March 11 – Chelsea hospital for soldiers is founded in England May 6 - Louis XIV of France moves his court to Versailles. ... // Events The third French and Indian War, known as King Georges War, breaks out at Port Royal, Nova Scotia The First Saudi State founded by Mohammed Ibn Saud Prague occupied by Prussian armies Ongoing events War of the Austrian Succession (1740-1748) Births January 10 - Thomas Mifflin, fifth President... Thomas Godfrey (1704-1749) was an optician and inventor in the American colonies, who, around 1730 invented the sextant. ... Events Building of the Students Monument in Aiud, Romania. ... Events While in debtors prison, John Cleland writes Fanny Hill (Memoirs of a Woman of Pleasure). ... A 16th century astrolabe. ...

Octant and logbook aboard the frigate Grand Turk

Contents

Image File history File links Download high resolution version (1176x900, 446 KB) Please see the file description page for further information. ... Image File history File links Download high resolution version (1176x900, 446 KB) Please see the file description page for further information. ... Grand Turk, at anchor in Oostende, Belgium The Grand Turk is a three-masted 6th rate frigate, well known as the from the TV series Hornblower (and also as the French ship Papillon). ...

Advantages

The specific feature that let the sextant displace the astrolabe is that celestial objects are measured relative to the horizon, rather than relative to the instrument. This allows much better precision. A 16th century astrolabe. ...


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 a viewing pointer, as it is in an astrolabe. A 16th century astrolabe. ...


The horizon and celestial object remain steady when viewed through a sextant, even when the user is on a moving ship. This occurs because the sextant views the (unmoving) horizon directly, and views the celestial object through two opposed mirrors that subtract the motion of the sextant from the reflection.


The sextant is not dependent upon electricity (unlike many forms of modern navigation) or anything human-controlled (like GPS satellites). For these reasons, it is considered an eminently practical back-up navigation tool for ships.


Adjustment

Due to the sensitivity of the instrument it is easy to knock the mirrors and put them out of adjustment. For this reason a sextant should be checked frequently for errors and adjusted accordingly.


There are four errors that can be adjusted by the navigator and they should be removed in the following order.


1. Perpendicularity error

This is when the index mirror is not perpendicular to the frame of the sextant. To test for this, place the index arm at about 35° on the arc and when looking into the index mirror the arc of the sextant should appear to continue unbroken into the mirror. If there is an error then the two views will appear to be broken. Adjust the mirror until the reflection and direct view of the arc appear to be continuous..


2. Side error

This occurs when the horizon glass/mirror is not perpendicular to the index mirror. To test for this, first zero the index arm then observe a star through the sextant. Then rotate the tangent screw back and forth so that the reflected image passes alternately above and below the direct view. If in changing from one position to another the reflected image passes directly over the unreflected image, no side error exists. If it passes to one side, side error exists. The user can hold the sextant on its side and observe the horizon to check the sextant during the day. If there are two horizons there is side error; adjust the horizon glass/mirror until the stars merge into one image or the horizons are merged into one. Horizon. ... Perpendicular is a geometric term that may be used as a noun or adjective. ... A mirror is a reflective surface that is smooth enough to form an image. ... For alternate meanings see star (disambiguation) Hundreds of stars are visible in this image taken by the Hubble Space Telescope of the Sagittarius Star Cloud in the Milky Way Galaxy. ...


3. Collimation error

This is when the telescope or monocular is not parallel to the plane of the sextant. To check for this you need to observe two stars 90° or more apart. Bring the two stars into coincidence either to the left or the right of the field of view. Move the sextant slightly so that the stars move to the other side of the field of view. If they separate there is collimation error. 50 cm refracting telescope at Nice Observatory. ... A monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and prisms; the use of prisms results in a lightweight telescope. ... Parallel is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. ... Two intersecting planes in R3 In mathematics, a plane is a fundamental two-dimensional object. ...


4. Index error

This occurs when the index and horizon mirrors are not parallel when the index arm is set to zero. To test for index error, zero the index arm and observe the horizon. If the reflected and direct image of the horizon are in line there is no index error. If one is above the other adjust the index mirror until the two horizons merge. This can be done at night with a star or with the moon.


Anatomy of a sextant

Marine Sextant
Using the sextant to measute the altitude of the Sun above the horizon
Using the sextant to measute the altitude of the Sun above the horizon

The arm moves the index mirror. The indicator points at the arc to show the measurement. The body ties everything together. Image File history File links Download high-resolution version (2341x2552, 631 KB) other version: vector version (SVG) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sextant ... Image File history File links Download high-resolution version (2341x2552, 631 KB) other version: vector version (SVG) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sextant ... Image File history File links Using_the_sextant_new. ... Image File history File links Using_the_sextant_new. ...


There are two types of sextants. Both types can give good results, and the choice between them is personal.


Traditional sextants have a half-horizon mirror. It divides the field 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, where the horizon can be difficult to see. However, one has to sweep the celestial object to assure 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 of 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. Modern sextants often have 5 cm or larger mirrors, while 19th-century sextants rarely had a mirror larger than 2.5 cm (one inch). In large part this is because precision flat mirrors have grown less expensive.


An artificial horizon is useful when the horizon is invisible. This occurs in fog, on moonless nights, in a calm, when sighting through a 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. This article or section should be merged with Attitude indicator The term Artificial Horizon is used to describe devices that can indicate the position of the horizon when it is not possible to see the actual horizon. ...


Most sextants also have filters for use when viewing the sun, and reducing the effects of haze.


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 lighted artificial horizon. A monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and prisms; the use of prisms results in a lightweight telescope. ...


Professional sextants use a click-stop degree measure, and a worm adjustment that reads to a minute, 1/60 of a degree. Most sextants also 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. This is about 200 yards. At sea, results within several miles, well within visual range, are acceptable. It has been suggested that milliarcsecond be merged into this article or section. ... A degree (in full, a degree of arc, arc degree, or arcdegree), usually symbolized °, is a measurement of plane angle, representing 1/360 of a full rotation. ... A vernier scale lets one read more precisely from a measurement scale. ... A nautical mile is a unit of length. ... This article is about the unit of measure known as the yard. ...


A change in temperature can warp the arc, creating inaccuracies. Many navigators purchase weatherproof cases so their sextant can be placed 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 so body heat does not warp the frame. Sextants for tropical use are 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 aluminum. Brass is lower-expansion than aluminum, but aluminum sextants are lighter and less tiring to use. Some say they are more accurate because one's hand trembles less. Invar, also called FeNi36, is an alloy of iron (64%) and nickel (36%) with some carbon and chromium. ...


Aircraft sextants are now out of 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, to compensate for random accelerations in the artificial horizon's fluid. Older aircraft sextants had two visual paths, one standard, another designed for use in open-cockpit aircraft that let one view from directly over the sextant in one's lap... An Airbus A380, currently the worlds largest airliner An aircraft is any vehicle or craft capable of atmospheric flight. ...


The "Bris" sextant

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 fixed-interval sextant. It's made of three narrow flat pieces of glass (microscope slides) permanently and rigidly mounted in a V-shape. When the sun or moon is viewed through the V, it is split into eight images. The sextant is small and rugged-enough that it can be kept in a film can (about 2 cm radius, 3 cm tall) on a lanyard around one's neck.


The "Bris" sextant is calibrated at a known geographic position with a good clock and a nautical almanac. As the day passes, one works the sight reductions backwards to develop exact angles for each of the images' tops and bottoms. The Sun and Moon have the same angular width from the surface of the Earth, and can use the same calibrations. An almanac (also spelled almanack, especially in Commonwealth English) is an annual publication containing tabular information in a particular field or fields often arranged according to the calendar. ... Bulk composition of the Moons mantle and crust estimated, weight percent Oxygen 42. ... Earth (IPA: , often referred to as the Earth, Terra, or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ...


In use, one waits until an image's edge touches the horizon, and then records the time and reduces the sight using the recorded angle for that edge of the image.


See also

Wikimedia Commons has media related to:

Image File history File links Commons-logo. ... Wikimedia Commons logo by Reid Beels The Wikimedia Commons (also called Commons or Wikicommons) is a repository of free content images, sound and other multimedia files. ... A 16th century astrolabe. ... Celestial Navigation is the 15th episode of The West Wing. ... Gago Coutinho (1869-1959) was a Portuguese aviation pioneer that, together with Sacadura Cabral (1881-1924), was the first to cross the South Atlantic Ocean by air in 1922, from Lisbon, in Portugal, to Rio de Janeiro, in Brazil. ... The Intercept Method or Marc St. ... Latitude, usually denoted symbolically by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. ... Longitude, sometimes denoted by the Greek letter λ, describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. ... Table of geography, hydrography, and navigation, from the 1728 Cyclopaedia. ... Octant Octant is a measuring instrument similar to a sextant. ... Ulugh Beg, here depicted on a Soviet stamp, was one of Islams greatest astronomers during the Middle Ages. ...

External links

Memorial statue for Nathaniel Bowditch, in Mount Auburn Cemetery, Cambridge, Massachusetts. ...


  Results from FactBites:
 
Sextant - LoveToKnow 1911 (1782 words)
Independently of Hadley and Newton the sextant was invented by Thomas Godfrey (1704-1749), a poor glazier in Philadelphia.
The sextant was formerly much used on land for determining latitudes in which case an artificial horizon (see below) is required, but it has now been largely superseded by the portable altazimuth cr theodolite, while at sea it continues to be indispensable.
The telescopes employed in sextants are of two kinds: the direct, for the more ordinary observations; and the inverting, for astronomical work, one of the eyepieces of which should be of high magnifying power, not less than 15 diameters.
Sextant - Academic Kids (1450 words)
A sextant is a measuring instrument used to measure the angle of elevation of a celestial object above the horizon.
The measurement is thus limited by the angular accuracy of the instrument and not the sine-error of the length of a viewing pointer, as it is in an astrolabe.
Sextants for tropical use are often painted white to reflect sunlight and remain relatively cool.
  More results at FactBites »

 
 

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