Sextant
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.
Encyclopedia
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.
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, and a quadrant is one with a longer scale .
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 , an English mathematician, and Thomas Godfrey , an American inventor. The sextant, along with the octant, replaced the
astrolabe as the main instruments for navigation.
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.
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.
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 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 or anything human-controlled . 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.
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.
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
The
arm moves the
index mirror. The
indicator points at the
arc to show the measurement. The body ties everything together.
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 . 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.
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.
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.
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 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 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.
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...
The "Bris" sextant
Sven Yrvind 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 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 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.
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
External links
- Her Majesty's Nautical Almanac Office: http://www.nao.rl.ac.uk/
- The History of HM Nautical Almanac Office: http://www.nao.rl.ac.uk/nao/history/
- The home page of Mr Sven Yrvind: http://www.yrvind.com/ and his self-biography in Swedish, "Konstruktören" , ISBN 91-518-4154-1.
- This on-line dealer has sextants, accessories and books: http://www.celestaire.com/
- Cassens & Plath GmbH manufactures and sells sextants, here the Bris-Minisextant:http://www.cassens-plath.de/catalog/096e.html
- A three page article on the Bris sextant appeared in magazine, June 1997.
- A two page article on the Bris sextant appeared in magazine, 22/1997: Mini-Sextant: Mit einem genial einfachen Gerät verblufft Weltumsegler Sven Lundin jetzt die gesamte Fachwelt.
- Chapter from the online edition of Nathaniel Bowditch's American Practical Navigator
- Simple do-it-yourself project.
