The nautical mile began as a sliver of the globe and ended as 1,852 metres
Sailors and pilots still measure distance in a unit that was meant to match one minute of latitude. Sixty of those minutes make a degree, so the planet's pole-to-pole girth comes out close to 21,600 of them. That tie to the map grid is exactly why the unit survived outside the metric system.
The word mile itself goes back to the Latin for a thousand paces. For a long time sea navigation was done largely by eye, until around 1500, when new instruments and a grid of latitude and longitude lines made angular reckoning practical. The first known statement that a degree holds 60 miles appears on a map by Nicolaus Germanus in a 1482 printing of Ptolemy's Geography. An earlier map of his had put the figure at 62 and a half, and why he changed it is unknown.
English navigators took up the 60-to-a-degree rule in the 16th century. William Bourne described it in 1574, and Robert Hues repeated it in 1594. Their mile, though, was the old English one of 5,000 feet, inherited from Ptolemy's undersized estimate of the Earth. A further wrinkle is that the planet bulges at the equator, so a minute of latitude runs to roughly 1,862 metres near the poles but only about 1,843 at the equator.
France settled the matter with arithmetic. The metre had been defined in 1791 as a ten-millionth of the distance from pole to equator, and dividing that quarter meridian into minutes gives about 1,851.85 metres, rounded to 1,852. The French Navy made it legal in 1906, and a hydrographic conference in Monaco fixed it internationally in 1929. Americans and Britons had meanwhile used slightly longer figures of 6,080.2 and 6,080 feet. The United States switched in 1954, Britain in 1970.
Several abbreviations compete: aviation uses NM, American engineers and government printers use nmi, and hydrographers use M. Speed follows naturally, since a knot is one nautical mile per hour. Someone has even worked out the Martian version, about 983 metres, which could help a future crew navigate by the stars.
Source: Nautical mile
