|Unit system||Non-SI unit|
|Symbol||M, NM, or nmi|
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 ( of a degree) of latitude along any line of longitude. Today the international nautical mile is defined as exactly 1852 metres. This converts to about 1.15 imperial/US miles. 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 of one degree of latitude. As one degree is of a circle, one minute of arc is of a circle (or, in radians, ). 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 1861 metres at the poles and 1843 metres at the Equator, with a mean value of 1852.3 metres. France and other countries measured the minute of arc at 45 degrees latitude, making the nautical mile 1852 metres.
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 exactly 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 1853 metres.
The metre was originally defined as 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 × × = 1855.3 metres) is known as the geographical mile.