View source for Knot (unit)

{{short description|Unit of speed}}
{{Use dmy dates|date=June 2017}}
{{Infobox unit
| name         = knot
| image        = Airspeed Indicator.svg
| caption      = An [[airspeed indicator]], which shows speed in knots
| standard     = {{ubl|meteorology|aviation|maritime}}
| quantity     = [[speed]]
| symbol       = kn
| symbol2      = kt
| units1       = [[Kilometres per hour|km/h]]
| inunits1     = 1.852
| units2       = [[Miles per hour|mph]]
| inunits2     = 1.15078
| units3       = [[Metre per second|m/s]]
| inunits3     = 0.514444
| units4       = [[Foot per second|ft/s]]
| inunits4     = 1.68781
}}
The '''knot''' ({{IPAc-en|n|ɒ|t}}) is a unit of [[speed]] equal to one [[nautical mile]] per hour, exactly {{val|1.852|u=km/h}} (approximately {{val|1.151|u=mph}} or {{val|0.514|u=m/s}}).<ref>{{cite book |title=A Dictionary of Aviation |first=David W. |last=Wragg |isbn=9780850451634 |edition=first |publisher=Osprey |year=1973 |page=173}}</ref><ref>{{cite book | title=RYA Navigation Handbook | last=Bartlett | first=Tim | orig-year=2003|date=July 2008 | publisher=[[Royal Yachting Association]] | location=Southampton }}</ref> The [[ISO 80000-3|ISO standard]] symbol for the knot is '''kn'''.<ref>{{cite web|title=ISO 80000-3:2006|url=http://www.iso.org/iso/catalogue_detail?csnumber=31888|publisher=[[International Organization for Standardization]]|access-date=20 July 2013}}</ref> The same symbol is preferred by the [[IEEE|Institute of Electrical and Electronics Engineers]] ([[IEEE]]), while '''kt''' is also common, especially in aviation, where it is the form recommended by the [[ICAO|International Civil Aviation Organization]] ([[ICAO]]).<ref>International Standards and Recommended Practices,  Annex 5 to the Convention on International Civil Aviation, "Units of measurement to be Used in Air and Ground Operations", ICAO, 4th Edition,  July 1979.</ref> The knot is a non-[[International System of Units|SI]] unit.<ref name="nonSi">{{cite web | title = Non-SI units accepted for use with the SI, and units based on fundamental constants | work = SI brochure (8th ed.) | publisher = [[International Bureau of Weights and Measures]] | url = http://www.bipm.org/en/publications/si-brochure/table8.html | quote = The knot is defined as one [[nautical mile]] per hour. There is no internationally agreed symbol, but the symbol kn is commonly used.}}</ref> The knot is used in [[meteorology]], and in [[Maritime transport|maritime]] and [[aviation|air]] navigation. A [[Watercraft|vessel]] travelling at 1&nbsp;knot along a [[Meridian (geography)|meridian]] travels approximately one [[Minute of arc|minute]] of geographic [[latitude]] in one hour.

==Definitions==
;1 international knot =
:1&nbsp;[[nautical mile]] per [[hour]] (by definition),
:{{convert|1|kn|m/h|3|abbr=off|disp=out|comma=gaps}} (exactly),<ref name="nonSi" />
:{{convert|1|kn|m/s|5|abbr=off|disp=out}} (approximately),
:{{convert|1|kn|mph|5|abbr=off|disp=out}} (approximately),
:{{convert|1|kn|in/s|5|abbr=off|disp=out}} (approximately)
:{{convert|1|kn|ft/s|5|abbr=off|disp=out}} (approximately).

The length of the internationally agreed nautical mile is {{val|1852|u=m|fmt=gaps}}. The US adopted the international definition in 1954, having previously used the US nautical mile ({{val|1853.248|u=m}}).<ref name="usnm">{{cite web|url=http://physics.nist.gov/Pubs/SP447/app4.pdf |title=Appendix 4 The international nautical mile |access-date=2007-08-02 |author=Louis E. Barbow and Lewie V. Judson |year=1976 |work=Weights and Measures Standards of the United States, A brief history |publisher=NIST Physics Laboratory |url-status=dead |archive-url=https://web.archive.org/web/20070926000328/http://physics.nist.gov/Pubs/SP447/app4.pdf |archive-date=2007-09-26 }}</ref> The UK adopted the international nautical mile definition in 1970, having previously used the UK [[British Admiralty|Admiralty]] nautical mile ({{val|6080|u=ft|fmt=gaps}} or {{val|1853.184|u=m}}).
{{Speed conversions}}

==Usage==
The speeds of vessels relative to the [[fluid]]s in which they travel (boat speeds and [[air speed]]s) can be measured in knots. If so, for consistency, the speeds of navigational fluids ([[ocean current]]s, [[tidal stream (marine science)|tidal streams]], [[river current]]s and [[wind speed]]s) 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) can also be given in knots. Since 1979, the [[International Civil Aviation Organization]] list the knot as permitted for temporary use in aviation, but no end date to the temporary period has been agreed {{asof|2024|lc=y}}.<ref>{{Cite web |url= https://www.icao.int/safety/airnavigation/nationalitymarks/annexes_booklet_en.pdf |title= The Convention on International Civil Aviation Annexes 1 to 18 |access-date=11 November 2024}}</ref>

== Origin ==<!-- This section is linked from [[Data logging]] -->
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.<ref name="Abbott1858">{{cite book|author=Jacob Abbott|title=Rollo on the Atlantic|url=https://books.google.com/books?id=NQYXAAAAYAAJ|access-date=30 November 2011|year=1858|publisher=DeWolfe, Fiske, & Co., Publishers}}</ref> [[Knot]]s tied at a distance of {{convert|47|ft|3|in|m|4|lk=on}} from each other, passed through a sailor's fingers, while another sailor used a 30-second [[Hourglass|sand-glass]] (28-second sand-glass is the currently accepted timing) to time the operation.<ref>{{cite book|editor1-last=Kemp|editor1-first=Peter|title=The Oxford Companion to Ships and the Sea|url=https://archive.org/details/oxfordcompaniont00kemp|url-access=registration|publisher=Oxford University Press|year=1976|isbn=0-19-282084-2|page=[https://archive.org/details/oxfordcompaniont00kemp/page/454 454]}}</ref> 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 {{convert|20+1/4|in/s|km/h|disp=or|5}}. The difference from the modern definition is less than 0.02%.

Derivation of knots spacing:

<math>1~\textrm{kn} = 1852~\textrm{m/h} = 0.5144~\textrm{m/s}</math>, so in <math>28</math> seconds that is <math>14.40</math> metres per knot.

==Modern use==
[[File:World Scale from DMA Series 1150 map.png|thumb|right|Graphic scale from a [[Mercator projection]] world map, showing the change with latitude|429x429px]]

Although the unit ''knot'' 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.

On a standard nautical chart using [[Mercator projection]], the horizontal (East–West) 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 [[Linear scale|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 [[Caliper#Divider caliper|dividers]] and the latitude scales on the sides of the chart. Recent British [[Admiralty chart]]s have a latitude scale down the middle to make this even easier.<ref>E.g. BA Chart 73, ''Puerto de Huelva and Approaches'', 2002</ref>

Speed is sometimes incorrectly expressed as "knots per hour",<ref name="Kipling">{{cite web|url=http://www.kipling.org.uk/rg_knotsanhour.htm|title='Knots an hour'|date=22 July 2009|access-date=1 December 2012|quote=Since the 1890s or thereabouts, it has been drummed into the young seaman that a knot is a unit of speed, namely, one nautical mile per hour; and that consequently only the uneducated speak of "knots per hour" or "knots an hour". It was therefore inevitable that Kipling’s frequent use of this expression should grieve a number of seafaring readers, as the pages of the Kipling Journal testify.|last=Wilson|first=Alastair|publisher=[[Kipling Society|The Kipling Society]]}}</ref> which would mean "nautical miles per hour per hour" and thus would refer to [[acceleration]].

=== 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.<ref>For example, Part 23 of the Federal Aviation Regulations, amendment 23–7, 14 September 1969</ref>

The following abbreviations are used to distinguish between various measurements of [[airspeed]]:<ref>{{cite web|url=http://edocket.access.gpo.gov/cfr_2009/janqtr/14cfr1.2.htm|title=Abbreviations and symbols.|website=edocket.access.gpo.gov}}</ref>
* TAS is "knots [[true airspeed]]", the airspeed of an aircraft relative to undisturbed air
* KIAS is "knots [[indicated airspeed]]", the speed shown on an aircraft's [[pitot-static system|pitot-static]] [[airspeed indicator]]
* CAS is "knots [[calibrated airspeed]]", the indicated airspeed corrected for [[position error]] and instrument error
* EAS is "knots [[equivalent airspeed]]", the calibrated airspeed corrected for [[Adiabatic process|adiabatic]] [[compressible flow]] for the particular altitude

The indicated airspeed is close to the true airspeed only at sea level in standard conditions and at low speeds. At {{convert|11,000|m|ft|abbr=on|comma=gaps}}, an indicated airspeed of 300 kn may correspond to a true airspeed of 500 kn in standard conditions.

==See also==
{{Portal|Transport}}
* [[Beaufort scale]]
* [[Hull speed]], which deals with theoretical estimates of practical maximum speed of displacement hulls
* [[Knot count]]
* [[Knotted cord]]
* [[Metre per second]]
* [[Orders of magnitude (speed)]]
* [[Rope (unit)]]

==References==
{{Reflist}}

{{DEFAULTSORT:Knot (Unit)}}
[[Category:Navigation]]
[[Category:Units of velocity]]
[[Category:Water transport]]