Nautical mile corresponds approximately to one minute of arc of latitude along any meridian. The international nautical mile was defined by the First International Extraordinary Hydrographic Conference, Monaco (1929) as exactly 1852 metres. Soviet Union accepted this definition in 1931, United States in 1959, and United Kingdom in 1970.
International nautical mile consists of 10 cables (1 cable=185.2 meters ).
The sea mile is the distance of one minute of arc of latitude at a given latitude and along the current meridian. Because the Earth being oblate (lense like) spheroid, the length of a nautical mile is longest at the poles, and shortest at equator, see article Ellipsoid for more details on oblate ellipsoids.
At first glance it may seem contrary - one minute arc length must be longer at equator, because the equatorial Earth radius is longer than polar one. It's true for geocentric latitude (), but the geographic latitude () is not geocentric. It's the angle between the equatorial plane and a line that is normal to the reference ellipsoid. See the picture.
The following calculator approximates the sea mile length at a given geographic latitude:
The knot is a unit of speed, widely used in navigation. A vessel travelling at 1 knot along a meridian travels one minute of geographic latitude in one hour (or one sea mile per hour). The name of the unit knot, for nautical mile per hour, was derived from the method of speed measurement using chip log. A chip log consists of a wooden board attached to a line. The log-line has a number of knots tied in it at uniform spacings (47 feet and 3 inches or 14.4 meters). When the navigator wished to determine the speed of his vessel, a sailor dropped the chip log over the stern of the ship. The log-line was allowed to run out for a fixed period of time. The speed of the ship was indicated by the number of knots on the line, passed through in 28 seconds.
Currently international knot is defined as 1.852 km/h (one nautical mile per hour).