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Concept
Nautical mile
Summary
A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute (1/60 of a degree) of latitude at the equator. Today the international nautical mile is defined as . The derived unit of speed is the knot, one nautical mile per hour.
There is no single internationally agreed symbol, with several symbols in use.
NM is used by the International Civil Aviation Organization.
nmi is used by the Institute of Electrical and Electronics Engineers and the United States Government Publishing Office.
M is used as the abbreviation for the nautical mile by the International Hydrographic Organization.
nm is a non-standard abbreviation used in many maritime applications and texts, including U.S. Government Coast Pilots and Sailing Directions. It conflicts with the SI symbol for nanometre.
The word mile is from the Latin phrase for a thousand paces: mille passus. Navigation at sea was done by eye until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude.
By the late 16th century English scientists 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. Robert Hues wrote in 1594 that the distance along a great circle was 60 miles per degree, that is, one nautical mile per arcminute. Edmund Gunter wrote in 1623 that the distance along a great circle was 20 leagues per degree. So Hues explicitly used nautical miles while Gunter did not.
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 at the poles and 1,843 metres at the Equator. 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.
Official source
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