Knot | Measurement | Britannica

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External Websites

• NASA - Knots Versus Miles per Hour
• MIT School of Engineering - Why is speed at sea measured in knots?
• Royal Museums Greenwich - Knots: measuring speed at sea

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knot, in navigation, measure of speed at sea, equal to one nautical mile per hour (approximately 1.15 statute miles per hour). Thus, a ship moving at 20 knots is traveling as fast as a land vehicle at about 23 mph (37 km/hr).

The term knot derives from its former use as a length measure on ships’ log lines, which were used to measure the speed of a ship through the water. Such a line was marked off at intervals by knots tied in the rope. Each interval, or knot, was about 47 feet (14.3 meters) long. When the log was tossed overboard, it remained more or less stationary while its attached log line trailed out from the vessel as the latter moved forward. After 28 seconds had elapsed, the number of knots that had passed overboard was counted. The number of knots that ran out in 28 seconds was roughly the speed of the ship in nautical miles per hour.

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The nautical mile is a unit defined as 1,852 meters (6,076 feet). This is the length on Earth’s surface of one minute (1/60 of a degree) of arc along a longitude. The distance covered in one degree of latitude is 60 nautical miles. The value of 1,852 meters for a nautical mile was adopted in 1929 by the First International Extraordinary Hydrographic Conference in Monaco.

The Editors of Encyclopaedia BritannicaThis article was most recently revised and updated by Erik Gregersen.