Editing Nautical mile

{{Short description|Unit of distance (1,852 m)}}
{{About|distance measurement at sea|distance measurement on land|mile|the U.S. unit based on the Equator|geographical mile}}
{{Infobox unit
| name       = Nautical mile
| image      = Nautic mile definition v2 English.svg
| caption    = Historical definition – 1 nautical mile
| quantity   = [[length]]
| symbol     = M, NM,{{efn|Alternative meanings of the abbreviation "nm" or "NM" are listed [[Nm (disambiguation)#Units of measure|here]].}} or nmi
| dimension  = L
| namedafter = 
| units1     = [[metre]]
| inunits1   = 1,852<ref name="BIPM">{{cite book |editor1-last=Göbel |editor1-first=E. |editor2-last=Mills |editor2-first=I.M. |editor3-last=Wallard |editor3-first=Andrew |title=The International System of Units (SI) |edition=8th |year=2006 |language=en |isbn=92-822-2213-6 |url=http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |location=Paris |publisher=[[Bureau International des Poids et Mesures]] |access-date=2017-06-20 |page=127 |archive-date=2017-08-14 |archive-url=https://web.archive.org/web/20170814094625/http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |url-status=dead }}</ref>
| units2     = [[Foot (unit)|foot]]
| inunits2   = ≈&nbsp;6,076.11549
| units3     = [[statute mile]]
| inunits3   = ≈&nbsp;1.15078
| units4     = [[Cable length|cable]]
| inunits4   = 10
}}

A '''nautical mile''' is a [[unit of length]] used in air, marine, and space [[navigation]], and for the definition of [[territorial waters]].<ref>{{cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=first |publisher=Osprey |year=1973 |page=200}}</ref><ref>{{cite web |url=https://www.britannica.com/science/mile |title=mile {{!}} unit of measurement |website=Encyclopædia Britannica |access-date=2016-06-10 |archive-date=2022-10-25 |archive-url=https://web.archive.org/web/20221025042119/https://www.britannica.com/science/mile |url-status=live }}</ref><ref>{{cite web |url=https://www.un.org/depts/los/convention_agreements/texts/unclos/part2.htm |title=UNITED NATIONS CONVENTION ON THE LAW OF THE SEA |website=www.un.org |access-date=2016-06-10 |archive-date=2017-12-18 |archive-url=https://web.archive.org/web/20171218221300/http://www.un.org/Depts/los/convention_agreements/texts/unclos/part2.htm |url-status=live }}</ref> Historically, it was defined as the [[meridian arc]] length corresponding to one [[minute of arc|minute]] ({{sfrac|1|60}} of a degree) of [[latitude]] at the [[equator]], so that [[Earth's circumference|Earth's polar circumference]] is very near to 21,600 nautical miles (that is 60 minutes × 360 degrees). Today the international nautical mile is defined as exactly {{convert|1852|m|ft mi|disp=x| (about |)}}.<ref>{{cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=first |publisher=Osprey |date=1973 |page=200 }}</ref> The derived unit of speed is the [[knot (unit)|knot]], one nautical mile per hour.

== Unit symbol ==
There is no single internationally agreed symbol, with several symbols in use.<ref name="BIPM" />

* '''NM''' is used by the [[International Civil Aviation Organization]].<ref>{{cite web |title = WS SIGMET Quick Reference Guide |url = http://www.icao.int/APAC/Documents/edocs/WS-SIGMET.pdf |website = ICAO |access-date = 2016-06-09 |archive-date = 2020-11-21 |archive-url = https://web.archive.org/web/20201121232112/https://www.icao.int/APAC/Documents/edocs/WS-SIGMET.pdf |url-status = live }}</ref><ref>International Standards and Recommended Practices, Annex 5 to the Convention on International Civil Aviation, “Units of measurement to be Used in Air and Ground Operations”, ICAO, Fifth Edition, July 2010.</ref>
* '''nmi''' is used by the [[Institute of Electrical and Electronics Engineers]]<ref>{{cite web |title = APPENDIX A: SYMBOLS AND PREFIXES |url = http://www.ieee-pes.org/appendix-a-symbols-and-prefixes |publisher = IEEE |access-date = 2016-06-09 |archive-date = 2021-04-23 |archive-url = https://web.archive.org/web/20210423091326/https://www.ieee-pes.org/appendix-a-symbols-and-prefixes |url-status = live }}</ref> and the [[United States Government Publishing Office]].<ref>{{cite web |title = U.S. Government Printing Office Style Manual |url = https://www.gpo.gov/fdsys/pkg/GPO-STYLEMANUAL-2000/html/GPO-STYLEMANUAL-2000-13.htm |publisher = U.S. Government Printing Office |access-date = 2016-06-10 |archive-date = 2018-11-19 |archive-url = https://web.archive.org/web/20181119011624/https://www.gpo.gov/fdsys/pkg/GPO-STYLEMANUAL-2000/html/GPO-STYLEMANUAL-2000-13.htm |url-status = live }}</ref>
* '''M''' is used as the abbreviation for the nautical mile by the [[International Hydrographic Organization]].<ref>{{cite book |title=Symboles, Abréviations et Termes utilisés sur les cartes marines |trans-title=Symbols, Abbreviations and Terms used on Charts |year=2016 |publisher=Service Hydrographique et Océanographique de la Marine (SHOM) |language=fr, en |edition=6th |url=https://www.iho.int/iho_pubs/standard/S-4/INT1_FR_Ed6_2016.pdf |access-date=2018-01-04 |volume=1D (INT1) |archive-url=https://web.archive.org/web/20160821103500/https://www.iho.int/iho_pubs/standard/S-4/INT1_FR_Ed6_2016.pdf |archive-date=2016-08-21 |url-status=dead }} also available as {{cite book |title = Symbols and Abbreviations used on ADMIRALTY Paper Charts|publisher=[[United Kingdom Hydrographic Office]] |year=2016 |edition=6th |at=section B, line 45 |isbn=978-0-70-774-1741 |volume=NP5011 }}</ref>
* '''nm''' is a non-standard abbreviation used in many maritime applications and texts, including U.S. Government Coast Pilots and Sailing Directions.<ref>{{cite book | title = Dutton's Navigation and Piloting| publisher=[[Naval Institute Press]]|location=[[Annapolis, MD]] | year=1985|edition=14th|isbn=0-87021-157-9}}</ref> It conflicts with the SI symbol for [[nanometre]].

== History ==
[[File:Nauticalmilecomparison.svg|thumb|left|350px|Visual comparison of a kilometre, [[Mile#Statute|statute mile]] and nautical mile]]

The word ''mile'' is from the Latin phrase for a thousand paces: {{lang|la|[[Mile#Roman mile|mille passus]]}}. Navigation at sea was done by eye<ref name=":0">{{cite web |url=http://www.encyclopedia.com/doc/1G2-3407500195.html |title=Mile, Nautical and Statute – FREE Mile, Nautical and Statute information {{!}} Encyclopedia.com: Find Mile, Nautical and Statute research |website=www.encyclopedia.com |access-date=2016-06-10 |archive-date=2016-06-07 |archive-url=https://web.archive.org/web/20160607165353/http://www.encyclopedia.com/doc/1G2-3407500195.html |url-status=live }}</ref> until around 1500 when navigational instruments were developed and cartographers began using a [[coordinate system]] with [[Circle of latitude|parallels]] of [[latitude]] and [[Meridian (geography)|meridians]] of [[longitude]].

The earliest reference of 60 miles to a degree is a map by [[Nicolaus Germanus]] in a 1482 edition of [[Ptolemy]]'s [[Geography (Ptolemy)|''Geography'']] indicating one degree of longitude at the Equator contains "{{lang|la|milaria 60}}".<ref name="Germanus 1482">{{cite book |last1=Germanus |first1=Nicolaus |title=Ptolemy's Cosmographia |date=1482 |publisher=Lienhart Holle |location=Ulm |page=245 |url=https://www.loc.gov/resource/rbc0001.2014rosen0088a/?sp=245 |access-date=8 October 2023 |archive-date=23 October 2023 |archive-url=https://web.archive.org/web/20231023101740/https://www.loc.gov/resource/rbc0001.2014rosen0088a/?sp=245 |url-status=live }}</ref> An earlier manuscript map by Nicolaus Germanus in a previous edition of ''Geography'' states "{{lang|la|unul gradul log. et latitud sub equinortiali formet stadia 500 que fanut miliaria 62{{sfrac|1|2}}}}" ("one degree longitude and latitude under the equator forms 500 [[Stadion (unit)|stadia]], which make 62{{sfrac|1|2}} miles").<ref name="Germanus 1460">{{cite book |last1=Germanus |first1=Nicolaus |title="Eleventh map of Asia," Ptolemy's Geographia |date=c. 1460 |url=https://digitalcollections.nypl.org/items/510d47da-e69b-a3d9-e040-e00a18064a99 |access-date=8 October 2023 |archive-date=23 October 2023 |archive-url=https://web.archive.org/web/20231023100609/https://digitalcollections.nypl.org/items/510d47da-e69b-a3d9-e040-e00a18064a99 |url-status=live }}</ref> Whether a correction or convenience, the reason for the change from 62{{sfrac|1|2}} to 60 miles to a degree is not explained. Eventually, the ratio of 60 miles to a degree appeared in English in a 1555 translation of [[Pietro Martire d'Anghiera]]'s Decades: "[Ptolemy] assigned likewise to every degree three score miles."<ref>{{cite book |last1=Anghiera |first1=Pietro Martire d' |title=The Decades of the Newe Worlde Or West India |date=1555 |publisher=Guilielm. Powell |location=London |page=323 |url=https://books.google.com/books?id=wcqc87Pb600C |access-date=2023-10-08 |archive-date=2023-10-24 |archive-url=https://web.archive.org/web/20231024211454/https://www.google.com/books/edition/The_Decades_of_the_Newe_Worlde_Or_West_I/wcqc87Pb600C |url-status=live }}</ref>

By the late 16th century English geographers and navigators knew that the ratio of distances at sea to degrees was constant along any [[great circle]] (such as the [[equator]], or any meridian), assuming that Earth was a sphere. In 1574, [[William Bourne (mathematician)|William Bourne]] stated in ''A Regiment for the Sea'' the "rule to raise a degree" practised by navigators: "But as I take it, we in England should allowe 60 myles to one degrée: that is, after 3 miles to one of our Englishe leagues, wherefore 20 of oure English leagues shoulde answere to one degrée."<ref name="Bourne">{{cite book |last1=Bourne |first1=William |title=A regiment for the sea |date=1574 |publisher=Thomas Hacket |location=London |page=39 |url=https://quod.lib.umich.edu/e/eebo/A16510.0001.001?view=toc |access-date=6 October 2023 |archive-date=23 October 2023 |archive-url=https://web.archive.org/web/20231023100102/https://quod.lib.umich.edu/e/eebo/A16510.0001.001?view=toc |url-status=live }}</ref> Likewise, [[Robert Hues]] wrote in 1594 that the distance along a great circle was 60 miles per degree.<ref name=Waters>{{citation |last=Waters |first=David W. |title=The Art of Navigation in England in Elizabethan and Early Stuart Times |year=1958 |url=https://archive.org/details/in.ernet.dli.2015.51856/page/n493 |page=374|mode=cs1}}</ref> However, these referred to the old English mile of 5000 feet and league of 15,000 feet, relying upon Ptolemy's underestimate of the [[Earth's circumference]].<ref name="Robert Hues">{{cite book |last1=Hues |first1=Robert |title=A learned treatise of globes, both cœlestiall and terrestriall with their several uses |date=1659 |publisher=J.S. for Andrew Kemb |location=London |pages=157, 163 |url=https://quod.lib.umich.edu/e/eebo/A44885.0001.001/1:8.1 |access-date=6 October 2023 |archive-date=24 October 2023 |archive-url=https://web.archive.org/web/20231024211451/https://quod.lib.umich.edu/cgi/t/text/pageviewer-idx?cc=eebo;c=eebo;idno=a44885.0001.001;node=A44885.0001.001:8.1;seq=1;page=root;view=text |url-status=live }}</ref> In the early seventeenth century, English geographers started to acknowledge the discrepancy between the angular measurement of a degree of latitude and the linear measurement of miles. In 1624 [[Edmund Gunter]] suggested 352,000 feet to a degree (5866{{sfrac|2|3}} feet per [[arcminute]]).<ref name="Gunther">{{cite book |last1=Gunther |first1=Edmund |title=The Works of Edmund Gunther |date=1673 |publisher=A. C.[larke]. for Francis Eglesfield, at the Marigold in St. Paul's Church-yard |location=London |pages=280–281 |url=https://archive.org/details/worksofedmundgun00gunt/page/n317/mode/2up |access-date=6 October 2023}}</ref><ref name=Waters/> In 1633, [[William Oughtred]] suggested 349,800 feet to a degree (5830 feet per arcminute).<ref name="Oughtred">{{cite book |last1=Oughtred |first1=William |title=The Circles of Proportion and the Horizontall Instrument |date=1639 |publisher=The Circles of Proportion and the Horizontall Instrument. |location=London |page=27 |url=https://books.google.com/books?id=OGRnAAAAcAAJ |access-date=6 October 2023 |archive-date=24 October 2023 |archive-url=https://web.archive.org/web/20231024211455/https://www.google.com/books/edition/The_Circles_of_Proportion_and_the_Horizo/OGRnAAAAcAAJ |url-status=live }}</ref> Both Gunter and Oughtred put forward the notion of dividing a degree into 100 parts, but their proposal was generally ignored by navigators. The ratio of 60 miles, or 20 leagues, to  a degree of latitude remained fixed while the length of the mile was revised with better estimates of the earth's circumference. In 1637, Robert Norwood proposed a new measurement of 6120 feet for an arcminute of latitude, which was within 44 feet of the currently accepted value for a nautical mile.<ref name="Richard Norwood">{{cite book |last1=Norwood |first1=Richard |title=The Sea-man's Practice: Containing a Fundamental Problem in Navigation, Experimentally Verified |date=1699 |publisher=Richard Mount |location=London |page=43 |url=https://books.google.com/books?id=PjCnkysmO20C |access-date=6 October 2023 |archive-date=24 October 2023 |archive-url=https://web.archive.org/web/20231024211455/https://www.google.com/books/edition/The_Sea_man_s_Practice/PjCnkysmO20C |url-status=live }}</ref>

Since the Earth is not a perfect sphere but is an [[oblate spheroid]] with slightly flattened poles, a minute of latitude is not constant, but about 1,862 metres at the poles and 1,843 metres at the Equator.<ref name="rasc">{{cite web |last1=McNish |first1=Larry |title=RASC Calgary Centre - Latitude and Longitude |url=https://calgary.rasc.ca/latlong.htm |website=The Royal Astronomical Society of Canada |access-date=30 August 2019 |archive-date=30 August 2019 |archive-url=https://web.archive.org/web/20190830130927/https://calgary.rasc.ca/latlong.htm |url-status=live }}</ref> France and other metric countries state that in principle a nautical mile is an arcminute of a meridian at a latitude of 45°, but that is a modern justification for a more mundane calculation that was developed a century earlier. By the mid-19th century, France had defined a nautical mile via the original 1791 [[definition of the metre]], one ten-millionth of a [[quarter meridian]].<ref name=Annuaires1933>{{citation |author=Bureau des Longitudes |journal=Annuaire Pour l'An 1933 |title=Mesures employées sur les cartes marines |year=1933 |page=392 |url=https://gallica.bnf.fr/ark:/12148/bpt6k65733736/f426.image |quote=The nautical mile [''mille marin''] is in principle the length of the sexagesimal minute of a meridian at a latitude of 45°. ... If we assume that the metre is exactly the ten-millionth part of the terrestrial quarter meridian, it would be equal to 1,851.85 m. |mode=cs1 |access-date=2019-08-31 |archive-date=2019-08-31 |archive-url=https://web.archive.org/web/20190831035803/https://gallica.bnf.fr/ark:/12148/bpt6k65733736/f426.image |url-status=live }} – Translation by Wikipedia.</ref><ref name=Annuaries1848>{{citation |author=Bureau des Longitudes |journal=Annuaire Pour l'An 1848 |title=Mesures itinéraires |year=1848 |page=74 |url=https://gallica.bnf.fr/ark:/12148/bpt6k65412527/f80.image |mode=cs1 |access-date=2019-08-31 |archive-date=2019-08-31 |archive-url=https://web.archive.org/web/20190831042814/https://gallica.bnf.fr/ark:/12148/bpt6k65412527/f80.image |url-status=live }}</ref> So {{nowrap|{{sfrac|10,000,000 m|90 × 60}} {{=}} 1,851.85 m ≈ 1,852 m}} became the metric length for a nautical mile. France made it legal for the French Navy in 1906, and many metric countries voted to sanction it for international use at the 1929 International Hydrographic Conference.{{citation needed|date=June 2023}}

Both the United States and the United Kingdom used an average arcminute—specifically, a minute of arc of a great circle of a sphere having the same surface area as the [[Earth ellipsoid|Clarke 1866 ellipsoid]].<ref name=Galzebrook>{{citation |last=Blazebrook |first=Richard |title=A Dictionary of Physics |volume=1 |year=1922 |url=https://archive.org/details/dictionaryofappl025484mbp/page/n593 |page=587|publisher=Macmillan and Co, Limited }}</ref> The [[authalic radius|''authalic'' (equal area) radius]] of the Clarke 1866 ellipsoid is {{convert|6370997.2|m|ft}}.<ref name=Snyder>{{citation |last=Snyder |first=John P. |title=Map Projections: A Working Manual |year=1987 |page=16|mode=cs1}}</ref> The resulting arcminute is {{convert|1,853.2480|m|ft}}. The United States chose five significant digits for its nautical mile, 6,080.2 [[Foot (unit)|feet]], whereas the United Kingdom chose four significant digits for its Admiralty mile, 6,080 feet.

In 1929 the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in [[Monaco]] as exactly 1,852 metres (which is {{convert|1852|m|ft|2|disp=out}}).<ref name="BIPM" /> The United States did not adopt the international nautical mile until 1954.<ref>{{cite web |url=http://geodesy.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf |title=Refinement of values for the yard and the pound |last1=Astin |first1=A.V. |last2=Karo |first2=H. Arnold |date=June 25, 1959 |website=NOAA.gov |publisher=National Bureau of Standards |access-date=2018-07-07 |archive-url=https://web.archive.org/web/20130309210627/http://geodesy.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf |archive-date=March 9, 2013 |url-status=dead }}</ref> Britain adopted it in 1970,<ref>{{cite web |title=Nautical mile definition and meaning {{!}} Collins English Dictionary |url=https://www.collinsdictionary.com/dictionary/english/nautical-mile |website=Collins Dictionary |access-date=1 September 2019 |archive-date=1 September 2019 |archive-url=https://web.archive.org/web/20190901010636/https://www.collinsdictionary.com/dictionary/english/nautical-mile |url-status=live }}</ref> but legal references to the obsolete unit are now converted to 1,853 metres (which is {{convert|1853|m|ft|2|disp=out}}).<ref>{{cite web |url=http://www.legislation.gov.uk/uksi/1995/1804/schedule/made |title=The Units of Measurement Regulations 1995 |website=www.legislation.gov.uk |access-date=2016-06-10 |archive-date=2017-03-24 |archive-url=https://web.archive.org/web/20170324164025/http://www.legislation.gov.uk/uksi/1995/1804/schedule/made |url-status=live }}</ref>

== Similar definitions ==
The [[metre]] was originally defined as {{frac|1|10,000,000}} of the length of the meridian arc from the [[North pole]] to the equator (1% of a centesimal degree of latitude),{{Efn |No meridian was specified in either 1791, 1793, 1795 or 1799 . For example, the Law of 18 Germinal an III (April 7, 1795) states: "''Meter'', the measure of length equal to the ten-millionth part of a terrestrial meridian contained between the north pole and the equator."<ref name=evolution>{{citation |last1=Hallock |first1=William |last2=Wade |first2=Herbert T. |title=Outlines of the Evolution of Weights and Measures and the Metric System |year=1906 |page=54 |publisher=New York The Macmillan company |url=https://archive.org/details/outlinesofevolut00halluoft/page/54}}</ref>}} thus one kilometre of distance corresponds to one [[Centigrade (angle)|centigrad]] (also known as centesimal arc minute) of latitude. The Earth's circumference is therefore approximately 40,000&nbsp;km. The equatorial circumference is slightly longer than the polar circumference{{snd}} the measurement based on this ({{sfrac|40,075.017 km|360 × 60}} = 1,855.3 metres) is known as the [[geographical mile]].

Using the definition {{sfrac|1|60}} of a degree of latitude on [[Mars]], a Martian nautical mile equals to {{Cvt|983|m|yd}}. This is potentially useful for [[celestial navigation]] on a [[Human mission to Mars|human mission to the planet]], both as a shorthand and a quick way to roughly determine the location.<ref>{{Cite book |last=Zubrin |first=Robert |title=The Case for Mars: The Plan to Settle the Red Planet and Why We Must |title-link=The Case for Mars |publisher=Free Press |others=Richard Wagner |year=1996 |isbn=0-684-82757-3 |location=New York |page=162 |oclc=34906203 |author-link=Robert Zubrin}}</ref>

== See also ==
{{Portal|Geography}}
*[[Nautical measured mile]]
* [[Conversion of units]]
* [[Orders of magnitude (length)]]

== Notes ==
{{notelist}}

== References ==
{{reflist}}

[[Category:Units of measurement in navigation|Mile]]
[[Category:Units of length]]
[[Category:Customary units of measurement in the United States]]