|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 defined as exactly 1,852 metres (6,076.1 ft; 1.1508 mi). Historically, it was defined as one minute of latitude, which is equivalent to one sixtieth of a degree of latitude. Today, it is a non-SI unit which has a continued use in both air and marine navigation, and for the definition of territorial waters.
There is no 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). Latitude and Longitude are equivalent or "square" at the Equator. Other countries measure the minute of arc at 45 degrees latitude, giving the nautical mile a length of 6,076 feet (1,852 m).
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.
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 metres. It was abandoned in 1970 and, legally, references to the obsolete unit are now 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".