Nautical mile

Last updated
Nautical mile
Unit of length
SymbolM, NM [lower-alpha 1] , or nmi
Conversions
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. Today the international nautical mile is defined as exactly 1,852 metres (6,076 ft; 1.151 mi). The derived unit of speed is the knot, one nautical mile per hour.

Contents

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]

History

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 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.

By the late 16th century, Englishmen 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. [11] Edmund Gunter wrote in 1623 that the distance along a great circle was 20 leagues per degree. [11] Thus, 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. [12] 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. [13] [14] Thus 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.

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. [15] The authalic (equal area) radius of the Clarke 1866 ellipsoid is 6,370,997.2 metres (20,902,222 ft). [16] 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. [1] The United States did not adopt the international nautical mile until 1954. [17] Britain adopted it in 1970, [18] but legal references to the obsolete unit are now converted to 1,853 metres. [19]

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. [21]

See also

Notes

  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." [20]

Related Research Articles

Minute and second of arc 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, one minute of arc 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 a 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 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 40075014.1723 m making the geographical mile 1855.3247 m, 0.1 millimetres shorter. In any ellipsoid, the length of a degree of longitude at the equator is thus exactly 60 geographical miles.

Longitude 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 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, England on the island of Great Britain. Positive longitudes are east of the prime meridian, and negative ones are west.

Metre SI unit of length

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

Mile 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 British Commonwealth 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.

The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and based on the metre as the unit of length and either the kilogram as the unit of mass or the kilogram-force as the unit of force.</ref> and the world's most widely used system of measurement. Established and maintained by the General Conference on Weights and Measures (CGPM), it is the only system of measurement with an official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

Geographic coordinate system System to specify locations on Earth

The geographic coordinate system (GCS) is a spherical or ellipsoidal 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 of 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.

Celestial navigation 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 without having to rely solely on estimated positional calculations, commonly known as "dead reckoning", made in the absence of satellite navigation or other similar modern electronic or digital positioning means.

Gram Metric unit of mass

The gram is a unit of mass in the metric system.

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), 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.

Paris meridian Meridian line running through the Paris Observatory in Paris, France

The Paris meridian is a meridian line running through the Paris Observatory in Paris, France – now longitude 2°20′14.02500″ East. It was a long-standing rival to the Greenwich meridian as the prime meridian of the world. The "Paris meridian arc" or "French meridian arc" is the name of the meridian arc measured along the Paris meridian.

Longitude by chronometer

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.

Grave (unit) Predecessor to the kilogram

The grave, abbreviated gv, is the unit of mass used in the first metric system which was implemented in France in 1793. In 1795, the grave was renamed as the kilogram.

The French Geodesic Mission to the Equator 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.

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).

History of the metre Origins and previous definitions of the SI base unit for measuring 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.

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).

References

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  13. Bureau des Longitudes (1933). "Mesures employées sur les cartes marines". Annuaire Pour l'An 1933: 392. The nautical mile [mille marin] is in principle the length of the sexagesimal minute of a meridian at a latitude of 45°. ... If we assume that the metre is exactly the ten-millionth part of the terrestrial quarter meridian, it would be equal to 1,851.85 m. – Translation by Wikipedia.
  14. Bureau des Longitudes (1848). "Mesures itinéraires". Annuaire Pour l'An 1848: 74.
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  18. "Nautical mile definition and meaning | Collins English Dictionary". Collins Dictionary. Retrieved 1 September 2019.
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  20. Hallock, William; Wade, Herbert T. (1906), Outlines of the Evolution of Weights and Measures and the Metric System, New York The Macmillan company, p. 54
  21. Zubrin, Robert (1996). The Case for Mars: The Plan to Settle the Red Planet and Why We Must. Richard Wagner. New York: Free Press. p. 162. ISBN   0-684-82757-3. OCLC   34906203.{{cite book}}: CS1 maint: date and year (link)