nautical mile
Lengthexact by definition
The nautical mile is exactly 1,852 metres by international agreement, adopted at the International Hydrographic Conference in Monaco in 1929. It is the unit of distance in marine and air navigation worldwide. The factor is exact by definition, though the value was chosen to approximate a quantity that varies slightly.
Watch out: It is 1,852 m, not 1,609 m. Confusing it with the statute mile understates distance by about 15%. The United States used a slightly different value until 1954 and the United Kingdom until 1970, so very old charts may not use exactly 1,852 m.
| 1 nmi | 1,852 m |
The nautical mile was defined as one minute of arc of latitude measured along a meridian. That is why it remains the navigator's unit: one nautical mile of northward travel changes latitude by almost exactly one minute, so distance can be read straight off the latitude scale at the side of a chart.
About the nautical mile
The nautical mile is the only common distance unit designed around the shape of the planet rather than around a human body or a farming practice. One minute of arc of latitude, one nautical mile: that identity means a navigator can measure a distance on a chart using the latitude scale printed on its edge, with no separate distance scale and no conversion. Sixty nautical miles is one degree of latitude, and 21,600 nautical miles is the polar circumference of the Earth, because a circle holds 21,600 arcminutes.
The reason it needed an international agreement at all is that the Earth is not a sphere. It is flattened at the poles, so a minute of latitude is about 1,843 m near the equator and about 1,862 m near the poles, a spread of roughly 1%. Different nations rounded differently, and by the early twentieth century the British, American and French nautical miles all differed. The 1929 conference settled on exactly 1,852 m, close to the average value, and the unit has been a fixed length ever since. Its companion, the knot, is one nautical mile per hour, which is why ship and aircraft speeds are still quoted in it.