Edit made on August 13, 2011 by ColinWright at 22:45:32
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One Nautical Mile is defined as the distance covered by
one arc-minute on a Great Circle. This could be either
the equator, or a line of longitude.
That means there are 60*90 Nautical Miles from the North
Pole to the Equator (90 degrees of latitude, times 60
arc minutes per degree), so 5400NM is 10 million metres
(by the definition of the metre).
Hence the Nautical Mile is 1851.851... metres, and has
since been declared to be 1852 metres, or 6076 feet.
Because the Earth is not exactly a sphere, the distance
equating to one arc-minute varies according to where you
are. At the poles, where the surface of the Earth is closer
to the centre, it's about 6067 feet, while at the Equator
it's about 6087 feet. Thus practice intrudes on theory,
and things get messy.