Geographical mile

Template:Short description Template:Infobox unit The geographical mile is an international unit of length determined by 1 minute of arc (Template:Sfrac degree) along the Earth's equator. For the international ellipsoid 1924 this equalled 1855.4 metres.<ref name=Admiralty>Template:Cite book</ref> The American Practical Navigator 2017 defines the geographical mile as Template:Convert.<ref name=Bowditch>Template:Citation</ref> 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 Template:Convert 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 Template:Val,<ref>This is used by the international (ITRS) and American WGS 84) coordinate reference system for the world.</ref> 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.

The shape of the Earth is a slightly flattened sphere, which results in the Earth's circumference 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 (which was historically linked to the circumference measured through both poles); one geographic mile is equivalent to approximately Template:Convert.

Historical units

Historically, certain nations used slightly different divisions to create their geographical miles.

The Portuguese system derived their miles ({{#invoke:Lang|lang}}) as one third of their league of three separate values. When each equatorial degree was divided into 18 leagues, the geographical mile was equal to Template:Sfrac degree or about Template:Convert; when divided into 20 leagues, the geographical mile was equal to Template:Sfrac degree, approximating the values provided above; and when divided into 25 leagues, the geographical mile was equal to Template:Sfrac degree or about Template:Convert.

The geographical miles of the traditional Dutch ({{#invoke:Lang|lang}}), German ({{#invoke:Lang|lang}} or {{#invoke:Lang|lang}}), and Danish systems ({{#invoke:Lang|lang}}) all approximated their much longer milesTemplate:Mdashequivalent to English leaguesTemplate:Mdashby using a larger division of the equatorial degree. Instead of using one minute of arc, they all used fourTemplate:MdashTemplate:Sfrac degreeTemplate:Mdashto produce a distance now notionally equal to Template:Convert but actually differing slightly depending on official measurements and computations. (For example, the Danish unit was computed as equivalent to about Template:Convert by the astronomer Ole Rømer.)<ref>Template:Cite journal</ref>

Relationship with the nautical mile

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The geographical mile is closely related to the nautical mile, which was originally determined as 1 minute of arc along a great circle of the Earth<ref>Template:Cite book</ref> but is nowadays defined by treaty as exactly 1,852 m.<ref name=Admiralty /> The US National Institute of Standards and Technology 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."<ref>Template:Cite report</ref><ref>Template:Cite report</ref> This deprecated value of 6,080.2 feet is equivalent to Template:Convert. A separate reference identifies the geographic mile as being identical to the international nautical mile of 1,852 m and slightly shorter than the British nautical mile of Template:Convert.<ref>Template:Cite book</ref>

Scandinavians used their own version of the geographical mile as their nautical mile up to the beginning of the 20th century, causing it to be more well known as the sea mile in Danish ({{#invoke:Lang|lang}}), Norwegian ({{#invoke:Lang|lang}}), and Swedish ({{#invoke:Lang|lang}}).

Use

The unit is not used much in English-speaking countries but is cited in some United States laws. For example, Section 1301(a) of the Submerged Lands Act defines state seaward boundaries in terms of geographic miles. While debating what became the Land Ordinance of 1785, Thomas Jefferson'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".<ref name=proposal>Template:Cite book</ref>

See also

• Conversion of units
• Medieval weights and measures for details of the geographical league of France
• Mile for the various other miles in use
• Nautical mile