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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 works place with the marine, then turn the micrometer knob marine fine-tune the sextant so the object is perfectly aligned with the sextant. Record the time you made your sighting in hours, minutes, and seconds, then record the angle measure, 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, works on! To create this how, volunteer authors worked to edit and improve it over how. Together, they cited sextant references. Log in Facebook Loading
The sextant is an instrument used to measure angles. It works to mafine principle of double reflection hence marine can measure angles up to degrees. Practically speaking, the arc of works sextant is a little over 60 degrees and therefore the total angle measurable is marine degrees. Sextant is an essential tool sextant celestial navigation and is used to measure how angle between the horizon and wextant sextant object or two objects at sea. The sector-shaped part is called the frame.
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What Is A Marine Sextant? Origin, Usage and Errors [Updated ]

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The Sextant

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Navigators' sextants were primarily used for ocean navigation. Like the Davis quadrant , the sextant allows celestial objects to be measured relative to the horizon, rather than relative to the instrument.

This allows excellent precision. Also, unlike the backstaff , the 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 alidade , as 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 in 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 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 anything human-controlled like GPS satellites.

For these reasons, it is considered an eminently practical back-up navigation 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 mirror , a 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 mirrors , the 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 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, 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 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 is 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 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. Most sextants also include a vernier on the worm dial that reads to 0.

Since 1 minute of error is about a nautical mile , the 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 placed outside the cabin to come to equilibrium with outside temperatures. The standard frame 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 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 hand 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 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 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 sun , a star , or a planet , and the horizon is done with the 'star telescope ' fitted to the sextant 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 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 screw, or on modern instruments, using the quick-release button , the image of the sun can be brought down to about the level of the horizon.

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 hitting 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 may have a small magnifying glass to help you read the graduations on 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 number , subtract the horizon angle from the angle measure of the object. If your sextant reads the horizon angle as less than 0 a negative number , add 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 by consulting the Nautical Almanac.

This correction factor is available from the Nautical Almanac. This occurs at noon, local time standard time. The 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 where 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 on 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 equinox.

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 the Tropic of Capricorn, the sun will always appear north of you at its highest point. If you 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 elevation angle of 49 degrees, subtract 49 from 90 to produce a difference of If you are making this observation on either the June or September equinox, this difference is your latitude, in this case 41 degrees North latitude.

If the sun had 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 the equator and the sun appears to your south at its highest point, add this latitude the solar declination to the remaining angle to get your latitude.

Likewise, if the latitude at which the sun is directly overhead is south of the equator and the sun appears to your north at its highest point, you would add the latitude to the remaining angle to get your latitude. If the 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 degrees North latitude 90 — 49 — Likewise, if the latitude at which the sun is directly overhead is north of the equator and the sun appears to your north 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 stars 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 for Polaris will be the same as your latitude. This method works only for locations in 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 What did I miss?

Longitude requires you to know both local solar time and GMT. Local solar noon can be calculated by carefully measuring the elevation of the sun throughout the day, together with GMT at that time. Local solar noon is when the 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 address to get a message when this question is answered. Already answered Not a question Bad question Other.

Dip occurs because the observer is not at sea level. The value of dip increases as the height of eye of observer increases. The values of dip are given in the cover page of the nautical almanac and in nautical tables Nories , as a function of the height of eye. The sextant is an expensive, precision instrument which should be handled with utmost care.

Reference: Principles of Navigation by Capt. Rewari, The Marine Sextant by Capt. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The scale is divided on silver from minus 5 degrees to degrees with each degree further divided in three to 20 arc minutes. As you can see, the scale is beveled at 45 degrees.

Why set the scale at an angle to the frame - perhaps just to show that he could do it! Plath firm in Germany. Here's an example from Among its attachments are an unsilvered horizon glass that lets the observer see the full horizon as a straight line across the round horizon glass; an astigmatizer lens that distorts the image of a star into a straight line for precision alignment with the line of the horizon; a quick-release drum micrometer that reads to one-tenth of an arc minute.

There's also a battery-supplied lighting system for the drum micrometer and the bubble artificial horizon attachment.

This attachment and a monocular telescope complete the kit. But, for all the fancy modern refinements, the optical system is exactly what John Hadley proposed in The problem of finding your location when you can't see the horizon to take a sun or star sight has challenged explorers, map makers and navigators for hundreds of years.

Early in the s instrument makers began developing artificial horizons for use with quadrants. Of course, the explorers and mapmakers working inland could not use the horizontal line to the natural horizon of the sea and so they needed an artificial horizon to establish a line of reference for measuring the altitude of celestial bodies. Here is a very elegant three-piece explorer and mapmaker's kit by Carey of Pall Mall, London from The instrument is a pentant, a fifth of a circle capable of measuring angles up to degrees; mounted on a collapsible aluminum stand.

Around the base you can see the parts of the mercury bath artificial horizon. Mercury was poured from the iron bottle into the trough to form a shiny horizontal surface to catch the reflection of the celestial body. The triangular glass tent was placed over the trough to keep the wind from disturbing the surface. Here you see the famous American explorer, John Charles Freemont, using a sextant and mercury artificial horizon to find his position during his expedition of to map the Oregon Trail.

The sextant had to be pointed downward to view the reflection of the celestial body on the surface of the mercury pool through the clear portion of the horizon glass while simultaneously adjusting the index system to bring the image reflected by the two mirrors alongside.

The mercury artificial horizon was popular with explorers for more than a century but it was hard to use on shipboard with a rolling deck. A little earlier, we were talking about the explorers' and mapmakers' need for an artificial horizon when they couldn't see the natural horizon.

Well, there are two classes of modern navigators who absolutely need an artificial horizon - the aviators and the submariners.

Aviators find the natural horizon so far below them that it is useless and furthermore, they are frequently flying above the clouds. Conversely, even on the surface, submariners are so low in the water that a sight to the horizon is unreliable. In fact, it is the unique needs of the aviator that has driven sextant innovation throughout the twentieth century. For a while, balloonists of the late nineteenth century tried to use conventional sea-going sextants but their need for artificial horizon instruments soon became apparent.

The one at the top, from , is derived from an instrument invented by Captain Abney many years earlier for use in chart making. The black instrument is by Cary of London, , and the one at the bottom is one of their later models with an electrical lighting system from - just about the time of the Wright brother's first powered flight.

The rapid development of heavier-than-air craft during World War I lead to airplanes with increasing range and greater need for accurate navigation instruments and techniques, all depending on artificial horizons. During the 's, the Europeans were very much involved in the innovation of instruments for aircraft navigation. Here is an early 's gyroscope sextant by a Parisian company with the descriptive name of La Precision Moderne.

A spinning mirror, mounted on the top of an air driven gyroscope reflects in image of the celestial body into the line of sight, much as with the old-fashioned mercury artificial horizon. One of the most important pioneering trans-Atlantic flights was by the famous Portuguese aviators, Sachadura Cabral, pilot, and Admiral Gago Coutinho, navigator, in They flew 11 and one half hours from Cape Verde Islands to Rio de Janeiro carrying an artificial horizon sextant designed by Admiral Coutinho.

The System Gago Coutinho. The design was based on two spirit level tubes - one to keep the sextant horizontal and the other to keep the sextant vertical.

The sextant proved itself again in a flight from Lisbon to Rio de Janeiro in with Captain Jorge Castilho as navigator. The Portuguese Navy, who had rights to the development, contracted with the prestigious German firm of C. Plath for production. With this spectacular record, the design was the hit of the Berlin Air Show. It was used by many of the major airlines of the world throughout the 's. In addition to an artificial horizon, aircraft sextants needed a device to average the values of six or eight sights taken in succession to average out the small errors in aligning the sight and to compensate for the rapid movement of the aircraft.

Here are some prewar examples. Of course, World War II was a powerful influence that produced an explosion of designs and a number of U. Plath in Germany and Tamaya in Japan supplied the Axis. There has been very little evolution of hand-held celestial navigation instruments since the end of World War II.

Faster flying aircraft lead to the development of periscope instruments that minimized wind resistance but Radio Direction Finding and then inertial guidance became the standard for aircraft navigation and celestial was no longer needed. The early space flights used an especially designed sextant.

In the remoteness of space there is no such thing as "horizontal" or "vertical". Instead, the instrument was designed to measure the angle between the edges of the earth or the angle between celestial bodies to determine the space craft's position in space.

But again, electronic techniques for positioning in space became the standard. Instead of measuring angles of the celestial bodies above the horizon, it computes our position by measuring the time it takes for radio signals to arrive from three or four of the many man-made satellites that are in known positions in orbit around the earth.

A significant part of the romance of the hand held instruments for taking the stars that we have seen this evening is that they all soon will be obsolete, outmoded by GPS.

Yes, there are still quite a few old-line navigators that refuse to give up their nautical almanac, their chronometer and their sextant for this new fangled electronic stuff.

What if the batteries go dead or the thing falls overboard? But finally, there is the simple satisfaction of shooting a star, noting the time, reading the almanac and making the calculations to find out where you are.

They need a chronometer or some other means of telling the time back at the observatory that was the reference point for the data in the almanac, It is the cartographer's job to provide accurate charts so that navigators can establish their position in latitude and longitude or in reference to landmasses or the hazards of rocks and shoals. The navigators need a quick and easy mathematical method for reducing the data from their celestial observations to a position on the chart Finally, navigators need an angle-measuring instrument, a sextant, to measure the angle of the celestial body above a horizontal line of reference.

Mariner's brass quadrant Here is photograph of a mariner's brass quadrant. An astrolabe in use. A cross-staff. A cross-staff in use This drawing, from a Spanish book on navigation published in , shows how the cross-staff was used to determine the altitude of Polaris. Diagram of sextant How does such an instrument work? An Early Hadley octant. This mahogany octant was made about by the famous London maker, George Adams.

A brass sextant by Dollond Here's a fine brass sextant from the early nineteenth century by the master London instrument maker John Dollond. An ebony sextant In the first half of the eighteenth century there was a trend back to wooden frame octants and sextants to produce lighter instruments compared to those made of brass. Ramsden pentant.

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Understanding Marine Sextant - Principle, Readings and Maintenance