The geographical mile is a unit of length determined by 1

minute of arc A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

along the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

. For the international ellipsoid 1924 this equalled 1855.4

metre The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pref ...

s. ''The American Practical Navigator'' 2017 defines the geographical mile as . Greater precision depends more on choice of ellipsoid than on more careful measurement: the length of the

in the

World Geodetic System The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also descri ...

WGS-84 is , which makes the geographical mile 1855.3248 m,. Equatorial radius = 6,378,137 m. while the '' IERS Conventions (2010)'' takes the equator to be , making the geographical mile 1855.3247 m,. Equatorial radius = 6,378,136.6 m. 0.1 millimetres shorter. In any ellipsoid, the

length of a degree of longitude Longitude (, ) is a geographic coordinate system, geographic coordinate that specifies the east–west position of a point on the surface of the Earth, or another Celestial navigation, celestial body. It is an angular measurement, usually exp ...

at the equator is thus exactly 60 geographical miles. The shape of the Earth is a slightly flattened sphere, which results in the

Earth's circumference Earth's circumference is the distance around Earth. Measured around the Equator, it is . Measured around the poles, the circumference is . Measurement of Earth's circumference has been important to navigation since ancient times. The first kno ...

being 0.168% larger when measured around the equator as compared to through the poles. The geographical mile is slightly larger than the

nautical mile A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude. Today ...

(which was historically linked to the circumference measured through both poles); one geographic mile is equivalent to approximately .

Related units

It was closely related to the

, which was originally determined as 1 minute of arc along a

great circle In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point. Any arc of a great circle is a geodesic of the sphere, so that great circles in spherical geomet ...

of the Earth, but is nowadays defined as exactly 1,852 metres. The US National Institute of Standards and Technology (NIST) notes that: "The international nautical mile of 1,852 meters (6,076.115 49...feet) was adopted effective July 1, 1954, for use in the United States. The value formerly used in the United States was 6,080.20 feet = 1 nautical (geographical or sea) mile." (The deprecated value, 6,080.2 feet, is ) A separate reference identifies the geographic mile as being identical to the international nautical mile of 1,852 metres (and slightly shorter than the British nautical mile of ). The unit is not used much, but is cited in some

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

laws (e.g., Section 1301(a) of the Submerged Lands Act, which defines state seaward boundaries in terms of geographic miles). While debating what became the Land Ordinance of 1785,

Thomas Jefferson Thomas Jefferson (April 13, 1743 – July 4, 1826) was an American statesman, diplomat, lawyer, architect, philosopher, and Founding Fathers of the United States, Founding Father who served as the third president of the United States from 18 ...

's committee wanted to divide the public lands in the west into " hundreds of ten geographical miles square, each mile containing 6,086 and 4-10ths of a foot" and "sub-divided into lots of one mile square each, or 850 and 4-10ths of an acre". The Danish and German geographical mile (''geografisk mil'' and ''geographische Meile'' or ''geographische Landmeile'', respectively) is 4 minutes of arc, and was defined as approximately 7,421.5 metres by the astronomer

Ole Rømer Ole Christensen Rømer (; 25 September 1644 – 19 September 1710) was a Danish astronomer who, in 1676, made the first measurement of the speed of light. Rømer also invented the modern thermometer showing the temperature between two fix ...

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Danish Realm, Kingdom of Denmark ...

. In Norway and Sweden, this 4-minute geographical mile was mainly used at sea (''sjømil''), up to the beginning of the 20th century.

References

{{DEFAULTSORT:Geographical Mile Units of length

Related units

See also

References