A nautical mile or sea mile is a unit of length. It corresponds approximately to one minute of latitude along any meridian. It is a non-SI unit used especially by navigators in the shipping and aviation industries.^[1] It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the geographical mile.

Definition

The international standard definition is: 1 nautical mile = exactly.^[1]

Unit symbol

There is no widely accepted international standard symbol for the unit nautical mile. The preferred abbreviation of the IEEE is nmi,^[2] while M is used by the BIPM^[1] and the maritime authorities of the USA^[3] and Canada.^[4] For aviation use, the preferred abbreviation of the ICAO is NM.^[5] The abbreviation nm, though conflicting with the SI symbol for the nanometre, is also in widespread use.

Conversions to other units

One nautical mile converts to:

• (exact)
• 1.150779 mile (statute) http://www.boatsafe.com/tools/scale.htm (exact: )
• (exact:
• (exact: )
• (exact: )
• 10 common-definition cables (exact, as one common definition of "cable")
• 10.126859 "ordinary" (100-fathom) cables (exact: ordinary cables)
• 12.152231 US Navy (120-fathom) cables (exact: US Navy cables)
• 0.998383 equatorial arc minutes = traditional geographical miles (approx.)
• 0.9998834 mean meridian arc minutes = mean historical nautical miles (approx.)

History

Historical definition - 1 nautical mile

The historical definition differs from the length-based standard in that a minute of arc, and hence a nautical mile, is not a constant length at the surface of the Earth but gradually lengthens with increasing distance from the equator, as a corollary of the Earth's oblateness, hence the need for "mean" in the last sentence of the previous paragraph. This length equals about at the poles and at the Equator, a variation of one percent.^[7]

Other nations had different definitions of the nautical mile. This variety in combination with the complexity of angular measure described above along with the intrinsic uncertainty of geodetically derived units mitigated against the extant definitions in favor of a simple unit of pure length. International agreement was achieved in 1929 when the International Extraordinary Hydrographic Conference held in Monaco adopted a definition of one (1) international nautical mile as being equal to 1,852 metres exactly, in excellent agreement (for an integer) with both the above-mentioned values of historical metres and standard metres.

Since the 1929 agreement, all nations have now adopted the international definition. In the United States, which formerly used a value of , the new definition has been in use officially since July 1, 1954.^[6]

British nautical mile (admiralty)

The British definition of the nautical mile originally related to the length on the surface of the Earth just south of Great Britain. It was not specified according to a calibrated measurement of the Earth, but chosen as exactly 800 feet longer than a statute mile, namely . For disambiguation, this is sometimes called the "admiralty mile" after the British Admiralty. The precise definition of the foot varied slightly around the world until the international yard, always equal to exactly three feet, was standardized at exactly 0.9144 m in 1959, making the admiralty mile exactly . The Royal Hydrographic Office of the United Kingdom converted to the international definition in 1970.

US navy nautical mile (rocketry)

As a simpler approximation, designers of radar systems for ballistic and cruise missiles for use by the United States Navy in the 1950s would take as their equivalent of a nautical mile. In the past, some ship-borne computer systems developed for the Royal Navy also used the "data mile" of , and the more unusual "foot*", equivalent to about nine inches, defined as (223 mm).

Associated units

The derived unit of speed is the knot, defined as one nautical mile per hour. The term "log" is used to measure the distance a vessel has moved through the water, it can also be used to measure the speed through the water (see chip log) as the speed and distance are directly related.

The term knot and log originally are derived from the practice of using a "log" tied to a knotted rope as a method of gauging speed of a ship. The log would be thrown into the water and the rope trailed behind the ship. The number of knots that passed off the ship and into the water in a given time would determine the speed in "knots". The present day measurement of knots and log are determined using a mechanical tow, electronic tow, retractable hull-mounted unit, doppler, ultrasonics, or GPS.^[8][9]

See also

• conversion of units
• knot for the unit of speed
• mile for other types of mile
• orders of magnitude (length)

Notes

• (IUGG/WGS-84 data)
• (IAU data)

External links

• National Bureau of Standards: Refinement of values for the yard and the pound (1959)
• Measure Distances on a map in Nautical miles
• Nautical Unit Converter Nautical miles converted to and from miles and kilometres

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