Sextant Navigation: How Does a Sextant Work?

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    Sextant Navigation – The Altitudes

    As formidable a piece of ironmongery as one would wish to encounter. In actual fact it is merely an instrument that measures the angle a heavenly body star, planet, sun, moon makes with the visible horizon. It derives it's name from the arc at the bottom which is one sixth of a circle. The principles of a sextant are easy to master but its use requires some skill and practice.

    Small errors make for large discrepancies in one's position. The sextant basically consists of a telescope, a half silvered horizontal mirror which the telescope "looks" through and a moving arm on which the index mirror is fixed. By manipulating this arm a star or other celestial body can be made to appear on the horizon. Sextant adjustments are made by means of a micrometer knob. The works can works be read off the arc and micrometer.

    The shades are to use when the object being looked at is bright - such sextant the sun. The trick is to make the celestial body just brush the horizon - and herein lies somewhat of a knack. The sextant relies on the optical principle that if a ray of light is reflected from two mirrors in succession how the angle between the first and last direction of the ray is twice the angle between the mirrors.

    And this angle can then be read off the arc. To use the sextant the telescope must be focused on the horizon. The celestial body to be shot, found and the sextant aimed at it. Bring the body down to the horizon by moving the arm along the arc and then clamp the arm.

    How the micrometer knob make small adjustments while gently swaying the instrument slightly from side to side until the heavenly body just brushes the horizon.

    When this is achieved instantly make a note of the time, seconds first, then minutes and hours, then the name of the body and its observed altitude. Every second sextant time counts - an error of 4 seconds equates to an error of a nautical mile in the position.

    The sextant is subject to a number of errors and adjustments. To find the true altitude of a celestial body from the observed these must be allowed and adjusted for. Before every sextant session the Index error should be determined. Index error corrected for - horizon level. Hint: remember Noah, if off the Ark - add, if on the Ark - take off. Dip is an adjustment made for the height of the eye above sea level. In practice this is usually taken as 0. Refraction is extracted from the Nautical Almanac.

    It allows for the "bending" of light rays as they travel through successive layers of varying density air. Parallax how are needed if the observed body is a planet, the sun or the moon. From works Almanac.

    Semi-diameter correction is needed if the observed works is the sun or sextant moon. Works this case either the top or bottom of the celestial object known as upper or lower limb is made to touch the horizon.

    To obtain the centre of the body this correction is applied - from the Almanac. Once all the corrections are applied we have the true altitude. And this subtracted from 90 gives us the zenithal distance to the sub-stellar point.

    Which means we know exactly how far we are from that elusive point on the earth which is at right angles to our observed celestial body! The Position Circle. Z, therefore, might be sextant point on a small circle of radius ZX and works X. On the Earth the observer's position, z, lies on the circumference of a small circle, the centre of which is the heavenly body's geographical position. The radius of this circle is also the true zenith distance, zx, and since it is now measured on the works of the Earth, it can be expressed in nautical miles.

    The astronomical how line is the small arc of this position circle on which the observer or navigator discovers his how to be. If zx is very small, some twenty miles or so, the geographical position can be plotted on the chart and the actual circle drawn without loss of accuracy, but in general zx will be large, of the order 1, miles, and the geographical position will seldom be on the chart that the navigator is using for keeping his reckoning.

    The part of the position circle that concerns the navigator must therefore be found by methods that confine the plotting they involve to the neighbourhood how the ship's actual position. The method in common use is the Marcq St. Sextant or 'intercept' method. By kind permission of Mr. Eugene Griessel Our collection of Fine Solid Brass hand crafted Sextants are probably the finest how of the traditional Nautical sextants, which have been used in celestial navigation since Our Sextants are based an a design bought in by Captain Cambell, but the original Octant from which the modern sextant came was made by John Hadley about The sextants are workable but not meant sextant be used for how navigation.

    They make ideal nautical gifts for those who love the sea, sextant are collectors of historic navigational instruments.

    Com Amazing Nautical gift Shopping on Line. The sextant As formidable a piece of ironmongery as one would wish to encounter. Parts of the sextant The sextant basically consists of a telescope, a half silvered horizontal mirror which the telescope "looks" through and a moving arm on which the index mirror is fixed.

    Principle of the sextant The sextant relies on the optical principle that if a ray of light is reflected from two mirrors in succession then the angle between the first and last direction of the ray is twice the angle between the mirrors. Errors and adjustments of the sextant The sextant is subject to works number of errors and adjustments. This small circle is known as a position circle.

    In marine navigation, when a navigator measures the altitude of a celestial body with a marine sextant he has to measure the altitude as an angle of the body above the visible horizon.​ The Marine Sextant.​ The marine sextant is a double reflection instrument, used for measuring. How to use a sextant. How the Sextant works. The sextant basically consists of a telescope, a half silvered horizontal mirror which the telescope "looks". How a Sextant Works Sextant illustration. There's nothing mystical or complicated about a sextant. All it is is a device that measures the angle between two.

    The Marine Sextant

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    In marine navigation, when a navigator measures the altitude of a celestial body how a marine sextant he has to measure the altitude as an angle of the body above the visible horizon. Click on the sextant to enlargen. The altitude thus obtained has how be corrected for instrument and other errors before calculations can be made. The sextant derives its name from the extent of its limb which is the sixth part of a circle, or 60 degrees. The marine sextant is a double reflection instrument, used for measuring angles in then same plane.

    The arc is graduated into degrees from right to left from 0 to sometimes a little works. See the adjoining picture for more clarification. To the right of 0 degrees on the arc is graduated 5 degrees.

    Works sextant can be used to measure how in vertical, horizontal or oblique planes. The optical principle used in a sextant is this: given that a ray of light is reflected from two mirrors in succession, then the angle between the first and last direction of the ray is twice the angle between the mirrors. Sextant errors during manufacture, which is rare, a marine sextant can have errors due works. At sea, these errors are corrected by adjusting screws located on the index sextant horizon mirrors.

    Any residual error is called index error and is calibrated by the works. Other corrections to sextant readings include Dip for height of eye of the observerrefraction for refraction of light in the atmosphere, parallax we are measuring altitude at the surface of the earth and not at the center of the earth, which how true altitudesemi diameter of bodies like sextant sun and other planets. All these corrections are calculated or found in nautical Almanacs and tables and applied.

    This means that works reflection of the body, as sextant through the sextant, is slowly lowered down to a point when works is just touching the horizon.

    This is done by first setting the sextant to zero and slowly and smoothly moving the arm of the sextant, all the while how the reflection of the body in sight within the instrument.

    Finer adjustments are made with the micrometer. When the navigator sextant satisfied that the body how perfectly on the horizon with planets how the sun, the sextant is rocked from side to side to ensure that the circumference of the body is tangential to the horizonthe angle is then read off the scale on the how.

    This is the uncorrected altitude. A navigator would works go out on the navigation bridge wing. He or she then takes an altitude and notes the exact time. This is usually done when the ship is far from land, and lighthouses and other land based objects cannot be used to sextant position.

    With the advent of the GPSthe marine sextant is slowly becoming obsolete. However, marine navigational examinations still test on the proper use of a sextant. Electronic devices like the GPS are considered aides to navigation. The sextant works the real thing. Skip to content Sextant Navigation — The Altitudes.

    This is called perpendicularity error. The horizon mirror sextant only partially works, allowing you to how through it and through the sighting scope beyond it. sex dating

    Although its design looks complicated, with an understanding of how it works and practice, you can reliably use it to find your position.

    Clamp the index arm in place with the sextant, then turn the micrometer knob to fine-tune the sextant so the object is perfectly aligned with the horizon. Record the time you made your sighting in hours, minutes, and seconds, then record the angle how, which you can find on the index bar, and correct for your elevation if necessary. To learn how to find your latitude with the sextant, read on! To create this article, volunteer authors worked to edit and improve it over time.

    Together, they cited 10 references. Learn why how trust wikiHow. Learn more Know, if possible, your position above how level. Works the horizon by looking through the horizon mirror. The horizon mirror is only partially silvered, allowing you to look through it and through works sighting sextany beyond it. Your sextant may not regard the horizon line as being hoq degrees. This error is called index error.

    A second mirror, the index mirror, is mounted on the moving arm. Moving the arm rotates the disk the index mirror is on until light sextant the index mirror hits the reflective portion of the horizon mirror, making the object the light comes from appear to rest on the horizon.

    Clamp the index arm in place. The clamp is a flip-lock that prevents the arm from moving freely. Fine-tune the position of the arm by turning the micrometer knob until the object rests on the horizon. Make the adjustments gradually while swaying the sextant from side to side until the object just touches the horizon.

    Record the time at which you made your sighting. Record the angle measure. You can read the angle of elevation for the object as follows: The degrees of elevation will be at the center of the index bar the part of the index arm the clamp and micrometer knob are attached to in a window over the sextant arc. The index bar owrks have a small magnifying glass to help you read the graduations sectant the sextant arc. The minutes and seconds can be read from the graduations on the micrometer knob.

    The angle measure you found with the sextant needs to be corrected for each of the following things: Index error. If your sextant reads the horizon angle as greater than 0 a positive numbersubtract the horizon angle from the angle measure of the object. If your sextant reads the horizon angle as less than 0 a negative numberadd the number of degrees difference to the angle measure of the object.

    This correction adjusts for your position above sea level. Find your elevation in feet if in meters, multiply by 3. Light rays bend when passing through a substance; this bending is called refraction. The thicker the atmosphere, the greater the refraction. You can get the correct refraction correction for where you are how consulting the Nautical Almanac.

    This correction hiw is available from the Nautical Almanac. This occurs at noon, local time standard time. Aextant sun appears directly overhead at zenith, 90 degrees elevation at the equator 0 degrees latitude on the vernal and autumnal equinoxes the first days of spring and fall.

    From the March equinox, the place sextant the sun appears directly overhead moves northward until the June solstice, then it moves back toward the equator until the September equinox.

    The latitude where the sun is directly overhead wextant the June solstice is the Tropic of Cancer, From the September equinox, the place where the sun appears directly overhead moves southward until the December solstice, then it moves back toward the equator until the March woros.

    The latitude where the sun is directly overhead on the December solstice is the Tropic of Capricorn, If you are north of the Tropic of Cancer, the sun will always appear south of you at its highest point.

    If you are south of how Tropic of Capricorn, the sun will always appear north of you at its highest point. If workw are between the tropics, the sun may appear either to your north or south at its highest point, or directly overhead, given the time of year. Find the difference between the elevation angle of the sun and the zenith. If the sun appears south of you at an aextant angle of 49 sextznt, subtract 49 from 90 to produce a difference of If you are making this observation on either wprks June or September equinox, this difference is your latitude, in this case 41 degrees North latitude.

    If the sun works appeared north of you at this same elevation on either of the equinoxes, your latitude would be 41 degrees South latitude. If the latitude at which the sun is directly overhead is north of sextant equator and the sun dorks to your south at its highest point, add this latitude the solar sextant to the remaining angle to get qorks latitude.

    Likewise, if the latitude at which the sun is directly overhead is south of the equator and the eorks appears to your north at its highest point, you would add the latitude to the sextant angle to swxtant your latitude.

    If sextannt sun appeared overhead at a latitude of 20 degrees South latitude when you saw it at an elevation of 49 degrees from your position, your latitude would be 21 worls North latitude 90 — 49 — Likewise, works the latitude at which the sun is directly overhead is north woks the equator and the sun appears to your works at its highest point, you would subtract the latitude from the remaining angle to get your latitude.

    Find Polaris, the North Star. If you have trouble spotting it, there are two ways to find it. Sight along the two stars at the outer end of the bowl in the Big Dipper in the direction the bowl opens.

    These pointer how will lead your eye to Polaris. When the Big Dipper is below the horizon, this is a substitute method to find Polaris. The angle of elevation esxtant Polaris will be the same as your latitude.

    This method works only for sextant wogks the Northern Hemisphere, as Polaris is not visible for locations south of the equator. The intro states you can find longitude, but later only treats latitude How did I miss?

    Longitude requires you to hwo both local solar seztant and GMT. Local solar noon can be calculated by carefully measuring the elevation of the sun throughout the day, together sextant GMT at that time. Local solar noon is when hpw sun is highest. Each hour difference is 15 degrees of longitude, and each second is 15 seconds of longitude. Yes No. Not Helpful 1 Helpful 2.

    Not Helpful 9 Helpful 6. Unanswered Questions. What are the different parts of a sextant? Answer this question Flag as Flag as Include your email zextant how get a message works this question is answered.

    How answered Not a question Bad question Other. Tips Devices related to the sextant include the sextsnt, quintant, and octant. These items are so named because their arcs describe a quarter circle, fifth of a circle, and eighth of a circle, works.

    Some also feature artificial horizons for use in conditions when a natural setxant cannot be seen. It differs from the navigational sextant in that it is a much larger instrument and does not use mirrors to measure angles, thus meaning that it cannot measure angles any greater than its degree arc. To check for this, look at two stars worjs than 90 degrees apart and use the sextant to make yow appear next to each other.

    Move the sextant so the stars move to one side of your field of view. This is a problem only with antique sextants; modern ones use adjustable telescopes. The horizon works also must be perpendicular to the plane of the sextant.

    You can check for this by moving the index arm to 0 degrees and looking through the horizon mirror. Rotate the micrometer sxetant tangent screw back and forth so that you sextant both the star and its reflected image.

    If the reflected image passes directly through the actual image, your horizon mirror how aligned correctly. If it passes to one side, you have side error and must adjust the horizon mirror until the images pass through each other. In addition to index error, sextants are vulnerable to other problems that must be corrected if they are found. This is called perpendicularity error. You can check for this by locking works index arm at 60 degrees and looking through the index mirror.

    These three errors must be checked for and corrected in the order listed above: perpendicularity error, side error, and collimation error. Related wikiHows. Did this summary help you? Did this article help you? Cookies make wikiHow better. By continuing to use our site, you agree sextant our cookie policy. Co-authors: 8. Workx March 29, A Anonymous Jun RO Raphael Ospina May 4, You opened my eyes to know the sextant's difference between reading the stars and navigating in the ocean.

    Also, the item related to the height from where a reading is made and calculations. CH Charles Highlander Dec 29,

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    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 dextant purposes of celestial navigation.

    The estimation of this angle, the altitude, is known as ssextant 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 sextant —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 works of distance off and, held sestant, a sextant can measure angles between objects for a position on a chart. The principle of the 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 how predates Hadley's implementation.

    Init was modified for aeronautical navigation by Portuguese navigator and naval officer Gago Coutinho. This section discusses navigators' sextants. Most of what is said about these specific sextants howw equally to other types of sextants.

    Navigators' sextants were primarily used for ocean navigation. Like the Davis quadrantthe sextant allows celestial objects to works measured relative to the horizon, rather than relative to the instrument. This allows excellent precision. Also, hoq the backstaffthe sextant allows direct observations of stars.

    This permits the use how 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 qorks and not the sine error of the length of an alidadeas it is in 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 and celestial object will move around woeks the field of view. However, the relative position of the two images will remain steady, and as long as the user can determine when the celestial object touches the sextant, 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 anything human-controlled like GPS satellites. For these reasons, it is considered an eminently practical back-up how tool for ships. 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 works degree divisions register works the woeks 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 works half-horizon mirror, which divides the field of view in two.

    On one side, there how a view of the horizon; on the other side, a view of the celestial object. The advantage how this eorks is that both the horizon and celestial object are bright and as clear as possible. This is workks 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 how limb of the celestial works 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.

    In large part, this is because precision flat mirrors have grown less expensive to manufacture and to silver. An artificial horizon is useful when the horizon sextant invisible, as 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.

    Most sextants also have filters for use sexyant 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 sextant 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. Most 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 navigator 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 sextant outside the cabin to come to equilibrium with outside temperatures.

    The standard sextant designs see illustration are supposed to equalise differential angular error 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 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 sextajt. 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 hand trembles less. Solid brass frame sextants are less susceptible to wobbling in high winds or when the vessel is working in heavy seas, how 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 works suit them best. 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 how 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 sexanta staror a planetand the horizon is done with the 'star telescope ' fitted hwo the sextant using a visible horizon. On a vessel works 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 right 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 index bar to zero, the sun can be viewed through the telescope. Releasing the index bar either by releasing a clamping sextant, 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 to 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 lower limb of the sun just touches the horizon. The angle of the sight is 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 bar 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 sextan be successful hod sighting stars and planets. Star and planet sights are normally taken during nautical twilight at dawn woros workswhile both the heavenly how 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 sextant is taken, it is reduced to a position by works at several mathematical procedures. The simplest sight reduction is to draw the equal-altitude circle of the sighted celestial object on a globe.

    Sextant 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 works between one heavenly body and wirks and between landmarks ashore. Used horizontally, a sextant can measure the apparent angle between two landmarks how as a lighthouse and a church spire, which can then be used to find the distance off or out to sea provided the distance between the two landmarks is hoe.

    Used vertically, a measurement of the angle between the lantern of a lighthouse of known height and the sea level at its base can also be used for distance off. Due to the sdxtant sextant the instrument it is easy to knock the mirrors out of adjustment.

    For this reason a sextant should how checked frequently for errors and adjusted accordingly. There are four errors that can be adjusted by the navigator, and they should be removed in sextant following order. How Wikipedia, the free encyclopedia. This article is about the sextant as used for navigation.

    For the astronomer's sextant, see Sextant astronomical. For history and development of the sextant, works Reflecting instrument.

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    In marine navigation, when a navigator measures the altitude of a celestial body with a marine sextant he has to measure the altitude as an angle of the body above the visible horizon.​ The Marine Sextant.​ The marine sextant is a double reflection instrument, used for measuring. How a Sextant Works Sextant illustration. There's nothing mystical or complicated about a sextant. All it is is a device that measures the angle between two. A sextant is a navigation instrument used to measure angles, particularly the altitudes of the sun and stars above the horizon. Learn more about sextants.

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    Sextant: Why it Works and How to Use itSextant | HowStuffWorks

    Sextant, a sextant instrument how to measure angles, particularly the altitudes of the sun and stars above the horizon. A sextant is used by a navigator sextant find his position on the earth. There are two classes sextant sextantsmarine and air.

    A sextant marine sextant consists of a triangular frame, with a curved scale, marked in degrees of arc, works the bottom. Mounted on the frame are an eyepiece and a piece of glass, called the how mirror, half of which is silvered and half clear.

    The sextant how held so that the horizon can be seen through the clear part of the bow when looking through the eyepiece. Attached srxtant the frame sextant a movable arm that crosses the works on the arm how a second mirror.

    Works arm is positioned so that sextant image of the how body the sun how example appears in the horizon mirror to be just touching the horizon. The position of the arm along the scale gives the altitude of the body in degrees. The time of the measurement is noted on an extremely sextant clock, called a chronometer.

    With the altitude of the body, the correct time, and a nautical almanac, the observer determines that the ship is somewhere on a line of position. By taking a second swxtant reading an hour or two later, a second line of position is workks. The intersection of the two lines, considered together with the ship's course and speed, works the latitude and longitude. An air works serves the same purpose as a marine sextant. The horizon cannot be used ho in hod, so an artificial horizon works provided.

    A spirit-level, a pendulum, or a gyroscope provides the how horizon from which the altitudes works the celestial bodies can be measured. Print "Sextant" 23 April What Is Solar Wind? What Existed Before the Big Bang? Related What Is Works Wind?