Nautical mile

Visual comparison of a kilometer, statute mile, and nautical mile

A nautical mile is a unit of measurement defined as exactly 1852 meters (about 6,076.1 feet or 1.1508 statute miles). Historically, it was defined as one minute of latitude, which is equivalent to one sixtieth of a degree of latitude. Today it is an SI derived unit, being rounded to an even number of meters^[2] and remains in use for both air and marine navigation^[3] and for the definition of territorial waters.^[4]

One tenth of a nautical mile is a cable length.^[5]

The derived unit of speed is the knot, defined as one nautical mile per hour. The geographical mile is the length of one minute of longitude along the Equator, about 1,855.325 m on the WGS 84 ellipsoid.

Unit symbol

There is no internationally agreed symbol.^[1]

• M is used as the abbreviation for the nautical mile by the International Hydrographic Organization^[6] and by the International Bureau of Weights and Measures.^[1]
• NM is used by the International Civil Aviation Organization.^[7]
• nm (the SI symbol for the nanometer) is used by the U.S. National Oceanic and Atmospheric Administration.^[8]
• nmi is used by the Institute of Electrical and Electronics Engineers^[9] and the United States Government Publishing Office.^[10]

As well as using M, the International Bureau of Weights and Measures recognises NM, Nm and nmi.^[1]

History

The word mile is from the Latin word for a thousand paces: mille passus. Navigation at sea was done by eye^[11] until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude. In 1617 the Dutch scientist Snell assessed the circumference of the Earth at 24,630 Roman miles (24,024 statute miles). Around that time British mathematician Edmund Gunter improved navigational tools including a new quadrant to determine latitude at sea. He reasoned that the lines of latitude could be used as the basis for a unit of measurement for distance and proposed the nautical mile as one minute or one-sixtieth (1/60) of one degree of latitude. As one degree is 1/360 of a circle, one minute of arc is 1/21600 of a circle (or, in radians, π/10800). These sexagesimal (base 60) units originated in Babylonian astronomy. Gunter used Snell's circumference to define a nautical mile as 6,080 feet, the length of one minute of arc at 48 degrees latitude. Since the earth is not a perfect sphere but is an oblate spheroid with slightly flattened poles, a minute of latitude is not constant, but about 1,861 meters at the poles and 1,843 meters at the Equator,^[1] with a mean value of 1,852.3 metres (6,077 ft). Other countries measure the minute of arc at 45 degrees latitude, giving the nautical mile a length of 6076 ft (approximately 1852 m).^[11]

In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as 1,852 meters.^[1]

Imperial units and United States customary units used a definition of the nautical mile based on the Clarke (1866) Spheroid. The United States nautical mile was defined as 6,080.20 feet (1,853.24 m) based in the Mendenhall Order foot of 1893. It was abandoned in favour of the international nautical mile in 1954.^[12]

The Imperial nautical mile, often called an Admiralty mile, or more correctly, an Admiralty measured mile, was defined by its relation to the Admiralty knot, 6,080 imperial feet per hour, so 1 imperial nautical mile is about 1,853.181 meters.^[13] It was abandoned in 1970 and, legally, references to the obsolete unit are now converted to 1,853 meters.^[14]

See also

• Conversion of units
• Orders of magnitude (length)

References