A 'nautical mile' or 'sea mile' is a unit of
length. It is a non-
SI unit used by special interest groups such as
navigators in the
shipping and
aviation industries.
[Section 4.1 Table 8 in the ''International System of Units'' 8th ed. (2006) by the Bureau International des Poids et Mesures.] It is commonly used in
international law and
treaties, especially regarding the limits of
territorial waters. It developed from the
geographical mile. Since the nautical mile is roughly equal to one minute of angle at the equator, the length of the equator is roughly 21,600 nautical miles (40 000 km).
Definition
The
international standard definition is: 1 nautical mile = 1852
metres exactly.
[ This corresponds approximately to one minute of latitude along any meridian.]
Unit symbol
There is no official international standard symbol for the unit nautical mile. The symbols 'M', 'NM', 'nm', and 'nmi' are commonly used in some areas.
The abbreviation 'nm' can be confused with the official SI symbol for the nanometre. Similarly the SI symbol 'N m' is a Newton metre.
There are several national unit symbols in use (for example, 'mpk' (''meripeninkulma'', "sea league") in Finnish. In German, 'sm' is used for ''Seemeile'', "sea mile", though this abbreviation generates confusion in translations to English with the term for the other type of mile, the statute mile. In Icelandic, 'M' is used for ''sjómíla'', "sea mile". The People's Republic of China uses 'n mile' as the national standard symbol with no s added for plural.
Conversions to other units
One nautical mile converts to:
★ 1852 metres (exact)
★ 1.150779 mile (statute) [1] (exact: 57875/50292 miles)
★ 2025.372 yard (exact: 2315000/1143 yards)
★ 6076.1155 feet (exact: 2315000/381 feet)
★ 1012.6859 fathoms (exact: 1157500/1143 fathoms)
★ 10 common-definition cables (exact, as one common definition of "cable")
★ 10.126859 "ordinary" (100-fathom) cables (exact: 11575/1143 ordinary cables)
★ 12.152231 US Navy (120-fathom) cables (exact: 9260/762 US Navy cables)
★ 0.998383 equatorial arc minutes = traditional geographical miles (approx.)
★ 0.9998834 mean meridian arc minutes = mean historical nautical miles (approx.)
History
The nautical mile was historically defined as a minute of arc along a meridian of the Earth, making a meridian exactly 180×60 = 10 800 historical nautical miles. It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart. The originally intended definition of the metre as 10-7 of a half-meridian makes the mean historical nautical mile exactly (2)/10 800 = 1851.851851… historical metres. Based on the current IUGG meridian of 20 003 931.4585 (standard) metres the mean historical nautical mile is 1852.216 m.
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, whence the need for "mean" in the preceding sentence. According to WGS84, the radius of curvature in a meridian plane is 6 399 593.6258 metres at the poles and 6 335 439.3273 metres at the Equator. By the definition of geodetic latitude, the length of a minute of arc depends on the radius of curvature. This radius generally does not pass through Earth's center, except for the latitudes of 0° (equator) and 90° (poles). This length equals about 1861.57 metres at the poles and 1842.90 metres at the Equator, a variation of one percent.
The length of a minute of arc defined by ''geocentric latitude'' also depends on the radius of curvature along a meridian on the surface of the Earth, but a specific length occurs at a different latitude because a surface feature's geocentric latitude differs by as much as 12 arc-minutes (at 45°) from its geodetic latitude. The naive definition of an arc minute as the distance over which one's (three-dimensional) trajectory turns one minute when sailing due north corresponds to geodetic latitude; if instead one measures it by the turning of a line between the vessel and the center of the earth this corresponds to geocentric latitude.
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 1851.851 historical metres and 1852.216 standard metres.
Since the 1929 agreement, all nations have now adopted the international definition. The United States, formerly using a value of 1853.248 m (6080.2 ft), did not however adopt this definition until July 1, 1954.[1]
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 6080 feet. 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 1853.184 m. The Royal Hydrographic Office of the United Kingdom converted to the international definition in 1970.
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 6000 feet (1829 m) 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 6000 feet, and the more unusual "foot
★ ", equivalent to about nine inches, defined as 6000/8192 feet (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 or ultrasonics, or GPS.[2][3]
See also
★ conversion of units
★ orders of magnitude (length)
★ mile for other types of mile
Notes
1. Appendix 4 The international nautical mile Louis E. Barbrow and Lewis V. Judson
2.
3. David, Fairhall (2005). "Pass your day skipper (2nd Edition)"
References
★ Geodetic Reference System, , H., Moritz, Bulletin Geodesique, (IUGG/WGS-84 data)
★ Computational Spherical Astronomy, , Laurence G., Taff, John Wiley and Sons, , (IAU data)
External links
★ National Bureau of Standards: Refinement of values for the yard and the pound (1959)
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