Editing Geographical mile

{{Short description|Unit of length of approximately 1855.3 m}}
{{Infobox unit
| units1 = [[SI units]]
| inunits1 = ~1,855.3 [[metre|m]]
| units2 = [[imperial units|imperial]]/[[US customary units|US]] units
| inunits2 = ~{{convert|1855.3|m|mi|disp=out|lk=on}} or ~{{convert|1855.3|m|ft|disp=out|lk=on}}
}}
The '''geographical mile''' is an international unit of length determined by 1 [[minute of arc]] ({{sfrac|1|60}} [[degree (angle)|degree]]) along the [[Earth]]'s [[equator]]. For the [[Hayford ellipsoid|international ellipsoid 1924]] this equalled 1855.4 [[metre]]s.<ref name=Admiralty>{{cite book|title=Admiralty manual of navigation|author=Ministry of Defence Staff, Navy Dept, Great Britain Ministry of Defence|pages=7|publisher=H.M. Stationery Office|year=1987|isbn=9780117728806}}</ref> ''[[The American Practical Navigator]]'' 2017 defines the geographical mile as {{convert|6087.08|ft|m|3}}.<ref name=Bowditch>{{citation |chapter=Glossary of Marine Navigation |title=The American Practical Navigator |volume=II |edition=2017 |page=346 |publisher=National Geospatial-Intelligence Agency |url=https://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_pageLabel=msi_portal_page_62&pubCode=0002}}</ref> Greater precision depends more on the choice of the [[Earth's radius]] of the used [[Earth ellipsoid|ellipsoid]] than on more careful measurement, since the radius of the [[geoid]] varies more than {{convert|100|m|ft|3}} 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 [[Geodetic Reference System 1980|GRS80]] ellipsoid is {{val|6378137.0000|u=m|fmt=commas}},<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 {{convert|1855.3|m|nmi|5|disp=out|abbr=off}}.

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

The [[Portuguese units of measurement|Portuguese system]] derived their miles ({{lang|pt|milha geográfica}}) as one third of [[Portuguese league|their league]] of three separate values. When each equatorial degree was divided into 18 leagues, the geographical mile was equal to {{sfrac|1|54}} degree or about {{convert|2.06|km|mi}}; when divided into 20 leagues, the geographical mile was equal to {{sfrac|1|60}} degree, approximating the values provided above; and when divided into 25 leagues, the geographical mile was equal to {{sfrac|1|75}} degree or about {{convert|1.48|km|mi}}.

The geographical miles of the traditional [[Dutch units of measurement|Dutch]] ({{lang|nl|geografische mijl}}), [[German units of measurement|German]] ({{lang|de|geographische Meile}} or {{lang|de|Landmeile}}), and [[Danish units of measurement|Danish systems]] ({{lang|da|geografisk mil}}) all approximated their much longer miles{{mdash}}equivalent to English leagues{{mdash}}by using a larger division of the equatorial degree. Instead of using one minute of arc, they all used four{{mdash}}{{sfrac|1|15}} degree{{mdash}}to produce a distance now notionally equal to {{convert|7408|m|ft}} but actually differing slightly depending on official measurements and computations. (For example, the Danish unit was computed as equivalent to about {{convert|7421.5|m|ft}} by the astronomer [[Ole Rømer]].)<ref>{{cite journal|last1=Rabounski|first1=Dmitri|title=Biography of Ole Rømer|journal=The Abraham Zelmanov Journal|date=2008|volume=1|page=2|url=http://zelmanov.ptep-online.com/papers/zj-2008-b1.pdf |archive-url=https://web.archive.org/web/20110715130250/http://zelmanov.ptep-online.com/papers/zj-2008-b1.pdf |archive-date=2011-07-15 |url-status=live|access-date=1 February 2018|ref=ISSN 1654-9163}}</ref> 

==Relationship with the nautical mile==
{{main|Nautical mile}}
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>{{cite book|title=Mapping|author1=David Greenhood|author2=Gerard L. Alexander|pages=[https://archive.org/details/mapping00gree/page/51 51&ndash;52]|publisher=University of Chicago Press|year=1964|isbn=9780226306971|url-access=registration|url=https://archive.org/details/mapping00gree/page/51}}</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&nbsp;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>{{cite report |url=https://www.nist.gov/sites/default/files/documents/2017/04/28/hb44-15-web-final.pdf |archive-url=https://web.archive.org/web/20170505110701/https://www.nist.gov/sites/default/files/documents/2017/04/28/hb44-15-web-final.pdf |archive-date=2017-05-05 |url-status=live |work=[[NIST]] |title="NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, General Tables of Units of Measurement |page=C-15 (Appendix C, footnote 14) |access-date=April 9, 2019 |date=November 2014 }}</ref><ref>{{cite report |url=https://nvlpubs.nist.gov/nistpubs/Legacy/MP/nbsmiscellaneouspub214.pdf |archive-url=https://web.archive.org/web/20170201042337/http://nvlpubs.nist.gov/nistpubs/Legacy/MP/nbsmiscellaneouspub214.pdf |archive-date=2017-02-01 |url-status=live |work=[[NIST|National Bureau of Standards]] |title="Units of Weight and Measure (United States Customary and Metric) Definition and Tables of Equivalents" |page=4 |access-date=May 5, 2021 |date=July 1, 1955 }}</ref> This deprecated value of 6,080.2 feet is equivalent to {{convert|6,080.2|ft|m|2|disp=out}}. 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 {{convert|6,080|ft|m|2}}.<ref>{{cite book |editor-last=Weast |editor-first=Robert C. |title=CRC Handbook of Chemistry and Physics, 62nd edition, 1981-1982 |isbn=978-0849304620 |location=Boca Raton, Florida |publisher=CRC Press |page=F-297 }}</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 ({{lang|da|sømil}}), [[Norwegian units of measure|Norwegian]] ({{lang|nb|sjømil}}), and [[Swedish units of measurement|Swedish]] ({{lang|sv|sjömil}}).

==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 "[[Hundred (county subdivision)|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>{{cite book |url=https://memory.loc.gov/cgi-bin/ampage?collId=lljc&fileName=027/lljc027.db&recNum=83 |title=A Century of Lawmaking for a New Nation: U.S. Congressional Documents and Debates, 1774 - 1875 |section=Journal of Continental Congress, Vol. 27 |page=446 |date=May 28, 1784 |via=[[Library of congress]] |access-date=April 9, 2019 }}</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]]

==References==
{{Reflist}}

{{DEFAULTSORT:Geographical Mile}}
[[Category:Units of length]]