The Admiralty Nautical Mile
A distance defined by the size of the Earth rather than by a king’s thumb.
Origin
Fixed by the Admiralty at 6,080 feet, and used by the Royal Navy and merchant fleet until Britain adopted the international nautical mile in 1970.
The catch
Every other British unit was defined by a body part, an object or a quantity of water. The nautical mile alone was defined by the planet: one minute of latitude, so that a chart could be read as a ruler.
The Admiralty figure of 6,080 feet was a British approximation of one minute of latitude, which varies slightly with the Earth’s oblateness. In 1929 an international conference settled on 1,852 metres exactly, or 6,076.1 feet, and Britain adopted it in 1970. The difference of four feet in a mile mattered little in practice but a great deal in principle: navigation depends on everyone agreeing.