Nautical mile

A nautical mile is a unit of length. It is a non-SI unit accepted for use with the SI. Non-SI units accepted for use with the SI 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.

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 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 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). 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.

Conversions to other units

1 nautical mile converts to:

1.852 km (exact)
1.150779 mile (statute)
2025.372 yard
6076.1155 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. The geodesic description WGS84 is considered to be the best mathematical model of this oblate spheroid. According to WGS84 the length of one minute of arc along a meridian on the Earth's surface varies from 1852.2 m near the poles to 1855.3 m near 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 several metres.

(One minute of latitude represents one minute of arc along a line of longitude, which is a meridian based on a 40007 km longitudinal circumference. Note that the equatorial circumference is slightly greater at 40074 km.)

Other nations had different definitions of the NM. 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. This value is very close to the average length of one minute of latitude.

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

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 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). The Royal Hydrographic Office of the United Kingdom converted to the international definition in 1970.

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".

Again, for maritime navigation, nautical miles are sometimes divided into 10 cables, although other precise definitions of a cable have also been used.

External links

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.

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