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Nautical mile
Nautical mile
Nautical mile
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Nautical mile
Historical definition – 1 nautical mile
General information
Unit oflength
SymbolM, NM,[a] or nmi
Conversions
1 M, NM,[a] or nmi in ...... is equal to ...
   metre   1,852[1]
   foot   ≈ 6,076.11549
   statute mile   ≈ 1.15078
   cable   10

A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters.[2][3][4] Historically, it was defined as the meridian arc length corresponding to one minute (⁠1/60⁠ of a degree) of latitude at the equator, so that 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 1,852 metres (about 6,076 ft; 1.151 mi).[5] The derived unit of speed is the knot, one nautical mile per hour.

The nautical mile is not part of the International System of Units (SI), nor is it accepted for use with SI. However, it is still in common use globally in air, marine, and space contexts due to its correspondence with geographic coordinates.

Unit symbol

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There is no single internationally agreed symbol, with several symbols in use.[1]

History

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Visual comparison of a kilometre, statute mile and nautical mile

The word mile is from the Latin phrase for a thousand paces: mille passus. Navigation at sea was done by eye[12] 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 "milaria 60".[13] An earlier manuscript map by Nicolaus Germanus in a previous edition of Geography states "unul gradul log. et latitud sub equinortiali formet stadia 500 que fanut miliaria 62⁠1/2⁠" ("one degree longitude and latitude under the equator forms 500 stadia, which make 62⁠1/2⁠ miles").[14] Whether a correction or convenience, the reason for the change from 62⁠1/2⁠ 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."[15]

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."[16] Likewise, Robert Hues wrote in 1594 that the distance along a great circle was 60 miles per degree.[17] 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.[18] 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 (5866⁠2/3⁠ feet per arcminute).[19][17] In 1633, William Oughtred suggested 349,800 feet to a degree (5830 feet per arcminute).[20] 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.[21]

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.[22] 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.[23][24] So ⁠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]

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.[25] The authalic (equal area) radius of the Clarke 1866 ellipsoid is 6,370,997.2 metres (20,902,222 ft).[26] The resulting arcminute is 1,853.2480 metres (6,080.210 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 6,076.12 ft).[1] The United States did not adopt the international nautical mile until 1954.[27] Britain adopted it in 1970,[28] but legal references to the obsolete unit are now converted to 1,853 metres (which is 6,079.40 ft).[29]

Similar definitions

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The metre was originally defined as 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),[b] 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 – the measurement based on this (⁠40,075.017 km/360 × 60⁠ = 1,855.3 metres) is known as the geographical mile.

Using the definition ⁠1/60⁠ of a degree of latitude on Mars, a Martian nautical mile equals to 983 m (1,075 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.[31]

See also

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Notes

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Nautical mile

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from Grokipedia
A nautical mile is a unit of length used in maritime and aviation navigation, defined as exactly 1,852 meters (approximately 6,076 feet or 1.1508 statute miles). It originates from the average length of one minute (1/60th of a degree) of latitude on Earth's surface, providing a convenient measure for calculating distances over the curved globe. This unit facilitates precise charting and speed measurements, with one knot equaling one nautical mile per hour. Historically, the nautical mile evolved from ancient attempts to divide Earth's circumference into navigable segments, with early references dating to the 15th century when explorers like Christopher Columbus estimated degrees of latitude in terms of miles. By the 19th century, variations existed across nations—such as the U.S. nautical mile of 1,853.248 meters based on the Clarke Spheroid of 1866—but these were approximations tied to one minute of arc. The concept bridged the gap between terrestrial miles and angular measurements, aiding celestial navigation before modern GPS. In 1929, the First International Extraordinary Hydrographic Conference in Monaco standardized the international nautical mile at 1,852 meters to promote global consistency in hydrography and navigation. The United States officially adopted this definition on July 1, 1954, replacing its prior measure, while the United Kingdom followed in 1970. Today, it remains essential for defining territorial waters (e.g., 12 nautical miles for the territorial sea), aviation routes, and international maritime boundaries, underscoring its role in safe and standardized global travel.

Definition and Standards

International Definition

The international nautical mile is defined as exactly 1,852 meters. This precise value was adopted by the International Hydrographic Bureau (now the International Hydrographic Organization) at the First International Extraordinary Hydrographic Conference in Monaco in 1929, establishing a uniform standard for maritime nations. The definition gained further international acceptance, with widespread ratification among member states by 1954, including the United States' official adoption on July 1 of that year. The unit's basis lies in the geometry of the Earth, specifically as the distance corresponding to one minute (1/60th of a degree) of latitude along a meridian, which approximates the planet's mean radius. This conceptual foundation derives from dividing the Earth's meridional circumference into 360 degrees, with each degree subdivided into 60 minutes of arc, resulting in 21,600 minutes around the globe and thus approximately one nautical mile per minute of latitude. Following the 1929 agreement, the nautical mile's length was fixed at 1,852 meters, rendering it independent of variations in local Earth measurements, such as curvature or spheroid models like the Clarke spheroid of 1866. This standardization eliminated discrepancies arising from differing national calculations of arc length, ensuring consistency in global navigation without reliance on ongoing geodetic refinements.

Unit Symbol and Equivalences

The nautical mile is denoted by symbols such as "M" used by the International Hydrographic Organization (IHO) and "NM" used by the International Civil Aviation Organization (ICAO); other common symbols include "nm" and "nmi". There is no single universally mandated symbol, but these are widely recognized and applied in nautical and aviation contexts to avoid confusion with other units like the meter (m). The international nautical mile is exactly equivalent to 1,852 meters, a standard adopted by the IHO in 1929. The equivalence of 1 NM = 1,852 m is per NIST SP 811. This length corresponds to approximately 6,076.11549 international feet or 1.15077945 statute miles. These conversions arise because the nautical mile is derived from angular measurements on Earth's surface (one minute of latitude), whereas the statute mile stems from historical land-based systems, resulting in non-integer ratios. To convert nautical miles to statute miles, multiply the value in nautical miles by 1.15077945; for example, 1 NM ≈ 1.15078 mi. This factor provides precise interoperability between maritime navigation and terrestrial distance measurements.

Historical Evolution

Ancient and Early Modern Origins

The division of the circle into 360 degrees originated with ancient Babylonian astronomers around 2000 BCE, who subdivided the ecliptic path of the Sun into 360 parts to facilitate sexagesimal calculations in their positional astronomy. This system, using base-60 divisions into degrees, minutes, and seconds, was adopted by Greek scholars, notably Hipparchus in the 2nd century BCE, who extended it to general angular measurements. In the 2nd century CE, Claudius Ptolemy applied this framework to Earth's surface in his Geography, introducing a coordinate system of latitude and longitude measured in degrees and minutes from the equator and a reference meridian, enabling the conceptual mapping of distances along great circles. During the medieval period, Arabic scholars preserved and refined Ptolemy's work through translations, such as al-Khwarizmi's 9th-century version of the Geography, which integrated astronomical observations and influenced Islamic cartography and navigation across the Indian Ocean and Mediterranean. This knowledge reached Europe via translations in the 12th century, notably through the Toledo School, where it informed Renaissance mapmaking. Early sea charts, including 13th-century portolan charts produced in Italian workshops, incorporated minutes of latitude as a practical unit for estimating distances, with rhumb lines and coastal scales calibrated to approximate one minute equaling one nautical mile for dead reckoning in coastal waters. In the 17th and 18th centuries, improved instruments for celestial observation enhanced latitude determination at sea, building on earlier quadrant and astrolabe methods. John Harrison's marine chronometer, completed in 1761 after decades of development, revolutionized positional accuracy by solving the longitude problem, which complemented latitude measurements to yield reliable meridian arc data and spurred refinements in nautical distance units. These advances produced practical estimates of the nautical mile at around 6,080 feet, derived from empirical surveys of one minute of latitude in mid-latitudes, as used in British Admiralty tables for voyage planning. A pivotal development occurred in 1791 when the French Academy of Sciences, tasked with reforming measurement units amid the Revolution, proposed a provisional nautical mile tied to the new meter—defined as one ten-millionth of the Earth's meridian quadrant—aligning it conceptually with one minute of arc while advancing decimal standardization.

19th and 20th Century Standardization

In the 19th century, national efforts to standardize the nautical mile focused on linking it to precise geodetic measurements of the Earth's surface. Britain established a nautical mile of 6,080 feet, a fixed imperial unit for maritime navigation based on the standardized yard. The United States, meanwhile, developed its own definition in the late 19th century, setting the nautical mile at 6,080.20 feet (1,853.248 meters), derived from one minute of arc on the Clarke spheroid of 1866, a mathematical model of the Earth that improved accuracy for North American triangulation surveys. Advances in geodesy during this period, particularly Alexander Ross Clarke's 1866 spheroid with an equatorial radius of 20,925,832 U.S. feet, enabled more reliable calculations of the mean length of one minute of arc along a meridian, providing a conceptual foundation for national nautical mile definitions and highlighting the variability due to the Earth's oblate shape. This work influenced efforts to approximate a universal value. The transition to international standardization accelerated in the 20th century through diplomatic conferences. The First International Extraordinary Hydrographic Conference in Monaco in 1929 defined the international nautical mile as exactly 1,852 meters, a metric-aligned value approximating the mean minute of latitude and adopted by a large number of maritime nations under the International Hydrographic Bureau. This fixed length aimed to facilitate global navigation by eliminating variations from local geodetic models. By 1954, the international nautical mile gained further traction with formal adoption by additional nations, including the United States, where government agencies shifted from the domestic value of 6,080.20 feet (1,853.248 meters) to 1,852 meters to align with international practices and the metric system. This decision by the Departments of Commerce and Defense marked a key step in harmonizing the unit across 25 hydrographic organizations, enhancing interoperability in maritime and aviation applications while preserving the conceptual tie to one minute of arc.

Role in Maritime and Aviation Navigation

In maritime navigation, the nautical mile serves as the standard unit for measuring distances at sea, with ship speeds expressed in knots, defined as one nautical mile per hour. Nautical charts are scaled using this unit, such as general charts at ratios from 1:150,000 to 1:600,000, where one inch on the chart represents a corresponding distance in nautical miles, facilitating precise plotting of courses and positions. In aviation, the nautical mile is essential for flight planning, where great-circle distances between waypoints are calculated in nautical miles to determine fuel requirements and estimated times en route. Aeronautical charts, including sectional charts scaled at 1:500,000 (one inch equaling approximately 6.86 nautical miles), use this unit for route measurements. VOR radials are calibrated in nautical miles, with usable ranges varying by altitude—for instance, 40 to 45 nautical miles at 1,000 feet above ground level—while distance-measuring equipment (DME) directly provides slant-range distances in nautical miles, aiding in position fixes. The nautical mile's alignment with angular measurements enhances its utility in dead reckoning and rhumb line calculations, as one degree of latitude corresponds to 60 nautical miles, allowing navigators to equate linear distance directly with arc minutes on the Earth's surface. This equivalence simplifies traditional navigation by converting differences in latitude to distances without additional conversion factors; for example, a 30-minute difference in latitude yields a distance of 30 nautical miles along a meridian. On Mercator projections common in nautical charts, rhumb lines—constant bearing paths—appear as straight lines, further streamlining course plotting when using nautical miles.

Integration with Modern Systems

In modern navigation, the Global Positioning System (GPS) provides vessel and aircraft positions in latitude and longitude coordinates, which are routinely converted to nautical miles for calculating distances and speeds using great-circle distance methods such as the haversine formula scaled to the international nautical mile of 1852 meters. This integration allows real-time computation of routes and fuel requirements, with tools like NOAA's latitude/longitude distance calculator directly outputting results in nautical miles to support precise maritime and aviation planning. Electronic Chart Display and Information Systems (ECDIS), mandated by the International Maritime Organization (IMO) under SOLAS Chapter V, Regulation 19 for passenger ships of 500 gross tonnage and above, tankers of 3,000 gross tonnage and above, and cargo ships of 10,000 gross tonnage and above engaged on international voyages, with phased implementation for existing ships, use the nautical mile as the default unit for displaying vectors, ranges, and bearings in compliance with performance standards. These systems integrate electronic navigational charts (ENCs) based on the WGS84 datum, enabling automated route monitoring where distances are expressed in nautical miles to align with SOLAS regulations and reduce collision risks through vector-based alerts. In aviation, the nautical mile is seamlessly integrated into flight management systems (FMS) and inertial navigation systems (INS), which output positions, waypoints, and required navigation performance (RNP) specifications in nautical miles for lateral guidance. Modern INS units, often hybridized with GPS, achieve position accuracies better than 0.6 nautical miles per hour, supporting long-haul flights where FMS computes great-circle paths and time-to-top-of-descent in nautical miles to optimize fuel efficiency and adherence to air traffic routes. The nautical mile's alignment with the WGS84 ellipsoid model, the standard for GPS and ECDIS, ensures high precision in the 2020s, as one minute of latitude approximates exactly 1 nautical mile with variations under 10 meters across latitudes, facilitating error-free conversions in digital systems without significant adjustments for Earth's oblateness. This compatibility enhances overall navigational reliability, particularly when speeds are measured in knots derived from nautical miles per hour.

Variations Across Countries

United States and United Kingdom Versions

In the United Kingdom, the nautical mile was historically defined by the British Admiralty as 6,080 feet, equivalent to 1,853.184 meters, a standard established in the mid-19th century and known as the Admiralty mile. This definition persisted until 1970, when the UK Hydrographic Office adopted the international nautical mile of exactly 1,852 meters as part of broader metrication efforts, converting nautical charts and related measurements accordingly. In the United States, the nautical mile was defined prior to 1954 as 6,080.20 feet, or 1,853.248 meters, based on the 1893 Mendenhall Order establishing the US survey foot. On July 1, 1954, all US government agencies, including the Departments of Defense and Commerce, adopted the international nautical mile of exactly 1,852 meters, equivalent to 6,076.11549 international feet, aligning precisely with the global standard set in 1929. The UK Admiralty mile was approximately 0.064% longer than the international nautical mile, while the pre-1954 US definition was about 0.069% longer; the US version matched the international length exactly upon adoption in 1954. Both nations transitioned to the international standard—the UK in 1970 and the US in 1954—fully incorporating it into maritime and aviation practices by the 1970s and 1980s, though legacy references to the older definitions occasionally appear in historical charts and military documents.

Other Historical or Regional Definitions

Several obsolete definitions existed in other European countries, stemming from local adaptations of the minute-of-arc concept, often adjusted for national degrees or fathoms. These variations were phased out as part of international harmonization efforts in the 20th century.

References

  1. https://oceanservice.noaa.gov/facts/nautical-mile-knot.html
  2. https://ihr.iho.int/articles/the-nautical-mile/
  3. https://www.britannica.com/science/nautical-mile
  4. https://usma.org/laws-and-bills/adoption-of-international-nautical-mile
  5. https://nauticalcharts.noaa.gov/data/us-maritime-limits-and-boundaries.html
  6. https://physics.nist.gov/cuu/pdf/sp811.pdf
  7. https://physics.nist.gov/cuu/pdf/sp330.pdf
  8. https://digitalcommons.ursinus.edu/cgi/viewcontent.cgi?article=1006&context=triumphs_precalc
  9. https://sites.math.rutgers.edu/~cherlin/History/Papers2000/hunt.html
  10. https://www.astronomy.ohio-state.edu/pogge.1/Ast161/Au06/Unit2/mapsky.html
  11. https://encyclopedia.pub/entry/31294
  12. https://brill.com/display/book/9789004446038/BP000011.xml?language=en
  13. https://sailingissues.com/navcourse1.html
  14. https://press.uchicago.edu/books/hoc/HOC_V1/HOC_VOLUME1_chapter19.pdf
  15. https://www.usni.org/magazines/naval-history-magazine/2019/october/john-harrison-and-longitude-problem
  16. https://www.rmg.co.uk/stories/time/harrisons-clocks-longitude-problem
  17. https://www.rmg.co.uk/stories/maritime-history/library-archive/18th-century-sailing-times-between-english-channel-coast
  18. https://journals.lib.unb.ca/index.php/ihr/article/download/28097/1882520852/1882521100
  19. https://www.roma1.infn.it/~dagos/history/sm/node4.html
  20. https://www.govinfo.gov/content/pkg/GOVPUB-C13-e4ebfdc3446aa55ca5b94ddf1ce10956/pdf/GOVPUB-C13-e4ebfdc3446aa55ca5b94ddf1ce10956.pdf
  21. https://usma.org/wp-content/uploads/2015/06/Nautical-Mile.pdf
  22. https://nauticalcharts.noaa.gov/publications/coast-pilot/files/cp2/CPB2_C01_WEB.pdf
  23. https://www.faa.gov/sites/faa.gov/files/18_phak_ch16.pdf
  24. https://nauticalcharts.noaa.gov/publications/docs/shore-and-sea-boundaries/volume-2/cse-library-shalowitz-v2-p2-ch6.pdf
  25. https://www.movable-type.co.uk/scripts/latlong.html
  26. https://www.nhc.noaa.gov/gccalc.shtml
  27. https://iho.int/uploads/user/pubs/standards/s-52/S-52%2520Edition%25206.1.1%2520-%2520June%25202015.pdf
  28. https://www.imo.org/en/OurWork/Safety/Pages/Electronic-Chart-Display-and-Information-System-%28ECDIS%29.aspx
  29. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.232%2882%29.pdf
  30. http://hibdz.skydiving.co.uk/757/767_tech/flight_management_navigation/lnav.htm
  31. https://skybrary.aero/articles/inertial-reference-system-irs
  32. https://aviation.stackexchange.com/questions/2619/whats-the-precision-of-a-modern-inertial-measurement-unit-and-how-is-it-obtain
  33. https://www.fws.gov/r7/nwr/Realty/data/LandMappers/Public/Help/HTML/Degreesandgrounddistance.html
  34. https://ukma.org.uk/what-is-metric/uk-progress/uk-metric-timeline/
  35. https://geodesy.noaa.gov/library/pdfs/Special_Publication_No_238_Supp_1954.pdf
  36. https://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf
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