Nautical mile

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Nautical mile
Unit system Non-SI unit
Unit of Length
SymbolM, NM, or nmi
1 M, NM, or nmi in ...... is equal to ...
    metre    1852 [1]
    foot    ≈6076
    statute mile    ≈1.151
    cable    10
Historical definition – 1 nautical mile Nautic mile definition v2 English.svg
Historical definition – 1 nautical mile
Visual comparison of a kilometre, statute mile, and nautical mile Nauticalmilecomparison.svg
Visual comparison of a kilometre, statute mile, and nautical mile

A nautical mile is a unit of measurement used in both air and marine navigation, [2] and for the definition of territorial waters. [3] Historically, it was defined as one minute (1/60) of a degree of latitude. Today it is defined as exactly 1852 metres. The derived unit of speed is the knot, one nautical mile per hour.

Navigation The process of monitoring and controlling the movement of a craft or vehicle from one place to another

Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another. The field of navigation includes four general categories: land navigation, marine navigation, aeronautic navigation, and space navigation.

Territorial waters Coastal waters that are part of a nation-states sovereign territory

The term territorial waters is sometimes used informally to refer to any area of water over which a state has jurisdiction, including internal waters, the territorial sea, the contiguous zone, the exclusive economic zone and potentially the continental shelf. In a narrower sense, the term is used as a synonym for the territorial sea.

Latitude The angle between zenith at a point and the plane of the equator

In geography, latitude is a geographic coordinate that specifies the north–south position of a point on the Earth's surface. Latitude is an angle which ranges from 0° at the Equator to 90° at the poles. Lines of constant latitude, or parallels, run east–west as circles parallel to the equator. Latitude is used together with longitude to specify the precise location of features on the surface of the Earth. On its own, the term latitude should be taken to be the geodetic latitude as defined below. Briefly, geodetic latitude at a point is the angle formed by the vector perpendicular to the ellipsoidal surface from that point, and the equatorial plane. Also defined are six auxiliary latitudes which are used in special applications.


Unit symbol

There is no single internationally agreed symbol. [1]

International Hydrographic Organization Intergovernmental organization

The International Hydrographic Organization (IHO) is the inter-governmental organisation representing hydrography.

International Bureau of Weights and Measures an intergovernmental organization established by the Metre Convention, through which Member States act together on matters related to measurement science and measurement standards (BIPM)

The International Bureau of Weights and Measures is an intergovernmental organization that was established by the Metre Convention, through which member states act together on matters related to measurement science and measurement standards. The organisation is usually referred to by its French initialism, BIPM. The BIPM's headquarters is based at Sèvres, France. It has custody of the International Prototype of the Kilogram and houses the secretariat for this organization as well as hosting its formal meetings.

International Civil Aviation Organization specialized agency of the United Nations, coordinates the field of aviation policy

The International Civil Aviation Organization, is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth. Its headquarters is located in the Quartier International of Montreal, Quebec, Canada.

While using M itself, the International Bureau of Weights and Measures recognises that NM, Nm and nmi are also in use. [1]


The word mile is from the Latin word for a thousand paces: mille passus. Navigation at sea was done by eye [10] until around 1500 when navigational instruments were developed and cartographers began using a coordinate system with parallels of latitude and meridians of longitude.

Latin Indo-European language of the Italic family

Latin is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets, and ultimately from the Phoenician alphabet.

Coordinate system System for uniquely determining the position of a point in a space of dimension n, by an n-tuplet of scalars

In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the position of the points or other geometric elements on a manifold such as Euclidean space. The order of the coordinates is significant, and they are sometimes identified by their position in an ordered tuple and sometimes by a letter, as in "the x-coordinate". The coordinates are taken to be real numbers in elementary mathematics, but may be complex numbers or elements of a more abstract system such as a commutative ring. The use of a coordinate system allows problems in geometry to be translated into problems about numbers and vice versa; this is the basis of analytic geometry.

Circle of latitude Geographic notion

A circle of latitude on Earth is an abstract east–west circle connecting all locations around Earth at a given latitude.

In 1617 the Dutch scientist Willebrord Snell assessed the circumference of the Earth at 24,630 Roman miles (24,024 statute miles). Around that time British mathematician Edmund Gunter improved navigational tools including a new quadrant to determine latitude at sea. He reasoned that the lines of latitude could be used as the basis for a unit of measurement for distance and proposed the nautical mile as one minute or one-sixtieth (1/60) of one degree of latitude. As one degree is 1/360 of a circle, one minute of arc is 1/21600 of a circle (or, in radians, π/10800). These sexagesimal (base 60) units originated in Babylonian astronomy. Gunter used Snell's circumference to define a nautical mile as 6,080 feet, the length of one minute of arc at 48 degrees latitude. 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 1861 metres at the poles and 1843 metres at the Equator, [1] with a mean value of 1852.3 metres. France [11] and other countries measured the minute of arc at 45 degrees latitude, making the nautical mile 1852 metres. [10]

Willebrord Snellius Dutch astronomer and mathematician

Willebrord Snellius was a Dutch astronomer and mathematician, known in the English-speaking world as Snell. In the west, especially the English speaking countries, his name is attached to the law of refraction of light.

Edmund Gunter, was an English clergyman, mathematician, geometer and astronomer of Welsh descent. He is best remembered for his mathematical contributions which include the invention of the Gunter's chain, the Gunter's quadrant, and the Gunter's scale. In 1620, he invented the first successful analogue device which he developed to calculate logarithmic tangents.

Quadrant (instrument) navigation instrument

A quadrant is an instrument that is used to measure angles up to 90°. Different versions of this instrument could be used to calculate various readings, such as longitude, latitude, and time of day. It was originally proposed by Ptolemy as a better kind of astrolabe. Several different variations of the instrument were later produced by medieval Muslim astronomers.

The Admiralty measured mile, or British nautical mile, 6,080 feet, was derived from the Admiralty knot, 6,080 imperial feet per hour. The U.S. nautical mile was 6,080.20 feet, based in the Mendenhall Order foot of 1893.

In 1929, the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as exactly 1,852 metres. [1] The United States did not adopt the international nautical mile until 1954. [12] Britain adopted it in 1970, and references to the obsolete unit are converted to 1853 meters‬‬. [13]

Similar definitions

The metre was originally defined as 110000000 of the meridian arc from the North pole to the equator passing through Dunkirk. 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 × 1/60 × 1/360 = 1855.3 metres is known as the geographical mile.

See also

Related Research Articles

Minute and second of arc angle units

A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn – it is for this reason that the Earth's circumference is almost exactly 21,600 nautical miles. A minute of arc is π/10800 of a radian.

The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the 1924 International Spheroid this equalled 1855.4 metres. The American Practical Navigator 2017 defines the geographical mile as 6087.08 feet (1855.342 m). Greater precision depends more on choice of ellipsoid than on more careful measurement: the length of the equator in the World Geodetic System WGS-84 is 40075016.6856 m which makes the geographical mile 1855.3248 m, while the IERS Conventions (2010) takes the equator to be 40075020.4555 m making the geographical mile 1855.3250 m, 1.2 millimetres longer. In any ellipsoid, the length of a degree of longitude at the equator is thus exactly 60 geographical miles.

Imperial units system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced

The system of imperial units or the imperial system is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The Imperial units replaced the Winchester Standards, which were in effect from 1588 to 1825. The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, although some imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire. The imperial system developed from what were first known as English units, as did the related system of United States customary units.

Mile unit of length

The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, and standardised as exactly 1,609.344 metres by international agreement in 1959.

The minute is a unit of time or angle. As a unit of time, the minute is most of times equal to ​160 of an hour, or 60 seconds. In the UTC time standard, a minute on rare occasions has 61 seconds, a consequence of leap seconds. As a unit of angle, the minute of arc is equal to ​160 of a degree, or 60 seconds. Although not an SI unit for either time or angle, the minute is accepted for use with SI units for both. The SI symbols for minute or minutes are min for time measurement, and the prime symbol after a number, e.g. 5′, for angle measurement. The prime is also sometimes used informally to denote minutes of time.

History of geodesy

Geodesy (/dʒiːˈɒdɨsi/), also named geodetics, is the scientific discipline that deals with the measurement and representation of the Earth. The history of geodesy began in pre-scientific antiquity and blossomed during the Age of Enlightenment.

Nautical chart topographic map of a maritime area and adjacent coastal regions

A nautical chart is a graphic representation of a sea area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land, 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.

Knot (unit) 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); 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. Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.

Degree (angle) angle unit; π/180 radians

A degree, usually denoted by °, is a measurement of a plane angle, defined so that a full rotation is 360 degrees.

A chip log, also called common log, ship log, or just log, is a navigation tool mariners use to estimate the speed of a vessel through water. The word knot, to mean nautical mile per hour, derives from this measurement method.

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, and was decimally divided. It was influential in the adoption of the metric system in 1799.

The obsolete Finnish units of measurement consist mostly of a variety of units traditionally used in Finland that are similar to those that were traditionally used in other countries and are still used in the United Kingdom and the United States.

Emmerke is a part of the municipality of Giesen in the district of Hildesheim, Lower Saxony, in north-western Germany.

The Arab, Arabic, or Arabian mile was a historical Arabic unit of length. Its precise length is disputed, lying between 1.8 and 2.0 km. It was used by medieval Arab geographers and astronomers. The predecessor of the modern nautical mile, it extended the Roman mile to fit an astronomical approximation of 1 minute of an arc of latitude measured along a north-south meridian. The distance between two pillars whose latitudes differed by 1 degree in a north-south direction was measured using sighting pegs along a flat desert plane.

Equator Intersection of a spheres surface with the plane perpendicular to the spheres axis of rotation and midway between the poles

An equator of a rotating spheroid is its zeroth circle of latitude (parallel). It is the imaginary line on the spheroid's surface, equidistant from its poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid's surface with the plane perpendicular to its axis of rotation and midway between its geographical poles.

The Navy Ten Nautical Miler is a 10-nautical-mile (19 km) run held at Naval Support Activity Mid-South in Millington, Tennessee. It is unique because it is the only race measured in nautical miles as opposed to statute miles.

ISO 80000-3:2006 is an ISO standard entitled Quantities and units – Part 3: Space and time, superseding ISO 31-1 and ISO 31-2. It is a part of the group of standards called ISO/IEC 80000, which together form the International System of Quantities.

Earths circumference

Earth's circumference is the distance around the Earth, either around the equator or around the poles.


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