Nautical mile

The international nautical mile is defined as exactly 1,852 metres. This definition was adopted in 1929 by the International Extraordinary Hydrographic Conference, Monaco. The USA adopted it in 1954.

Prior to the adoption of the international nautical mile, the nautical mile formerly used in the U.S. and the U.K was 6080 feet, or 1853.18 metres.

It was previously defined as 1/60 of a degree (a minute of arc) along a great circle of the Earth, which is closely related to the geographical mile. However, since the earth is not perfectly spherical, this distance varies depending on the great circle chosen; thus several different definitions of the nautical mile have existed in the past. For purposes of practical navigation, the former definition is still widely used: the length of a minute of arc of latitude is found on any nautical chart and can be taken to represent approximately one nautical mile on that particular chart.

One knot is a unit of speed defined as one nautical mile per hour.

See also: geographical mile, mile, conversion of units