Nautical mile

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Nautical mile
Unit of length
SymbolM, NM, [lower-alpha 1] or nmi
1 M, NM, [lower-alpha 2] or nmi in ...... is equal to ...
    metre    1,852 [1]
    foot    ≈6,076
    statute mile    ≈1.151
    cable    10

A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. [2] [3] Historically, it was defined as the meridian arc length corresponding to one minute (1/60 of a degree) of latitude at the equator. Today the international nautical mile is defined as 1,852 metres (about 6,076 ft; 1.151 mi). [4] The derived unit of speed is the knot, one nautical mile per hour.


Unit symbol

Historical definition - 1 nautical mile Nautic mile definition v2 English.svg
Historical definition – 1 nautical mile

There is no single internationally agreed symbol, with several symbols in use. [1]


Visual comparison of a kilometre, statute mile and nautical mile Nauticalmilecomparison.svg
Visual comparison of a kilometre, statute mile and nautical mile

The word mile is from the Latin phrase for a thousand paces: mille passus . Navigation at sea was done by eye [11] until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude.

By the late 16th century English scientists knew that the ratio of distances at sea to degrees was constant along any great circle (such as the equator, or any meridian), assuming that Earth was a sphere. Robert Hues wrote in 1594 that the distance along a great circle was 60 miles per degree, that is, one nautical mile per arcminute. [12] Edmund Gunter wrote in 1623 that the distance along a great circle was 20 leagues per degree. [12] So Hues explicitly used nautical miles while Gunter did not.

Since the Earth is not a perfect sphere but is an oblate spheroid with slightly flattened poles, a minute of latitude is not constant, but about 1,861 metres at the poles and 1,843 metres at the Equator. [13] France and other metric countries state that in principle a nautical mile is an arcminute of a meridian at a latitude of 45°, but that is a modern justification for a more mundane calculation that was developed a century earlier. By the mid-19th century, France had defined a nautical mile via the original 1791 definition of the metre, one ten-millionth of a quarter meridian. [14] [15] So 10,000,000 m/90 × 60 = 1,851.85 m ≈ 1,852 m became the metric length for a nautical mile. France made it legal for the French Navy in 1906, and many metric countries voted to sanction it for international use at the 1929 International Hydrographic Conference.[ citation needed ]

Both the United States and the United Kingdom used an average arcminute—specifically, a minute of arc of a great circle of a sphere having the same surface area as the Clarke 1866 ellipsoid. [16] The authalic (equal area) radius of the Clarke 1866 ellipsoid is 6,370,997.2 metres (20,902,222 ft). [17] The resulting arcminute is 1,853.2480 metres (6,080.210 ft). The United States chose five significant digits for its nautical mile, 6,080.2 feet, whereas the United Kingdom chose four significant digits for its Admiralty mile, 6,080 feet.

In 1929 the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as exactly 1,852 metres (which is 6,076.12 ft). [1] The United States did not adopt the international nautical mile until 1954. [18] Britain adopted it in 1970, [19] but legal references to the obsolete unit are now converted to 1,853 metres (which is 6,079.40 ft). [20]

Similar definitions

The metre was originally defined as 110,000,000 of the length of the meridian arc from the North pole to the equator, [lower-alpha 3] thus one kilometre of distance corresponds to one centigrad (also known as centesimal arc minute) of latitude. The Earth's circumference is therefore approximately 40,000 km. The equatorial circumference is slightly longer than the polar circumference  the measurement based on this (40,075.017 km/360 × 60 = 1,855.3 metres) is known as the geographical mile.

Using the definition 1/60 of a degree of latitude on Mars, a Martian nautical mile equals to 983 m (1,075 yd). This is potentially useful for celestial navigation on a human mission to the planet, both as a shorthand and a quick way to roughly determine the location. [22]

See also


  1. Alternative meanings of the abbreviation "nm" or "NM" are listed here.
  2. Alternative meanings of the abbreviation "nm" or "NM" are listed here.
  3. No meridian was specified in either 1791, 1793, 1795 or 1799. For example, the Law of 18 Germinal an III (April 7, 1795) states: "Meter, the measure of length equal to the ten-millionth part of a terrestrial meridian contained between the north pole and the equator." [21]

Related Research Articles

<span class="mw-page-title-main">Minute and second of arc</span> Units for measuring angles

A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, or complete rotation, one arcminute is 1/21600 of a turn. The nautical mile (nmi) was originally defined as the arc length of a minute of latitude on a spherical Earth, so the actual Earth circumference is very near 21600 nmi. A minute of arc is π/10800 of a radian.

The geographical mile is an international unit of length determined by 1 minute of arc along the Earth's equator. For the international ellipsoid 1924 this equalled 1855.4 metres. The American Practical Navigator 2017 defines the geographical mile as 6,087.08 feet (1,855.342 m). Greater precision depends more on the choice of the Earth's radius of the used ellipsoid than on more careful measurement, since the radius of the geoid varies more than 100 metres (328.084 ft) along the equator. In any ellipsoid, the length of a degree of longitude at the equator is exactly 60 geographical miles. The Earth's radius at the equator in the GRS80 ellipsoid is 6,378,137.0000 m, which makes the geographical mile 1,855.3248 m. The rounding of the Earth's radius to metres in GRS80 has an effect of 0.0001 m.

<span class="mw-page-title-main">Latitude</span> Geographic coordinate specifying north–south position

In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole, with 0° at the Equator. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude and longitude are used together as a coordinate pair to specify a location on the surface of the Earth.

<span class="mw-page-title-main">Longitude</span> Geographic coordinate that specifies the east-west position of a point on the Earths surface

Longitude is a geographic coordinate that specifies the east–west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek letter lambda (λ). Meridians are imaginary semicircular lines running from pole to pole that connect points with the same longitude. The prime meridian defines 0° longitude; by convention the International Reference Meridian for the Earth passes near the Royal Observatory in Greenwich, south-east London on the island of Great Britain. Positive longitudes are east of the prime meridian, and negative ones are west.

<span class="mw-page-title-main">Metre</span> SI unit of length

The metre is the base unit of length in the International System of Units (SI).

<span class="mw-page-title-main">Mile</span> Unit of length

The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a British imperial unit and United States customary unit of distance; both are based on the older English unit of length equal to 5,280 English feet, or 1,760 yards. The statute mile was standardised between the Commonwealth of Nations and the United States by an international agreement in 1959, when it was formally redefined with respect to SI units as exactly 1,609.344 metres.

<span class="mw-page-title-main">Geographic coordinate system</span> System to specify locations on Earth

The geographic coordinate system (GCS) is a spherical or geodetic coordinate system for measuring and communicating positions directly on the Earth as latitude and longitude. It is the simplest, oldest and most widely used of the various spatial reference systems that are in use, and forms the basis for most others. Although latitude and longitude form a coordinate tuple like a cartesian coordinate system, the geographic coordinate system is not cartesian because the measurements are angles and are not on a planar surface.

NM, nm, and variations may refer to:

<span class="mw-page-title-main">Celestial navigation</span> Navigation using astronomical objects to determine position

Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface of the Earth without relying solely on estimated positional calculations, commonly known as "dead reckoning." Celestial navigation is performed without using satellite navigation or other similar modern electronic or digital positioning means.

<span class="mw-page-title-main">Gram</span> Metric unit of mass

The gram is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.

<span class="mw-page-title-main">Nautical chart</span> Topographic map of a maritime area and adjacent coastal regions

A nautical chart or hydrographic chart is a graphic representation of a sea region or water body and adjacent coasts or banks. Depending on the scale of the chart, it may show depths of water (bathymetry) and heights of land (topography), natural features of the seabed, details of the coastline, navigational hazards, locations of natural and human-made aids to navigation, information on tides and currents, local details of the Earth's magnetic field, and human-made structures such as harbours, buildings, and bridges. Nautical charts are essential tools for marine navigation; many countries require vessels, especially commercial ships, to carry them. Nautical charting may take the form of charts printed on paper or computerized electronic navigational charts. Recent technologies have made available paper charts which are printed "on demand" with cartographic data that has been downloaded to the commercial printing company as recently as the night before printing. With each daily download, critical data such as Local Notices to Mariners are added to the on-demand chart files so that these charts are up to date at the time of printing.

<span class="mw-page-title-main">Knot (unit)</span> Unit of speed

The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h. The ISO standard symbol for the knot is kn. The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE), while kt is also common, especially in aviation, where it is the form recommended by the International Civil Aviation Organization (ICAO). The knot is a non-SI unit. The knot is used in meteorology, and in maritime and air navigation. A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.

Gabriel Mouton was a French abbot and scientist. He was a doctor of theology from Lyon, but was also interested in mathematics and astronomy. His 1670 book, the Observationes diametrorum solis et lunae apparentium, proposed a natural standard of length based on the circumference of the Earth, divided decimally. It was influential in the adoption of the metric system in 1799.

<span class="mw-page-title-main">Longitude by chronometer</span>

Longitude by chronometer is a method, in navigation, of determining longitude using a marine chronometer, which was developed by John Harrison during the first half of the eighteenth century. It is an astronomical method of calculating the longitude at which a position line, drawn from a sight by sextant of any celestial body, crosses the observer's assumed latitude. In order to calculate the position line, the time of the sight must be known so that the celestial position i.e. the Greenwich Hour Angle and Declination, of the observed celestial body is known. All that can be derived from a single sight is a single position line, which can be achieved at any time during daylight when both the sea horizon and the sun are visible. To achieve a fix, more than one celestial body and the sea horizon must be visible. This is usually only possible at dawn and dusk.

Decimal degrees (DD) is a notation for expressing latitude and longitude geographic coordinates as decimal fractions of a degree. DD are used in many geographic information systems (GIS), web mapping applications such as OpenStreetMap, and GPS devices. Decimal degrees are an alternative to using sexagesimal degrees. As with latitude and longitude, the values are bounded by ±90° and ±180° respectively.

The French Geodesic Mission to the Equator, also called the French Geodesic Mission to Peru and the Spanish-French Geodesic Mission, was an 18th-century expedition to what is now Ecuador carried out for the purpose of performing an arc measurement, measuring the length of a degree of latitude near the Equator, by which the Earth radius can be inferred. The mission was one of the first geodesic missions carried out under modern scientific principles, and the first major international scientific expedition.

<span class="mw-page-title-main">Equator</span> Imaginary line halfway between Earths North and South poles

The equator is a circle of latitude that divides a spheroid, such as Earth, into the northern and southern hemispheres. On Earth, it is an imaginary line located at 0 degrees latitude, about 40,075 km (24,901 mi) in circumference, halfway between the North and South poles. The term can also be used for any other celestial body that is roughly spherical.

The angstrom or ångström is a metric unit of length equal to 10−10 m; that is, one ten-billionth (US) of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874).

<span class="mw-page-title-main">History of the metre</span> Origins and values of the unit of length

The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus's publication of De revolutionibus orbium coelestium in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision then in use, they also preferred a decimal system to ease their calculations.

<span class="mw-page-title-main">Earth's circumference</span> Distance around the Earth

Earth's circumference is the distance around Earth. Measured around the equator, it is 40,075.017 km (24,901.461 mi). Measured around the poles, the circumference is 40,007.863 km (24,859.734 mi).


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