Nautical mile

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Nautical mile
Unit systemNon-SI unit
Unit ofLength
SymbolM, NM, or nmi 
Conversions
1 M, NM, or nmi in ...... is equal to ...
   metre   1852[1]
   foot   ≈6076.12
   statute mile   ≈1.15078
   cable   10
Historical definition – 1 nautical mile
Visual comparison of a kilometre, statute mile, and nautical mile

A nautical mile is a unit of measurement used in both air and marine navigation,[2] and for the definition of territorial waters.[3] Historically, it was defined as one minute (1/60) of a degree of latitude. Today it is defined as 1,852 metres (6,076.12 ft; 1.15078 mi). The derived unit of speed is the knot, one nautical mile per hour.

Unit symbol[edit]

There is no single internationally agreed symbol.[1]

While using M itself, the International Bureau of Weights and Measures recognises that NM, Nm and nmi are also in use.[1]

History[edit]

The word mile is from the Latin word for a thousand paces: mille passus. Navigation at sea was done by eye[10] until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude.

In 1617 the Dutch scientist Willebrord Snell assessed the circumference of the Earth at 24,630 Roman miles (24,024 statute miles). Around that time British mathematician Edmund Gunter improved navigational tools including a new quadrant to determine latitude at sea. He reasoned that the lines of latitude could be used as the basis for a unit of measurement for distance and proposed the nautical mile as one minute or one-sixtieth (1/60) of one degree of latitude. As one degree is 1/360 of a circle, one minute of arc is 1/21600 of a circle (or, in radians, π/10800). These sexagesimal (base 60) units originated in Babylonian astronomy. Gunter used Snell's circumference to define a nautical mile as 6,080 feet, the length of one minute of arc at 48 degrees latitude. 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 1,861 metres (6,110 ft) at the poles and 1,843 metres (6,050 ft) at the Equator,[1] with a mean value of 1,852.3 metres (6,077 ft). France[11] and other countries measured the minute of arc at 45 degrees latitude, making the nautical mile 1,852 metres (6,076 ft).[10]

The Admiralty measured mile, or British nautical mile, 6,080 feet, was derived from the Admiralty knot, 6,080 imperial feet per hour. The U.S. nautical mile was 6,080.20 feet, based in the Mendenhall Order foot of 1893.

In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as 1,852 metres.[1] The United States did not adopt the international nautical mile until 1954.[12] Britain adopted it in 1970, and references to the obsolete unit are converted to 1,853 metres.[13]

Today[edit]

Despite the existence of precise modern definitions, in the early 21st century the old definitions are still in use. The Royal Yachting Association says in its manual for day skippers: "1 (minute) of Latitude = 1 sea mile", followed by "For most practical purposes distance is measured from the latitude scale, assuming that one minute of latitude equals one nautical mile".[14]

One tenth of a nautical mile is a cable length.[15]

Similar definitions[edit]

The metre was originally defined as ​110000000 of the meridian arc from the North pole to the equator passing through Dunkirk. 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 × 1/60 × 1/360 = 1,855.3 metres (6,087 ft)) is known as the geographical mile.

See also[edit]

References[edit]

  1. ^ a b c d e f Göbel, E.; Mills, I.M.; Wallard, Andrew, eds. (2006). The International System of Units (SI) (PDF) (8th ed.). Paris: Bureau International des Poids et Mesures. p. 127. ISBN 92-822-2213-6. Retrieved 2017-06-20.
  2. ^ "mile | unit of measurement". Encyclopædia Britannica. Retrieved 2016-06-10.
  3. ^ "UNITED NATIONS CONVENTION ON THE LAW OF THE SEA". www.un.org. Retrieved 2016-06-10.
  4. ^ Symboles, Abréviations et Termes utilisés sur les cartes marines [Symbols, Abbreviations and Terms used on Charts] (PDF) (in French and English). 1D (INT1) (6 ed.). Service Hydrographique et Océanographique de la Marine (SHOM). 2016. Retrieved 2018-01-04. also available as Symbols and Abbreviations used on ADMIRALTY Paper Charts. NP5011 (6th ed.). United Kingdom Hydrographic Office. 2016. section B, line 45. ISBN 978-0-70-774-1741.
  5. ^ "WS SIGMET Quick Reference Guide" (PDF). ICAO. ICAO. Retrieved 2016-06-09.
  6. ^ 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, 4th Edition, July 1979.
  7. ^ "Law of the Sea". NOAA. Retrieved 2016-06-09.
  8. ^ "APPENDIX A: SYMBOLS AND PREFIXES". IEEE. Retrieved 2016-06-09.
  9. ^ "U.S. Government Printing Office Style Manual". U.S. Government Printing Office. Retrieved 2016-06-10.
  10. ^ a b "Mile, Nautical and Statute – FREE Mile, Nautical and Statute information | Encyclopedia.com: Find Mile, Nautical and Statute research". www.encyclopedia.com. Retrieved 2016-06-10.
  11. ^ "MILLE MARIN : définition de MILLE MARIN et synonymes de MILLE MARIN (français)". dictionnaire.sensagent.leparisien.fr. Retrieved 2018-12-23.
  12. ^ Astin, A.V.; Karo, H. Arnold (June 25, 1959). "Refinement of values for the yard and the pound" (PDF). NOAA. National Bureau of Standards. Retrieved 2018-07-07.
  13. ^ "The Units of Measurement Regulations 1995". www.legislation.gov.uk. Retrieved 2016-06-10.
  14. ^ Hopkinson, Sara (2012). RYA day skipper handbook - sail. Hamble: The Royal Yachting Association. p. 76. ISBN 9781-9051-04949.
  15. ^ Fenna, Donald (2002), "cable, cable length, cable's length", A Dictionary of Weights, Measures, and Units, Oxford: Oxford University Press, pp. 35, ISBN 0-19-860522-6, OCLC 62608533, retrieved 12 January 2017. Also "fathom", from the same work (pp. 88–89, retrieved 12 January 2017).