Nautical mile
A nautical mile is a unit of length. It is not an SI unit. It is accepted for use with the SI. The nautical mile is required 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.
Definition
The international standard definition is: 1 nautical mile = 1852 metres (exactly).
Unit symbol
There is no official international standard symbol for the unit nautical mile. The symbols NM, nm and nmi are commonly used in some areas (not to be confused with nm, the official symbol for nanometer).
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 without using a symbol, saying in a footnote: "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 = 1852 billion nanometres). For example, listings of aircraft flight ranges will have nm next to km (kilometres).
There are several national unit symbols in use, for example mpk (meripeninkulma, "sea league") in Finnish and sm (Seemeile, "sea mile") in German.
Conversions to other units
1 nautical mile of 1852 metres converts to:
- 1.150 779 45 statute miles
- 6,076.115 49 feet
History
The nautical mile was historically defined as a minute of arc along a great circle of the Earth. It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart. However, like all planets, the Earth is not a perfect sphere. It bulges at the equator like a spinning top, so the length of one minute of arc on the Earth's surface varies from 1862 m at the poles to 1843 m at the Equator. The Earth's surface also has bumps and hollows like a potato. Thus, there is no fixed relationship between angle and arc length along the Earth's surface; one minute of arc can vary in length by tens of metres.
The British definition related to the length on the surface of the Earth just south of Great Britain. It was 6080 feet (1853.184 metres). The Royal Hydrographic Office of the United Kingdom converted to the international definition in 1970.
In the United States, the nautical mile was defined as 6080.2 ft (1853.249 m). It adopted the international definition in 1954.
The precise definition of feet varied around the world. That was resolved in 1959 when a single definition of the international yard was agreed upon.
Other nations had different definitions. International agreement was achieved in 1929, when the International Extraordinary Hydrographic Conference, Monaco adopted a definition of 1 international nautical mile = 1,852 metres, roughly the average length of one minute of latitude i.e. one minute of arc along a line of longitude (a meridian). In subsequent years, nations converted to use of the international definition.
Associated units
The derived unit of speed is the knot, defined as one nautical mile per hour.
The term "knot" derived from the practice of using 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 term stuck since "knot" is so close to "naut" in "nautical".
Again, for maritime navigation, nautical miles are sometimes divided into 10 cables, although other precise definitions of a cable have also been used.
See also
- conversion of units
- orders of magnitude (length)
- mile for other types of mile