Knot = nautical mile per hour
Nautical mile (British Admiralty mile) = 6080 feet (=100 fathoms).
In the sailing days a knotted line was used to measure the speed through the water. In 1637 the English navigator Richard Norwood recommended the use of a line knotted at intervals of 47 1/4 feet (14.3 metres) and a 28-second sandglass; if the first knot appeared as the sand ran out, the ship's speed was 6,076 feet (1,852 metres [one nautical mile]) per hour, or one knot.
1 nautical mile is the distance subtended at the equator by 1 minute of latitude. Hence the circumference of the earth at the equator is 21600nm or 24856 statute miles.
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edited for appalling typing
[This message has been edited by Code Blue (edited 22 January 2001).]