A unit of distance equal to 1852 metres.
One nautical mile on the surface of the earth subtends approximately one minute of arc at the centre. The earth is not a true sphere so a minute of arc can represent distances that vary by tens of metres.
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- "The nautical mile is a special unit employed for marine and aerial navigation to express distance. The conventional value given above was adopted by the First International Extraordinary Hydrographic Conference, Monaco, 1929, under the name 'International nautical mile'. As yet there is no internationally agreed symbol. This unit was originally chosen because one nautical mile on the surface of the Earth subtends approximately one minute of angle at the centre."
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- -- http://www.bipm.fr/pdf/si-brochure.pdf
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- "On July 1, 1954, it was announced that the Secretary of Commerce and the Secretary of Defense had agreed officially that the International Nautical Mile would henceforth be used within their respective departments. The International Nautical Mile is based on the meter and is equal to 1852 meters."
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- -- http://geodesy.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf
For those like me who find it hard to remember an apparently random number, here's a derivation.
- A NauticalMile was intended to be 1 arc minute of a great circle
- A metre was defined to be one ten millionth of the distance from the North Pole to the Equator through Paris, which is 90 degrees of longitude, or 60*90 arc minutes.
- Hence 90 degrees is 10 million metres
- Hence 5400 arc minutes is 10 million metres
- so 1 arc minutes = 1851.851851 ... metres.
The "officially defined" figure of 1852 is pretty close.
Using the yard-meter conversion factor effective July 1, 1959, the international nautical mile is equivalent to 6076.11549 international feet. In British practice, the 'Admiralty measured mile' (exactly 6080 ft) is used, especially when defining a knot (one such mile per hour).
Nautical Mile