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Sextant

A sextant is a measuring instrument used to measure the angle of elevation of a celestial object, traditionally the Sun above the horizon. Knowing the angle and time of day, traditionally mid-day for the sun, one can calculate the degree of latitude. See celestial navigation for more discussion.

Two different men independently invented the sextant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), an American inventor. The sextant replaced the astrolabe as the main instrument for navigation.

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, rather than relative to the instrument, 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 (unmoving) horizon directly, and views the celestial object through two opposed mirrors that subtract the motion of the sextant from the reflection.

Usage

After a sight is taken, it is reduced to a position by following any of several mathematical procedures. The simplest sight reduction is to draw the equal-elevation circle of the sighted celestial object on a globe. The intersection of that circle with a dead-reckoning track, or another sighting gives a precise location.

A sextant is a delicate instrument. If dropped, the arc might bend. After one has been dropped, its accuracy is suspect. Recertification is possible with surveying instruments and a large field, or with precision optical instruments. Repair is not possible.

To avoid worries about bent arcs, serious navigators traditionally buy their sextants new. Traditional wisdom is that a used sextant is probably bent. Many navigators refuse to share their sextant, to assure that its integrity is traceable.

Most sextants come with a neck-lanyard, and all but the cheapest come with a case. Traditional care is to put on the neck lanyard before removing the sextant from its case, and to always case the sextant between sights.

Never purchase a used sextant that lacks a case, because this is a sure sign of abuse.

Mirrors are adjusted for parallelism with precision spacers to be placed on a flat surface. No accuracy adjustment is possible or needed.

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's 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.

Most sextants also have filters for measuring 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 minutes. Since a degree 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, well within visual range.

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.

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 "V" Sextant

Gunnarson developed his "V" sextant as part of his quest for low-cost. low-technology equipment for ocean crossings. The "V" 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 2cm radius, 3cm tall) on a lanyard around one's neck.

The "V" 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.