# Nautical mile

Nautical mile
Unit system SI derived unit
Unit of Length
Symbol M, NM, or nmi
Unit conversions
1 M, NM, or nmi in ...... is equal to ...
meter    1852[1]
foot    ≈6076.12
statute mile    ≈1.15078
cable    10
Graphic scale from a Mercator projection world map, showing the change with latitude
Visual comparison of a kilometer, statute mile, and nautical mile

A nautical mile is a unit of measurement defined as exactly 1852 meters (about 6,076.1 feet or 1.1508 statute miles). Historically, it was defined as one sixtieth of the distance between two parallels of latitude separated by one degree. Today it is an SI derived unit, being rounded to an even number of meters[2] and remains in use for both air and marine navigation[3] and for the definition of territorial waters.[4]

The derived unit of speed is the knot, defined as one nautical mile per hour. The geographical mile is the length of one minute of longitude along the Equator, about 1,855.325 m on the WGS 84 ellipsoid.

## Unit symbol

There is no internationally agreed symbol.[1]

## History

The word mile is from the Latin word for a thousand paces: mīlia. 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 Snell assessed the circumference 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 meters at the poles and 1,843 meters at the Equator,[1] with a mean value of 1,852.3 metres (6,077 ft). Other countries measure the minute of arc at 45 degrees latitude, giving the nautical mile a length of 6076 ft (approximately 1852 m).[10]

In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as 1,852 meters.[1]

Imperial units and United States customary units used a definition of the nautical mile based on the Clarke (1866) Spheroid. The United States nautical mile was defined as 6,080.20 feet (1,853.24 m) based in the Mendenhall Order foot of 1893. It was abandoned in favour of the international nautical mile in 1954.[11]

The Imperial nautical mile, often called an Admiralty mile, or more correctly, an Admiralty measured mile, was defined by its relation to the Admiralty knot, 6,080 imperial feet per hour, so 1 imperial nautical mile is about 1,853.181 meters.[12] It was abandoned in 1970 and, legally, references to the obsolete unit are now converted to 1,853 meters.[13]

## References

1. "The International System of Units (SI)" (PDF). Bureau International des Poids et Mesures. Retrieved 2016-06-09.
2. Administration, US Department of Commerce, National Oceanic and Atmospheric. "What is the difference between a nautical mile and a knot?". oceanservice.noaa.gov. Retrieved 2016-06-09.
3. "mile | unit of measurement". Encyclopedia Britannica. Retrieved 2016-06-10.
4. "UNITED NATIONS CONVENTION ON THE LAW OF THE SEA". www.un.org. Retrieved 2016-06-10.
5. Symbols, Abbreviations and Terms used on Charts (PDF) (6 ed.). International Hydrographic Organization. 2016. Retrieved 2016-06-09.
6. "WS SIGMET Quick Reference Guide" (PDF). ICAO. ICAO. Retrieved 2016-06-09.
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 2016-06-09.
10. "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. "REFINEMENT OF VALUES FOR THE YARD AND THE POUND" (PDF). NOAA. National Bureau of Standards. Retrieved 2016-06-10.
12. "imperial nautical mile". sizes.com. Retrieved 2016-06-10.
13. "The Units of Measurement Regulations 1995". www.legislation.gov.uk. Retrieved 2016-06-10.
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