Why ships and airplanes measure speed in nautical miles (knots) rather than miles and kilometers?

The knot is the unit of speed equivalent to one nautical mile an hour. The knot is the survival of the earliest practical method of ascertaining the speed of a ship. This method consisted of casting out a log line with a triangular piece of wood weighted with load attached to the end to keep it upright and retard its passage through water.

The line was divided into sections called knots because they were marked by pieces of cord worked in between the strands. Each knot was 14.4 meters (47 feet 3 inches) apart. At the end of the first interval was one knot, followed by two knots at the end of the second and so on. A nautical mile is one sixtieth of one degree of the Earth’s circumference or about 6,076 feet. Therefore, the number of knots that ran off the reel in 28 seconds, showed with fair accuracy the number of miles the vessel was sailing an hour. An interval of 28 seconds has the same relation to the hour (3,600 seconds) what the distance of 47 feet 3 inches has to 6,076 feet. (3600 / 28 = 128.57 and 6076 / 47.3 = 128.59). Therefore, if the log has pulled out 7 intervals of line, it would mean that the ship was moving at 7 knots, or 7 nautical miles an hour. The knot as a unit of speed has been retained, but the old method of measuring the speed has been replaced by automatic speedometers which register on dials.

One knot is equal to 1.852 kilometers. Ships and airlines use this unit of measurement because their captain want to know how many degrees of the Earth’s circumference they have traveled and this also helps them to work out their exact geographical location at a given time.

More reading:
Nautical mile (Wikipedia)
Knot (unit) (Wikipedia)
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