The knot is a unit of speed equal to one nautical mile per hour. Its kn abbreviation is preferred by American and Canadian maritime authorities, the Institute of Electrical and Electronics Engineers, and the International Bureau of Weights and Measures, however, the kt (knot) and kts (knots) abbreviations also are used. The knot is a non-SI unit accepted for use with the SI. World-wide, the knot is used in meteorology, and in maritime and aviation navigation — for example, a vessel travelling at 1 knot along a meridian, travels one-sixtieth of a degree of geographic latitude in one hour, however, this is only true if the water is static. If a vessel is heading (say) west at three knots (measured on GPS) and a tidal stream is heading east at three knots, the indicated speed from a ships log would be six knots, when the ground speed would be three. Therefore the speed measured is relative to the medium that is being passed through, ie, water or air. Mariners first used the term knot denoting the measure of how many knots of line paid out in a given time using the chip log.
1.852 km is the length of the internationally-agreed nautical mile. The U.S. adopted the international definition in 1954, having previously used the U.S. nautical mile (1,853.248 m). The U.K. adopted the international nautical mile definition in 1970, having previously used the U.K. Admiralty nautical mile (6,080ft [1,853.184 m]).
The speeds of vessels relative to the fluids in which they travel (boat speeds and air speeds) are measured in knots. For consistency, the speeds of navigational fluids (tidal streams, river currents and wind speeds) are also measured in knots. Thus, speed over the ground (SOG) (ground speed (GS) in aircraft) and rate of progress towards a distant point ('velocity made good', VMG) are also given in knots.
Although the unit knot does not fit within the primary SI system, its retention for nautical and aviation use is important for navigational reasons, since the length of a nautical mile is almost identical to a minute of latitude. As a result, distance in nautical miles on a navigational chart can easily be measured by using dividers and the latitude indicators on the side of the chart.
Nautical speed is sometimes erroneously expressed as "knots per hour" which would actually be a measure of acceleration, as in "nautical miles per hour per hour."
Prior to 1969, airworthiness standards for civil aircraft in the USA Federal Aviation Regulations specified that distances were to be in statute miles, and speeds in miles per hour. In 1969 these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots.
The following abbreviations are used to distinguish between various measurements of airspeed.