The scale of a sextant has a length of of a full circle (60°); hence the sextant's name (sextāns, -antis is the Latin word for "one sixth", "εξάντας" in Greek). An octant is a similar device with a shorter scale (of a circle, or 45°), whereas a quintant (or 72°) and a quadrant (or 90°) have longer scales.
Sir Isaac Newton (1643-1727) invented the principle of the doubly reflecting navigation instrument (a reflecting quadrant - see Octant (instrument)), but never published it. Two men independently developed the octant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), an optician in Philadelphia. The octant and later the sextant, replaced the Davis quadrant as the main instrument for navigation.
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.
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.
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.
The index 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, 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. Modern sextants often have 5cm or larger mirrors, while 19th century sextants rarely had a mirror larger than 2.5cm (one inch). In large part, this is because precision flat mirrors have grown less expensive to manufacture and to silver.
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 lit 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 . 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 .
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.
Now long out of production, aircraft recording sextants and octants were used from around 1930 through the late 1950s, when fast, high-flying jet aircraft spelled the end of low-altitude, long distance celestial navigation. Instead of using a plumb bob, a liquid-damped steel ball recorded the altitude of the object on a screen. Most aircraft octants had artificial horizons using a centered bubble (the so-called 'bubble octant') to permit taking a sight through a flush overhead window. Drum micrometers were used to determine the precise altitude reading. Some had mechanical averagers to make hundreds of measurements per sight for compensation of random accelerations in the artificial horizon's fluid.
Older aircraft instruments 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 and octants 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. After a sight is taken, it was reduced to a position by following any of several mathematical procedures. The simplest sight reduction was 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 gave a more precise location.
During World War II, the emergency or 'lifeboat sextant' was introduced for use by Allied naval and merchant marine personnel in emergencies; many were stored on lifeboats as part of their equipment. The lifeboat sextant was made in 3" and 4" sizes. Like the pocket sextant, sextants of this size were limited in their accuracy.
One type of 'sextant', the Bris sextant fits inside a small pillbox. The Bris is really an solar angular measuring device, consisting of a small collection of bonded mirrors; its primary use is for emergency navigation. Prerecorded sun images at a known location are compared with Bris sextant sightings when the sun touches the horizon, using sun tables from a nautical almanac to find latitude and longitude.
To avoid worries about bent arcs, serious navigators traditionally buy their sextants new. Common wisdom is that a used sextant is probably bent. Many navigators refuse to share their sextant, to ensure that its integrity is traceable. A used sextant lacking a case is very likely to have a bent arc.
Most sextants come with a neck-lanyard; 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.
There are four errors that can be adjusted by the navigator and they should be removed in the following order.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 60° on the arc and hold the sextant horizontally with the arc away from you at arms length and look 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.Side error:This occurs when the horizon glass/mirror is not perpendicular to the plane of the instrument. 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.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.Index error:This occurs when the index and horizon mirrors are not parallel to each other 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.