|Unit system||Non-SI unit|
|Symbol||M, NM, or nmi|
|1 M, NM, or nmi in ...||... is equal to ...|
A nautical mile is a unit of measurement used in both air and marine navigation, and for the definition of territorial waters. Historically, it was defined as one minute (1/) of a degree of latitude. Today it is defined as 1,852 metres (6,076.12 ft; 1.15078 mi). The derived unit of speed is the knot, one nautical mile per hour.
There is no single internationally agreed symbol.
While using M itself, the International Bureau of Weights and Measures recognises that NM, Nm and nmi are also in use.
The word mile is from the Latin word for a thousand paces: mille passus. Navigation at sea was done by eye 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 Willebrord 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/) of one degree of latitude. As one degree is 1/ of a circle, one minute of arc is 1/ of a circle (or, in radians, π/). 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 metres (6,110 ft) at the poles and 1,843 metres (6,050 ft) at the Equator, with a mean value of 1,852.3 metres (6,077 ft). France and other countries measured the minute of arc at 45 degrees latitude, making the nautical mile 1,852 metres (6,076 ft).
The Admiralty measured mile, or British nautical mile, 6,080 feet, was derived from the Admiralty knot, 6,080 imperial feet per hour. The U.S. nautical mile was 6,080.20 feet, based in the Mendenhall Order foot of 1893.
In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as 1,852 metres. The United States did not adopt the international nautical mile until 1954. Britain adopted it in 1970, and references to the obsolete unit are converted to 1,853 metres.
Despite the existence of precise modern definitions, in the early 21st century the old definitions are still in use. The Royal Yachting Association says in its manual for day skippers: "1 (minute) of Latitude = 1 sea mile", followed by "For most practical purposes distance is measured from the latitude scale, assuming that one minute of latitude equals one nautical mile".
The metre was originally defined as 1⁄10000000 of the meridian arc from the North pole to the equator passing through Dunkirk. The Earth's circumference is therefore approximately 40,000 km. The equatorial circumference is slightly longer than the polar circumference – the measurement based on this (40,075.017 km × 1/ × 1/ = 1,855.3 metres (6,087 ft)) is known as the geographical mile.