Geographical mile

From Wikipedia, the free encyclopedia

The geographical mile is a unit of length determined by 1 minute of arc along the Earth's equator. For the 1924 International Spheriod this equalled 1855.4 metres.^[1] Any greater precision depends more on choice of standard than on more careful measurement: the length of the equator in the World Geodetic System WGS-84 is 40,075,016.6856 m which makes the geographical mile 1855.3248 m, while the International Astronomical Union standard IAU-2000 takes the equator to be 40,075,035.5351 m making the geographical mile 1855.3257 m, almost a millimetre longer.

The unit is not used much; it is closely related to the nautical mile, which was originally determined as 1 minute of arc along a great circle of the Earth^[2] but is nowadays defined as exactly 1852 metres.^[1]

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 7421.5 metres by the astronomer Ole Rømer 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.

[edit] References

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2009)

1. ^ ^{a b} Ministry of Defence Staff, Navy Dept, Great Britain Ministry of Defence (1987). Admiralty manual of navigation. H.M. Stationery Office. pp. 7. ISBN 0117728802. 
2. ^ David Greenhood and Gerard L. Alexander (1964). Mapping. University of Chicago Press. pp. 51–52. ISBN 0226306976. 

[edit] 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