Nautical Mile

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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.

Close enough.

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.

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