Nautical mile

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A nautical mile or sea mile is a unit of length. It is accepted for use with the International System of Units, but it is not an SI unit.[1] The nautical mile is used around the world for maritime and aviation purposes. It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the geographical mile.

Contents

[edit] Definition

The international standard definition is: 1 nautical mile = 1852 metres exactly.

[edit] 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 (not to be confused with nm, the official symbol for nanometre).

The International Bureau of Weights and Measures (BIPM) in its International System of Units brochure lists the nautical mile in the table of units "currently accepted" for use with SI using M as a symbol, mentioning in a footnote other symbols, and stating that as yet there is no internationally agreed symbol.

Although nm is the official symbol for nanometre, there is little confusion because it is used in very different contexts, and differs by twelve orders of magnitude (one nautical mile = 1.852 trillion nanometres). Listings of aircraft flight ranges typically include both the "nm" (nautical mile) and "km" (kilometre) equivalents next to each other.

There are several national unit symbols in use (for example, mpk (meripeninkulma, "sea league") in Finnish, sm (Seemeile, "sea mile") in German, and M (sjómíla, "sea mile") in Icelandic). The People's Republic of China uses n mile as the national standard symbol with no s added for plural.

[edit] Conversions to other units

One nautical mile converts to:

[edit] History

The nautical mile was historically defined as a minute of arc along a meridian of the Earth. It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart.

According to WGS84, 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, length of the minute of arc depends on the radius of curvature; distance to the angle (1 minute, in this case), not to the Earth's centre. This length equals to approx. 1861.57 metres at the poles and 1842.90 metres at the Equator.

Length of a minute of arc defined by the geocentric latitude depends on the distance from the Earth's center (and curvature). This length is greater at the Equator.

Other nations had different definitions of the nautical mile. 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. This value is very close (within 0.01 percent) to the average length of one minute of latitude (1852.235 m), i.e. the sea-level surface distance between the Earth's poles is marginally over 180×60=10800 nautical miles.

Since the 1929 agreement, all nations have now adopted the international definition.

In the United States, the nautical mile was defined in the nineteenth century as 6080.2 ft (1853.249 m). This was chosen by a measurement of the Earth made by an American. It adopted the international definition in 1954.

The precise definition of the foot varied slightly around the world until a standard definition of the international yard, always equal to exactly 3 feet, was agreed upon in 1959.

The British definition related to the length on the surface of the Earth just south of Great Britain. It was 6080 feet exactly (1853.184 metres). But it was not specified according to a calibrated measurement of the Earth, but chosen as exactly 800 feet longer than a statute mile. For disambiguation, this is sometimes called the "admiralty mile" after the British Admiralty. 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 US Navy in the 1950s would take 6000 feet as their equivalent of a nautical mile. 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 9 inches, defined as 6000/8192 feet.

[edit] 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 yacht has moved through the water, it can also be used to measure the speed 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 rope 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. [2][3]

Also in maritime navigation, nautical miles can be divided into 10 cables, although the present day definition of the cable uses a much more precise method.

[edit] See also

[edit] External links

  1. ^ Non-SI units accepted for use with the SI, and units based on fundamental constants - At one time, the nautical mile was discouraged for use by the BIPM, but it is now officially accepted for use (as of the 8th edition of the SI brochure), since its use is "expected to continue indefinitely", as well as having an exact SI definition.
  2. ^ Origin of Naval Terminology. Naval Historical Center, US Naval Dept. Library (May 03, 2006).
  3. ^ David, Fairhall (2005). "Pass your day skipper (2nd Edition)"
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