Mile - Nautical Mile

The nautical mile was originally defined as one minute of arc along a meridian of the Earth. Navigators use dividers to step off the distance between two points on the navigational chart, then place the open dividers against the minutes-of-latitude scale at the edge of the chart, and read off the distance in nautical miles. The Earth is not perfectly spherical but an oblate spheroid, so the length of a minute of latitude increases by 1% from the equator to the poles. Using the WGS84 ellipsoid, the commonly accepted Earth model for many purposes today, one minute of latitude at the WGS84 equator is 6,046 feet and at the poles is 6,107.5 feet. The average is about 6,076 feet (about 1,852 metres or 1.15 statute miles).

In the United States the nautical mile was defined in the 19th century as 6,080.2 feet (1,853.249 m), whereas in the United Kingdom, the Admiralty nautical mile was defined as 6,080 feet (1,853.184 m) and was about one minute of latitude in the latitudes of the south of the UK. Other nations had different definitions of the nautical mile, but it is now internationally defined to be exactly 1,852 metres.