Editing Celestial navigation (section)

==Modern celestial navigation==
The celestial line of position concept was discovered in 1837 by [[Thomas Hubbard Sumner]] when, after one observation, he computed and plotted his longitude at more than one trial latitude in his vicinity and noticed that the positions lay along a line. Using this method with two bodies, navigators were finally able to cross two position lines and obtain their position, in effect determining both latitude and longitude. Later in the 19th century came the development of the modern (Marcq St. Hilaire) [[intercept method]]; with this method, the body height and azimuth are calculated for a convenient trial position and compared with the observed height. The difference in arcminutes is the nautical mile "intercept" distance that the position line needs to be shifted toward or away from the direction of the body's subpoint. (The intercept method uses the concept illustrated in the example in the "How it works" section above.) Two other methods of reducing sights are the [[longitude by chronometer]] and the [[ex-meridian]] method.

While celestial navigation is becoming increasingly redundant with the advent of inexpensive and highly accurate satellite navigation receivers ([[GNSS]]), it was used extensively in aviation until the 1960s and [[marine navigation]] until quite recently. However, since a prudent mariner never relies on any sole means of fixing their position, many national maritime authorities still require deck officers to show knowledge of celestial navigation in examinations, primarily as a backup for electronic or satellite navigation. One of the most common current uses of celestial navigation aboard large merchant vessels is for compass calibration and error checking at sea when no terrestrial references are available.

In 1980, French Navy regulations still required an independently operated timepiece on board so that, in combination with a sextant, a ship's position could be determined by celestial navigation.<ref>[https://clockdoc.org/gs/handler/getmedia.ashx?moid=57430&dt=3&g=1 The marine chronometer in the age of electricity by David Read, September 2015]</ref>

The [[U.S. Air Force]] and [[U.S. Navy]] continued instructing military aviators on celestial navigation use until 1997, because:
* celestial navigation can be used independently of ground aids.
* celestial navigation has global coverage.
* celestial navigation cannot be jammed (although it can be obscured by clouds).
* celestial navigation does not give off any signals that could be detected by an enemy.<ref>[[U.S. Air Force]] Pamphlet (AFPAM) 11-216, Chapters 8–13</ref>

The [[United States Naval Academy]] (USNA) announced that it was discontinuing its course on celestial navigation (considered to be one of its most demanding non-engineering courses) from the formal curriculum in the spring of 1998.<ref>[https://query.nytimes.com/gst/fullpage.html?res=9504EED61038F93AA15756C0A96E958260&n=Top%2fReference%2fTimes%20Topics%2fOrganizations%2fU%2fUnited%20States%20Naval%20Academy Navy Cadets Won't Discard Their Sextants] {{webarchive|url=https://web.archive.org/web/20090213125009/http://query.nytimes.com/gst/fullpage.html?res=9504EED61038F93AA15756C0A96E958260&n=Top%2FReference%2FTimes%20Topics%2FOrganizations%2FU%2FUnited%20States%20Naval%20Academy |date=2009-02-13 }}, [[The New York Times]] By DAVID W. CHEN
Published: May 29, 1998</ref> In October 2015, citing concerns about the reliability of GNSS systems in the face of potential hostile [[hacker (computer security)|hacking]], the USNA reinstated instruction in celestial navigation in the 2015 to 2016 academic year.<ref>[http://www.capitalgazette.com/news/naval_academy/ph-ac-cn-celestial-navigation-1014-20151009-story.html Seeing stars, again: Naval Academy reinstates celestial navigation] {{webarchive|url=https://web.archive.org/web/20151023034855/http://www.capitalgazette.com/news/naval_academy/ph-ac-cn-celestial-navigation-1014-20151009-story.html |date=2015-10-23 }}, [[Capital Gazette]] by Tim Prudente Published: 12 October 2015</ref><ref>{{cite news|first1=Andrea|last1=Peterson|title=Why Naval Academy students are learning to sail by the stars for the first time in a decade|url=https://www.washingtonpost.com/news/the-switch/wp/2016/02/17/why-naval-academy-students-are-learning-to-sail-by-the-stars-for-the-first-time-in-a-decade/|newspaper=[[The Washington Post]]|date=17 February 2016|url-status=live|archive-url=https://web.archive.org/web/20160222081514/https://www.washingtonpost.com/news/the-switch/wp/2016/02/17/why-naval-academy-students-are-learning-to-sail-by-the-stars-for-the-first-time-in-a-decade/|archive-date=22 February 2016}}</ref>

At another federal service academy, the US Merchant Marine Academy, there was no break in instruction in celestial navigation as it is required to pass the US Coast Guard License Exam to enter the [[United States Merchant Marine|Merchant Marine]]. It is also taught at [[Harvard University|Harvard]], most recently as Astronomy&nbsp;2.<ref>[http://scholar.harvard.edu/psadler/classes/astronomy-2-celestial-navigation – ''Astronomy 2 Celestial Navigation'' by Philip Sadler] {{webarchive|url=https://web.archive.org/web/20151122121834/http://scholar.harvard.edu/psadler/classes/astronomy-2-celestial-navigation |date=2015-11-22 }}</ref>

Celestial navigation continues to be used by private yachtsmen, and particularly by long-distance cruising yachts around the world. For small cruising boat crews, celestial navigation is generally considered an essential skill when venturing beyond visual range of land. Although [[satellite navigation]] technology is reliable, offshore yachtsmen use celestial navigation as either a primary navigational tool or as a backup.

Celestial navigation was used in commercial aviation up until the early part of the jet age; early [[Boeing 747]]s had a "sextant port" in the roof of the cockpit.<ref>{{cite news|last1=Clark|first1=Pilita|title=The future of flying|url=http://www.ft.com/intl/cms/s/2/dbd7f5d4-e3ad-11e4-9a82-00144feab7de.html#slide0|access-date=19 April 2015|work=[[Financial Times]]|date=17 April 2015|url-status=live|archive-url=https://web.archive.org/web/20150614034443/http://www.ft.com/intl/cms/s/2/dbd7f5d4-e3ad-11e4-9a82-00144feab7de.html#slide0|archive-date=14 June 2015}}</ref>  It was only phased out in the 1960s with the advent of [[inertial navigation]] and Doppler navigation systems, and today's satellite-based systems which can locate the aircraft's position accurate to a 3-meter sphere with several updates per second.

A variation on terrestrial celestial navigation was used to help orient the [[Apollo spacecraft]] en route to and from the Moon. To this day, space missions such as the [[Mars Exploration Rover#Cruise stage navigation components|Mars Exploration Rover]] use [[star tracker]]s to determine the [[Attitude control (spacecraft)|attitude]] of the spacecraft.

As early as the mid-1960s, advanced electronic and computer systems had evolved enabling navigators to obtain automated celestial sight fixes. These systems were used aboard both ships and US Air Force aircraft, and were highly accurate, able to lock onto up to 11&nbsp;stars (even in daytime) and resolve the craft's position to less than {{convert|300|ft|m}}. The [[SR-71]] high-speed [[reconnaissance aircraft]] was one example of an aircraft that used a combination of [[Astro-inertial navigation system|automated celestial and inertial navigation]]. These rare systems were expensive, however, and the few that remain in use today are regarded as backups to more reliable satellite positioning systems.

[[Intercontinental ballistic missile]]s use celestial navigation to check and correct their course (initially set using internal gyroscopes) while flying outside the Earth's [[atmosphere]]. The immunity to jamming signals is the main driver behind this seemingly archaic technique.

[[X-ray pulsar-based navigation|X-ray pulsar-based navigation and timing]] (XNAV) is an experimental navigation technique for space whereby the periodic [[X-ray]] signals emitted from [[X-ray pulsar|pulsars]] are used to determine the location of a vehicle, such as a spacecraft in deep space. A vehicle using XNAV would compare received X-ray signals with a database of known pulsar frequencies and locations. Similar to GNSS, this comparison would allow the vehicle to triangulate its position accurately (±5&nbsp;km). The advantage of using X-ray signals over [[radio waves]] is that [[X-ray telescopes]] can be made smaller and lighter.<ref>{{cite web |url=http://physicsworld.com/cws/article/news/2013/jun/04/pulsars-map-the-way-for-space-missions |title=Pulsars map the way for space missions |date=4 June 2014 |first=Tushna |last=Commissariat |publisher=[[Physics World]] |url-status=live |archive-url=https://web.archive.org/web/20171018145025/http://physicsworld.com/cws/article/news/2013/jun/04/pulsars-map-the-way-for-space-missions |archive-date=18 October 2017 }}</ref><ref>{{cite magazine |url=http://www.technologyreview.com/view/515321/an-interplanetary-gps-using-pulsar-signals |title=An Interplanetary GPS Using Pulsar Signals |date=23 May 2013 |magazine=[[MIT Technology Review]] |access-date=29 August 2017 |archive-date=29 November 2014 |archive-url=https://web.archive.org/web/20141129034100/http://www.technologyreview.com/view/515321/an-interplanetary-gps-using-pulsar-signals/ |url-status=dead }}</ref><ref>{{cite journal |last1=Becker |first1=Werner |last2=Bernhardt |first2=Mike G. |last3=Jessner |first3=Axel |arxiv=1305.4842 |title=Autonomous Spacecraft Navigation With Pulsars |journal=Acta Futura |date=2013-05-21 |volume=7 |issue=7 |pages=11–28 |doi=10.2420/AF07.2013.11|bibcode=2013AcFut...7...11B |s2cid=118570784 }}</ref> On 9&nbsp;November 2016 the [[Chinese Academy of Sciences]] launched an experimental pulsar navigation satellite called [[XPNAV 1]].<ref>{{cite web |url=http://space.skyrocket.de/doc_sdat/xpnav-1.htm |title=XPNAV 1 |work=Gunter's Space Page |first=Gunter |last=Krebs |access-date=2016-11-01 |url-status=live |archive-url=https://web.archive.org/web/20161101165228/http://space.skyrocket.de/doc_sdat/xpnav-1.htm |archive-date=2016-11-01 }}</ref><ref>{{cite web|url=http://spaceflight101.com/long-march-11-launches-xpnav-1-satellite/|title=Chinese Long March 11 launches first Pulsar Navigation Satellite into Orbit|date=10 November 2016|publisher=Spaceflight101.com|url-status=live|archive-url=https://web.archive.org/web/20170824052232/http://spaceflight101.com/long-march-11-launches-xpnav-1-satellite/|archive-date=24 August 2017}}</ref> SEXTANT (Station Explorer for X-ray Timing and Navigation Technology) is a [[NASA]]-funded project developed at the [[Goddard Space Flight Center]] that is testing XNAV on-orbit on board the [[International Space Station]] in connection with the [[Neutron Star Interior Composition Explorer|NICER]] project, launched on 3&nbsp;June 2017 on the [[SpaceX CRS-11]] ISS resupply mission.<ref name="nasa-nicer-manifest">{{cite web |url=https://heasarc.gsfc.nasa.gov/docs/nicer/news/nicer_whatsnew.html |title=NICER Manifested on SpaceX-11 ISS Resupply Flight |series=NICER News |publisher=NASA |date=December 1, 2015 |access-date=June 14, 2017 |quote=Previously scheduled for a December 2016 launch on SpaceX-12, NICER will now fly to the International Space Station with two other payloads on SpaceX Commercial Resupply Services (CRS)-11, in the Dragon vehicle's unpressurized Trunk. |url-status=live |archive-url=https://web.archive.org/web/20170324025933/https://heasarc.gsfc.nasa.gov/docs/nicer/news/nicer_whatsnew.html |archive-date=March 24, 2017 }}</ref>