Nautical mile

There is no single internationally agreed symbol, with several symbols in use.

• NM is used by the International Civil Aviation Organization.{{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 }}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.
• nmi is used by the Institute of Electrical and Electronics Engineers{{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 }} and the United States Government Publishing Office.{{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 }}
• M is used as the abbreviation for the nautical mile by the International Hydrographic Organization.{{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 }}
• nm is a non-standard abbreviation used in many maritime applications and texts, including U.S. Government Coast Pilots and Sailing Directions.{{cite book | title = Dutton's Navigation and Piloting| publisher=Naval Institute Press|location=Annapolis, MD | year=1985|edition=14th|isbn=0-87021-157-9}} It conflicts with the SI symbol for nanometre.

File:Nauticalmilecomparison.svg and nautical mile]]

The word mile is from the Latin phrase for a thousand paces: {{lang|la|mille passus}}. Navigation at sea was done by eye{{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 }} until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and 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 indicating one degree of longitude at the Equator contains "{{lang|la|milaria 60}}".{{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 }} 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 stadia, which make 62{{sfrac|1|2}} miles").{{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 }} 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."{{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 }}

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 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."{{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 }} Likewise, Robert Hues wrote in 1594 that the distance along a great circle was 60 miles per degree.{{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}} 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.{{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 }} 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).{{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}} In 1633, William Oughtred suggested 349,800 feet to a degree (5830 feet per arcminute).{{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 }} 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.{{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 }}

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.{{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 }} 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.{{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.{{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 }} 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 Clarke 1866 ellipsoid.{{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 }} The authalic (equal area) radius of the Clarke 1866 ellipsoid is {{convert|6370997.2|m|ft}}.{{citation |last=Snyder |first=John P. |title=Map Projections: A Working Manual |year=1987 |page=16|mode=cs1}} The resulting arcminute is {{convert|1,853.2480|m|ft}}. The United States chose five significant digits for its nautical mile, 6,080.2 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}}). The United States did not adopt the international nautical mile until 1954.{{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 }} Britain adopted it in 1970,{{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 }} but legal references to the obsolete unit are now converted to 1,853 metres (which is {{convert|1853|m|ft|2|disp=out}}).{{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 }}

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."{{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}}}} thus one kilometre of distance corresponds to one centigrad (also known as centesimal arc minute) of latitude. The Earth's circumference is therefore approximately 40,000 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 the planet, both as a shorthand and a quick way to roughly determine the location.{{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}}

{{Portal|Geography}}

• Nautical measured mile
• Conversion of units
• Orders of magnitude (length)

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Mile

Category:Units of length

Category:Customary units of measurement in the United States