Nautical mile

The nautical mile is a unit of length used in air, marine, and space navigation, as well as to define territorial waters. It was originally based on the length of one minute (1/60 of a degree) of latitude along a meridian at the equator, making Earth’s polar circumference nearly 21,600 nautical miles. Today, the international nautical mile is set at exactly 1,852 metres (about 6,076 feet or 1.151 miles). Speed measured in nautical miles per hour is called a knot. Although it is not part of the International System of Units (SI), the nautical mile remains widely used in air, marine, and space contexts because it aligns with geographic coordinates.

No single symbol is agreed upon internationally. The International Civil Aviation Organization uses NM; the Institute of Electrical and Electronics Engineers and the U.S. Government Publishing Office use nmi; the International Hydrographic Organization uses M; and nm appears in some maritime texts, though it conflicts with the SI symbol for nanometre.

The word “mile” comes from the Latin for a thousand paces (*mille passus*). Until around 1500, navigation at sea relied on sight, after which instruments and coordinate systems with latitude and longitude were developed. The earliest known reference to 60 miles per degree appears on a 1482 map by Nicolaus Germanus in an edition of Ptolemy’s *Geography*, stating one degree of longitude at the Equator contains 60 miles. An earlier manuscript by Germanus gave 62.5 miles per degree, but the reason for the change is unknown. The 60-mile ratio appeared in English in a 1555 translation of Pietro Martire d’Anghiera’s *Decades*. By the late 16th century, English geographers and navigators understood that the ratio of distance to degrees was constant along any great circle, assuming a spherical Earth. In 1574, William Bourne wrote that English navigators allowed 60 miles per degree, using the old English mile of 5,000 feet and league of 15,000 feet, based on Ptolemy’s underestimate of Earth’s circumference. In the early 1600s, discrepancies between angular and linear measurements were acknowledged. Edmund Gunter in 1624 proposed 352,000 feet per degree (5,866.67 feet per arcminute), and William Oughtred in 1633 suggested 349,800 feet per degree (5,830 feet per arcminute). Both proposed dividing a degree into 100 parts, but navigators largely ignored this. The ratio of 60 miles per degree remained, while the mile’s length was revised. In 1637, Robert Norwood proposed 6,120 feet per arcminute, within 44 feet of the modern value.

Because Earth is an oblate spheroid, a minute of latitude varies—about 1,862 metres at the poles and 1,843 metres at the Equator. Metric countries like France define a nautical mile as an arcminute of a meridian at 45° latitude, a justification for a calculation developed earlier. By the mid-19th century, France defined a nautical mile using the 1791 definition of the metre (one ten-millionth of a quarter meridian), yielding 1,851.85 metres, rounded to 1,852 metres. France made this legal for its navy in 1906, and many metric countries approved it internationally at the 1929 International Hydrographic Conference. The United States and United Kingdom previously used an average arcminute based on the Clarke 1866 ellipsoid’s authalic radius (6,370,997.2 metres), giving 1,853.2480 metres. The U.S. used 6,080.2 feet, and the U.K. used 6,080 feet for its Admiralty mile. In 1929, the international nautical mile was set at exactly 1,852 metres (6,076.12 feet). The U.S. adopted it in 1954, and Britain in 1970, though legal references to the old unit are now converted to 1,853 metres.

The metre was originally defined as one ten-millionth of the meridian arc from the North Pole to the Equator, so one kilometre corresponded to one centigrad of latitude.