Editing Sextant (section)

==Design==
The frame of a sextant is in the shape of a [[Circular sector|sector]] which is approximately {{Fraction|1|6}} of a circle (60°),<ref>{{cite book|title=Great Collections : treasures from Art Gallery of NSW, Australian Museum, Botanic Gardens Trust, Historic Houses Trust of NSW, Museum of Contemporary Art, Powerhouse Museum, State Library of NSW, State Records NSW.|last1=A.)|first1=McPhee, John (John|last2=NSW.|first2=Museums and Galleries|year=2008|publisher=Museums & Galleries NSW|isbn=9780646496030|pages=56|oclc=302147838}}</ref> hence its name (''sextāns, sextantis'' is the [[Latin]] word for "one sixth"). Both smaller and larger instruments are (or were) in use: the  [[Octant (instrument)|octant]], [[reflecting instrument#Quintant and others|quintant]] (or [[reflecting instrument#Quintant and others|pentant]]) and the (doubly reflecting) quadrant<ref>This article treats the doubly reflecting quadrant, not its predecessor described at [[Quadrant (instrument)|quadrant]].</ref> span sectors of approximately {{Fraction|1|8}} of a circle (45°), {{Fraction|1|5}} of a circle (72°) and  {{Fraction|1|4}} of a circle (90°), respectively. All of these instruments may be termed "sextants".

[[File:Marine sextant.svg|thumb|Marine sextant]]
[[File:Using sextant swing.gif|thumb|Using the sextant to measure the [[Celestial coordinate system#Altitude|altitude]] of the Sun above the horizon]]
[[File:Käpt'n Jonny Arndt bei einer Horizontalwinkelmessung..jpg|thumb|Sextants can also be used by navigators to measure horizontal angles between objects.]]

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 scales of the octant, sextant, quintant and quadrant are graduated from below zero to 90°, 120°, 140° and 180° respectively. For example, the sextant illustrated has a scale graduated from −10° to 142°, which is basically a quintant: the frame is a sector of a circle subtending an angle of 76° at the pivot of the index arm.

The necessity for the doubled scale reading follows from 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. When the index arm moves by an angle, say 20°, the angle between the fixed ray and the normal also increases by 20°. But the angle of incidence equals the angle of reflection so the angle between the object ray and the normal must also increase by 20°. The angle between the fixed ray and the object ray must therefore  increase by 40°. This is the case shown in the graphic.

There are two types of horizon mirrors on the market today. Both types give good results.

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 and/or a star being sighted 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 5&nbsp;cm or larger mirrors, while 19th-century sextants rarely had a mirror larger than 2.5&nbsp;cm (one inch). In large part, this is because precision flat mirrors have grown less expensive to manufacture and to [[silvering|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. There are two common designs of artificial horizon. An artificial horizon can consist simply of a pool of water shielded from the wind, allowing the user to measure the distance between the body and its reflection, and divide by two. Another design allows the mounting of a fluid-filled tube with bubble directly to the sextant.

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.{{Citation needed|date=January 2015}}

Professional sextants use a click-stop degree measure and a worm adjustment that reads to a [[minute of arc|minute]], 1/60 of a [[degree (angle)|degree]]. Most sextants also include a [[vernier scale|vernier]] on the worm dial that reads to 0.1 minute. Since 1 minute of error is about a [[nautical mile]], the best possible accuracy of celestial navigation is about {{convert|0.1|nmi|m|-1}}. 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 {{convert|0.25|nmi|m|-1|adj=on}}.<ref>''Dutton's Navigation and Piloting'', 12th edition. G.D. Dunlap and H.H. Shufeldt, eds. Naval Institute Press 1972, {{ISBN|0-87021-163-3}}</ref>

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 they 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 [[Periscope|periscopic]] with only a small projection above the [[fuselage]]. With these, the navigator pre-computed their sight and then noted the difference in observed versus predicted height of the body to determine their position.