What Is A Marine Sextant? Origin, Usage and Errors [Updated ]
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.
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