Nautical mile

The international nautical mile is defined as exactly 1852 meters, adopted in 1929 to standardize national values that had differed slightly. It approximates one minute of arc of latitude along a meridian, which is why a latitude scale on a Mercator chart doubles as a distance scale local to that latitude. Speed at sea is measured in knots, one nautical mile per hour. The unit is not part of the SI but is retained for navigation and aviation because of its direct geometric link to angular position on the globe.