geographic mile

length Distinct from the statute mile (the familiar mile of English), the geographical mile is a longer unit. Though the common word ‘mile’ derives from the Latin for one thousand, and the statute mile derives from the Roman mile of a thousand paces, the geographic mile has a very much earlier and different provenance, belonging to dynastic Egypt if not earlier, where it was sized geodesically. It is the distance along a meridian of 1 minute of latitude, or, more generally, the distance along any great circle that subtends 1 minute of angle at Earth's centre. Because Earth is an oblate spheroid, with equatorial radius 0.34% greater than polar radius, this real distance varies from 1 842.~ m to 1 862.~ m. The locally specific size is usually called a sea mile in navigation. The geographic mile and the nautical mile are mean values. The latter has long been officially standardized, since 1954 internationally at 1 852 m (6 076.~ ft). The geographic mile has usually been interpreted as equal to the nautical mile of the map-maker, though the Central Bureau of Longitude promulgated the figure of 1 852 m for it a hundred years ago.
[Glazebrook R. T. (ed.) Dictionary of Applied Physics, Vol. 1: Mechanics, Engineering, Heat (London: Macmillan, 1922)]

Historically, the geographic mile was often divided into 1 000 fathoms (q.v. for extended discussion). Three miles routinely make 1 league.

The kilometre, when created in 1792, was, within its context, effectively a geographic mile. The metre was defined as ¹/_10000000 of the meridional distance from Equator to Pole, making the kilometre equal to ¹/₁₀₀₀₀ of that distance. Since the decimalization efforts of the time were also applied to angles, but in centesimal rather than truly decimal steps, the 90 degrees of latitude from Equator to Pole became 100 grad, each divided into 100 centesimal minutes, each of 100 centesimal seconds. Thus, within the nascent metric system, there were 10 000 minutes of latitude from Equator to Pole, identical with the number of kilometres.