Editing Navigation (section)

===Celestial navigation===
{{Main|Celestial navigation}}
[[File:Sun-Moon path.PNG|thumb|upright=1.2|A celestial fix will be at the intersection of two or more circles.]]

Celestial navigation systems are based on observation of the positions of the [[Sun]], [[Moon]], [[planet]]s and [[list of selected stars for navigation|navigational stars]] using a [[sextant]] or similar navigation instrument.<ref name="s102">{{cite web | title=Celestial Navigation | website=Time and Navigation | url=https://timeandnavigation.si.edu/navigating-at-sea/navigating-without-a-clock/celestial-navigation | access-date=2025-02-24}}</ref> By knowing which point on the rotating Earth a celestial object is above and measuring its height above the observer's horizon, the navigator can determine his distance from that subpoint using mathematical calculation.<ref name="q565">{{cite book | last=Cunliffe | first=Tom | title=Celestial Navigation | publisher=*Wiley Nautical | publication-place=London | date=2010-09-28 | isbn=978-0-470-66633-3}}</ref> A [[nautical almanac]] and a source of time, typically a [[marine chronometer]] are used to compute the subpoint on Earth a celestial body is over, and a [[sextant]] is used to measure the body's angular height above the horizon.<ref name="q565"/> That height can then be used to compute distance from the subpoint to create a circular line of position. Alternatively sight reduction tables can be used.<ref name="j728">{{cite book | last=Prinet | first=Dominique F. | title=Celestial Navigation | publisher=FriesenPress | date=2014-07-18 | isbn=978-1-4602-4212-4}}</ref> A navigator shoots a number of stars in succession to give a series of overlapping lines of position. Where they intersect is the celestial fix. The Moon and Sun may also be used. The Sun can also be used by itself to shoot a succession of lines of position (best done around local noon) to determine a position.<ref name="bow269"/> Since the advent of GNSS, celestial navigation is less used for marine and air navigation, though it remains useful as a backup or as another method to cross-check the accuracy of electronic systems, particularly in the open ocean.<ref name="t697">{{cite book | last=Karl | first=John | title=Celestial navigation in the GPS age | publisher=Paradiese Cay Publ | publication-place=Arcata,CA | date=2007 | isbn=978-0-939837-75-5}}</ref><ref name="u717">{{cite web | last=Escobar | first=Lieutenant Juan J. | last2=Navy | first2=Chilean | title=Bring Celestial Navigation into the 21st Century | website=U.S. Naval Institute | date=2021-12-01 | url=https://www.usni.org/magazines/proceedings/2021/december/bring-celestial-navigation-21st-century | access-date=2025-02-24}}</ref>

====Marine chronometer====
{{main|Marine chronometer}}
[[File:Breguet marine clock-CnAM 16767-1-IMG 1525-white.jpg|thumb|right|Breguet marine chronometer]]
In order to accurately measure longitude, the precise time is required of a sextant sighting (down to the second, if possible) which is then recorded for subsequent calculation. Each second of error is equivalent to 15 seconds of longitude error, which at the equator is a position error of .25 of a nautical mile, about the accuracy limit of manual celestial navigation. The spring-driven marine chronometer is a precision timepiece used aboard ship to provide accurate time for celestial observations.<ref name="bow269">Bowditch, 2002:269.</ref> A chronometer differs from a spring-driven watch principally in that it contains a variable lever device to maintain even pressure on the mainspring, and a special balance designed to compensate for temperature variations.<ref name="bow269"/> A spring-driven chronometer is set approximately to Greenwich mean time (GMT) and is not reset until the instrument is overhauled and cleaned, usually at three-year intervals.<ref name="bow269"/> The difference between GMT and chronometer time is carefully determined and applied as a correction to all chronometer readings.<ref name="bow269"/> Spring-driven chronometers must be wound at about the same time each day.<ref name="bow269"/>

[[Quartz clock#Accuracy enhancement|Quartz crystal marine chronometers]] have replaced spring-driven chronometers onboard modern ships because of their greater accuracy.<ref name="bow269"/> They are maintained on GMT directly from radio time signals.<ref name="bow269"/> This eliminates chronometer error and watch error corrections.<ref name="bow269"/> Should the second hand be in error by a readable amount, it can be reset electrically.<ref name="bow269"/> The basic element for time generation is a quartz crystal oscillator.<ref name="bow269"/> The quartz crystal is temperature compensated and is hermetically sealed in an evacuated envelope.<ref name="bow269"/> A calibrated adjustment capability is provided to adjust for the aging of the crystal.<ref name="bow269"/>

The chronometer is typically designed to operate for a minimum of one year on a single set of batteries.<ref name="bow269"/> Observations may be timed and ship's clocks set with a comparing watch, which is set to chronometer time and taken to the bridge wing for recording sight times.<ref name="bow269"/> In practice, a wrist watch coordinated to the nearest second with the chronometer will be adequate.<ref name="bow269"/> A stop watch, either spring wound or digital, may also be used for celestial observations.<ref name="bow269"/> In this case, the watch is started at a known GMT by chronometer, and the elapsed time of each sight added to this to obtain GMT of the sight.<ref name="bow269"/> All chronometers and watches should be checked regularly with a radio time signal.<ref name="bow269"/> Times and frequencies of radio time signals are listed in publications such as [[Radio Navigational Aids]].<ref name="bow269"/>

====The marine sextant====
[[File:Marine sextant.svg|thumb|upright=1.2|The marine [[sextant]] is used to measure the elevation of celestial bodies above the horizon.]]
{{Further|Sextant}}
The second critical component of celestial navigation is to measure the angle formed at the observer's eye between the celestial body and the sensible horizon. The sextant, an optical instrument, is used to perform this function. The sextant consists of two primary assemblies. The frame is a rigid triangular structure with a pivot at the top and a graduated segment of a circle, referred to as the "arc", at the bottom. The second component is the index arm, which is attached to the pivot at the top of the frame. At the bottom is an endless vernier which clamps into teeth on the bottom of the "arc". The optical system consists of two mirrors and, generally, a low power telescope. One mirror, referred to as the "index mirror" is fixed to the top of the index arm, over the pivot. As the index arm is moved, this mirror rotates, and the graduated scale on the arc indicates the measured angle ("altitude").<ref name="q939">{{cite book | last=House | first=D.J. | title=Seamanship Techniques | publisher=Routledge | publication-place=London | date=2013-11-12 | isbn=978-1-135-08015-0 | page=350-252}}</ref> The second mirror, referred to as the "horizon glass", is fixed to the front of the frame. One half of the horizon glass is silvered and the other half is clear. Light from the celestial body strikes the index mirror and is reflected to the silvered portion of the horizon glass, then back to the observer's eye through the telescope. The observer manipulates the index arm so the reflected image of the body in the horizon glass is just resting on the visual horizon, seen through the clear side of the horizon glass.<ref name="q939"/>

There are three main errors that must be corrected in order to each usage for navigation.<ref name="q939"/> The main errors are perpendicular error, side error and index error.<ref name="q939"/> Adjustment of the sextant consists of checking and aligning all the optical elements to eliminate the overall "index error" (or index correction). Index correction should be checked, using the horizon or more preferably a star, each time the sextant is used.<ref name="q939"/> The practice of taking celestial observations from the deck of a rolling ship, often through cloud cover and with a hazy horizon, is by far the most challenging part of celestial navigation.<ref>{{Cite web |title=How Did Aviators "Shoot" the Sun and Stars? {{!}} Time and Navigation |url=http://timeandnavigation.si.edu/navigating-air/challenges/overcoming-challenges/celestial-navigation |access-date=2023-06-12 |website=timeandnavigation.si.edu |language=en}}</ref>

====Bubble octant====
Until the widespread usage of technologies such as inertial navigation systems, [[VHF omnidirectional range]] and GNSS, air navigators used the [[Bubble octant]] or bubble sextant.<ref name="t804">{{cite book | author=United States. Navy Department | title=Air Navigation: Flying Training | publisher=Air Training Command in accordance with AFR 5-6 | series=Air Force AFM | year=1983 | url=https://books.google.com/books?id=Te-mFCmtDOwC | access-date=2025-02-25 | page=16-1}}</ref> Using this instrument to take sights, mathematical calculations could then be carried out to determine the past position of the aircraft.<ref name="m792">{{cite book | last=Wolper | first=James S. | title=Understanding Mathematics for Aircraft Navigation | publisher=McGraw Hill Professional | date=2001-06-13 | isbn=978-0-07-163879-1 | page=109-150}}</ref>