Nautical mile
Length · Also called: Admiralty mile
That is
1.852 km
1 nautical mile (international)
The other way round: 1 km is 0.54 nautical miles.
- Period: 1929 onwards, adopted by the UK in 1970
- Region: worldwide
- Goods: navigation and aviation
Values
| Value | That is | Period | Used for |
|---|---|---|---|
| international | 1.852 km | 1929 onwards, adopted by the UK in 1970 | navigation and aviation · worldwide |
| AdmiraltyExactly 6080 feet. | 1.853 km | 19th century to 1970 | navigation · the UK and the Empire |
What the unit was used for
The nautical mile is a navigational convenience rather than an arbitrary length: it is one minute of arc of latitude, so a navigator steps distance straight off the latitude scale at the side of a chart.
Ninety degrees of latitude separate the Equator from each pole and every degree holds 60 minutes, which puts the pole 5400 nautical miles from the Equator.
Because the Earth is not a true sphere, nations computed slightly different values, and the British Admiralty fixed its own at exactly 6080 feet.
The UK dropped that in 1970 for the international nautical mile of exactly 1852 m, agreed at the hydrographic conference in Monaco in 1929.
A knot is one nautical mile per hour, and the name is literal: a weighted chip was thrown from the stern, a knotted line ran out behind the ship, and a hand counted the knots passing through his fingers while a sandglass ran, typically 30 seconds aboard a ship such as Cutty Sark.
In the system
1 nautical mile is about one minute of arc of latitude; 1 knot = 1 nautical mile per hour