;1 international knot = The length of the internationally agreed nautical mile is. The US adopted the international definition in 1954, having previously used the US nautical mile. The UK adopted the international nautical mile definition in 1970, having previously used the UKAdmiralty nautical mile.
Usage
The speeds of vessels relative to the fluids in which they travel are measured in knots. For consistency, the speeds of navigational fluids are also measured in knots. Thus, speed over the ground and rate of progress towards a distant point are also given in knots.
Origin
Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out. Knots tied at a distance of 47 feet 3 inches from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%. Derivation of knots spacing: , so in seconds that is metres per knot.
Modern use
Although the unitknot does not fit within the SI system, its retention for nautical and aviation use is important because the length of a nautical mile, upon which the knot is based, is closely related to the longitude/latitude geographic coordinate system. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship. Standard nautical charts are on the Mercator projection and the horizontal scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. Recent British Admiralty charts have a latitude scale down the middle to make this even easier. Speed is sometimes incorrectly expressed as "knots per hour", which would mean "nautical miles per hour per hour" and thus would refer toacceleration.
Aeronautical terms
Prior to 1969, airworthiness standards for civil aircraft in the United States 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:
KTAS is "knots true airspeed", the airspeed of an aircraft relative to undisturbed air
The indicated airspeed is close to the true airspeed only at sea level in standard conditions and at low speeds. At, an indicated airspeed of 300 kn may correspond to a true airspeed of 500 kn in standard conditions.
