View source for Unit of length

{{Short description|Reference value of length}}

[[File:Measurement_unit.jpg |thumb|right|A ruler, depicting two customary units of length, the centimeter and the inch]]

A '''unit of length''' refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the [[Metric system|metric units]], used in every country globally. In the United States the U.S. customary units are also in use. British [[Imperial units]] are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into [[International System of Units|SI]] and non-SI units.<ref>{{cite book|last=Cardarelli|first=François|title=Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins|url=https://archive.org/details/encyclopaediaofs0000card|url-access=registration|year=2003|publisher=Springer|isbn=9781852336820 }}</ref><ref name=Hinkelman>{{cite book|last=Hinkelman|first=Edward G.|author2=Sibylla Putzi  |title=Dictionary Of International Trade: Handbook Of The Global Trade Community|year=2005|publisher=World Trade Press|page=245|isbn=9781885073723}}</ref><ref>{{cite book|last=Judson|first=Lewis Van Hagen|title=Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue,233|year=1960|pages=3&ndash;4|publisher=U.S. Department of Commerce, National Bureau of Standards |url=https://books.google.com/books?id=Mialg1senzMC&q=Units+of+Weight+and+Measure+%28United+States+Customary+and+Metric%29 |access-date=16 October 2012}}</ref>

==Metric system==
{{main article|Metric system}}

===SI===
{{main article|International System of Units}}
The [[SI base unit|base unit]] in the International System of Units (SI) is the meter, defined as "the length of the path travelled by [[Physics:Speed of light|light]] in vacuum during a time interval of {{frac|299792458}} seconds."<ref name="Res1">{{cite web |url=http://www.bipm.org/en/CGPM/db/17/1/ |title=17th General Conference on Weights and Measures (1983), Resolution 1. |access-date=2012-09-19}}</ref> It is approximately equal to {{val|1.0936|u=yards}}. Other SI units are derived from the meter by adding [[Metric prefix|prefixes]], as in millimeter or kilometer, thus producing systematic decimal multiples and submultiples of the base unit that span many orders of magnitude. For example, a kilometer is {{val|1000|u=meters}}.

===Non-SI===
In the [[Centimetre–gram–second system of units|centimeter–gram–second system of units]], the basic unit of length is the centimeter, or {{frac|100}} of a meter.
Other non-SI units are derived from decimal multiples of the meter.
{| class="wikitable" style="margin-left: 20px;" background-color: #ffffff"
! Name
! Symbol
! SI value
|- align="left"
| fermi
| aligh="center" | fm
| femtometer
|-
| [[Ångström|ångström]] 
| Å
| 100 picometers
|-
| micron
| μm
| 1 [[Micrometre|micrometer]]
|-
| Norwegian/Swedish mil or myriameter
| 
| 10,000 meters
|-
| [[Physics:X unit|x unit]] 
| xu
| 0.1 picometer
|}

==Imperial/U.S.==
The basic unit of length in the imperial and U.S. customary systems is the [[Yard|yard]], defined as exactly {{val|0.9144|u=m}} by international treaty in 1959.<ref name=Hinkelman/><ref name="Fenna2002">{{cite book|author=Donald Fenna|title=A dictionary of weights, measures, and units|url=https://archive.org/details/dictionaryofweig0000fenn|url-access=registration|access-date=8 January 2012|date=2002|publisher=Oxford University Press|isbn=978-0198605225|pages=[https://archive.org/details/dictionaryofweig0000fenn/page/130 130]–131}}</ref>

Common imperial units and U.S. customary units of length include:<ref>{{harvnb|Cardarelli|2003|pp=29&ndash;30}}</ref>
* thou or mil ({{frac|1000}} of an inch)
* [[Inch|inch]] ({{val|25.4|u=mm}})
* [[Foot (unit)|foot]] (12&nbsp;inches, 0.3048 m)
* [[Yard|yard]] (3&nbsp;feet, 0.9144 m)
* (terrestrial) [[Mile|mile]] (5280&nbsp;feet, 1609.344 m) 
* (land) [[League (unit)|league]] {{convert|3|mile|m}}

==Marine==
In addition, the following are used by [[Engineering:Sailor|sailor]]s:
* [[Fathom|fathom]] (for depth; only in non-metric countries) (2 yards = 1.8288 m)
* [[Nautical mile|nautical mile]] (one minute of arc of latitude = {{val|1852|u=m}})

==Aviation==
Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.{{citation needed|date=March 2016}}

==Surveying==
[[File:Determination of the rute and the feet in Frankfurt.png|thumb|right|Determination of the rod, using the length of the left foot of 16 randomly chosen people coming from church service]]

[[Engineering:Surveying|Surveyors]] in the United States continue to use:
* [[Chain (unit)|chain]] (22 yards, or {{val|20.1168|u=m}})
* [[Rod (unit)|rod]] (also called pole or perch) (quarter of a chain, 5{{Frac|1|2}} yards, or {{val|5.0292|u=m}})

===Building trades===
The Australian building trades adopted the [[Metric system|metric system]] in 1966 and the units used for measurement of length are meters (m) and millimeters (mm). Centimeters (cm) are avoided as they cause confusion when reading plans. For example, the length two and a half meters is usually recorded as 2500&nbsp;mm or 2.5&nbsp;m; it would be considered non-standard to record this length as 250&nbsp;cm.<ref>{{Cite book|last=Wilks, Kevin Joseph.|url=https://www.worldcat.org/oclc/27702954|title=Metrication in Australia : a review of the effectiveness of policies and procedures in Australia's conversion to the metric system|date=1992|publisher=Australian Govt. Pub. Service|others=Australia. Department of Industry, Technology, and Commerce.|isbn=0-644-24860-2|location=Canberra|pages=94|oclc=27702954}}</ref><ref>{{Cite web|url=http://themetricmaven.com/wp-content/uploads/2012/03/Metrication-in-Australia-built-2013-06-24.pdf|title=Metrication in Australia}}</ref>

===Surveyor's trade===
American surveyors use a decimal-based system of measurement devised by [[Biography:Edmund Gunter|Edmund Gunter]] in 1620. The base unit is [[Gunter's chain]] of {{convert|66|ft|m}} which is subdivided into 4 rods, each of 16.5&nbsp;ft or 100 links of 0.66&nbsp;feet. A link is abbreviated "lk",  and links "lks", in old deeds and land surveys done for the government.

==Science==

===Astronomy===
{{main article|Astronomy:Astronomical system of units}}
[[Astronomy:Astronomy|Astronomical]] measure uses:
*[[Astronomy:Earth radius|Earth radius]] {{Earth radius}} ≈&nbsp;6,371&nbsp;km<ref name="Moritz2000">{{cite journal |last=Moritz |first=H. |date=March 2000 |title=Geodetic Reference System 1980 |journal=Journal of Geodesy |volume=74 |issue=1 |pages=128–133 |doi=10.1007/s001900050278 |bibcode = 2000JGeod..74..128. |s2cid=195290884 }}</ref>
* [[Astronomy:Lunar distance|Lunar distance]] ''LD''&nbsp;≈&nbsp;{{val|384402|u=km}}.<ref name="Apache Point">{{cite journal|last1=Battat|first1=J. B. R.|last2=Murphy|first2=T. W.|last3=Adelberger|first3=E. G.|title=The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range|journal=Astronomical Society of the Pacific|date=January 2009|volume=121|issue=875|pages=29–40|doi=10.1086/596748|jstor=10.1086/596748|bibcode = 2009PASP..121...29B |doi-access=free}}</ref> Average distance between the center of Earth and the center of the Moon.
*[[Astronomy:Astronomical unit|astronomical unit]] au. Defined as {{val|149597870700|u=m}}.<ref name=Nature2012>{{cite web
 | url = http://www.nature.com/news/the-astronomical-unit-gets-fixed-1.11416
 | title = The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number.
 | author = Geoff Brumfiel
 | date = 14 Sep 2012
 | access-date = 14 Sep 2012
}}</ref>  Approximately the distance between the Earth and Sun.
*[[Astronomy:Light-year|light-year]] ly&nbsp;≈&nbsp;{{val|9460730472580.8|u=km}}. The distance that light travels in a [[Physics:Vacuum|vacuum]] in one [[Astronomy:Julian year|Julian year]].<ref name="IAUgen">{{citation| url = http://www.iau.org/public_press/themes/measuring/ | title = The IAU and astronomical units | publisher = International Astronomical Union | access-date=2008-07-05}}</ref>
*[[Astronomy:Parsec|parsec]] pc&nbsp;≈&nbsp;{{val|30856775814671.9|u=km}} or about {{val|3.26156|u=ly}}
*[[Astronomy:Hubble's law#Hubble length|Hubble length]] 14.4 billion light-years or 4.55 gigaparsecs

===Physics===
In atomic physics, sub-atomic physics, and cosmology, the preferred unit of length is often related to a chosen fundamental physical constant, or combination thereof. This is often a characteristic radius or wavelength of a particle. Some common [[Natural units|natural units]] of length are included in this table:

{| class="wikitable" style="margin-left: 20px;" background-color: #ffffff"
! Atomic property
! Symbol
! Length, in meters
! Reference
|- align="left"
| The classical electron radius 
| align="center" |''r''<sub>''e''</sub>
| {{val|2.817940285|(31)|e=-15}}
| align="center" |<ref name="Bohr ruler">{{cite book|last1=Quinn|first1=T.J.|last2=Leschiutta|first2=S.|last3=Tavella|first3=P.|title=Recent advances in metrology and fundamental constants|date=August 2000|publisher=IOS Press, 2001. Proceedings of the International School of Physics "Enrico Fermi"|location=Amsterdam ; Washington, DC|pages=142–143|isbn=9781586031671|url=https://books.google.com/books?id=WE22Fez60EcC&q=bohr+ruler&pg=PA142}}</ref>
|-
| The Compton wavelength of the electron
| align="center" |''λ''<sub>''C''</sub>
| {{val|2.426310215|(18)|e=-12}}
| align="center" |<ref name="Bohr ruler"/>
|-
| The reduced Compton wavelength of the electron
| align="center" |{{strikethrough|''λ''}}<sub>''C''</sub>
| {{val|3.8615926764|(18)|e=-13}}
| align="center" |<ref>{{cite web |publisher=NIST |title=Compton wavelength over 2 pi |url=http://physics.nist.gov/cgi-bin/cuu/Value?ecomwlbar |work=The NIST Reference on Constants, Units, and Uncertainty |access-date=15 October 2012}}</ref>
|-
| The Compton wavelength (or reduced Compton wavelength) of any fundamental particle
| align="center" |{{strikethrough|''λ''}}<sub>''x''</sub>
| 
|
|-
| The [[Physics:Bohr radius|Bohr radius]] of the hydrogen atom (Atomic unit of length)
| align="center" |''a''<sub>''0''</sub>
| {{val|5.291772083|(19)|e=-11}}
| align="center" |<ref name="Bohr ruler"/>
|-
| The reduced wavelength of hydrogen radiation
| align="center" |1 / ''R''<sub>∞</sub>
| {{val|9.112670505509|(83)|e=-8}}
| align="center" |<ref name="Bohr ruler"/>
|-
| The [[Planck length]]
| align="center" |𝓁<sub>''P''</sub>
| {{val|1.616199|(97)|e=-35}}
| align="center" |<ref>{{cite web |publisher=NIST |title=Planck length |url=http://physics.nist.gov/cgi-bin/cuu/Value?plkl |work=The NIST Reference on Constants, Units, and Uncertainty |access-date=15 October 2012}}</ref>
|-
| [[Stoney units|Stoney unit]] of length
| align="center" | ''l''<sub>''S''</sub>
| {{val|1.381|e=-35}}
| align="center" |
|-
| Quantum chromodynamics ([[Physics:Quantum chromodynamics|QCD]]) unit of length
| align="center" | ''l''<sub>''QCD''</sub>
| {{val|2.103|e=-16}}
| align="center" |
|-
| Natural units based on the electronvolt
| align="center" | 1 eV<sup>−1</sup>
| {{val|1.97|e=-7}}
| align="center" |
|}

==Archaic==
Archaic units of distance include:
*[[Cana (unit)|cana]]
*[[Cubit|cubit]]
*[[Rope (unit)|rope]]
*[[League (unit)|league]]
*[[Li (unit)|li]] (China)
*[[Pace (unit)|pace]] (the "double pace" of about 5 feet used in [[History:Ancient Rome|Ancient Rome]])
*[[Verst|verst]] (Russia)

==Informal==
In everyday conversation, and in informal literature, it is common to see lengths measured in units of objects of which everyone knows the approximate width. Common examples are:
*[[Engineering:Double-decker bus|Double-decker bus]] (9.5–11 meters in length)
*Football field (100 yards in length)
*Thickness of a human hair (around 80 micrometers)

==Other==
Horse racing and other equestrian activities keep alive:
* [[Furlong|furlong]]&nbsp;=&nbsp;{{convert|1/8|mi|m|3}}
*[[Horse length|horse length]]&nbsp;≈&nbsp;{{convert|8|ft|m}}

==See also==
*{{Section link|List of conversion factors|Length}}
*[[List of examples of lengths]]
*{{Section link|List of unusual units of measurement|Length}}
*[[Medieval weights and measures]]
*[[Orders of magnitude (length)]]
*[[System of measurement]]
*Units of measurement

==References==
{{Reflist}}

==Further reading==
*{{cite book|last=Whitelaw|first=Ian|title=A Measure of All Things: The Story of Man and Measurement|year=2007|publisher=Macmillan|isbn=9780312370268}}

{{Units of length used in Astronomy}}

[[Category:Units of length| ]]

{{Sourceattribution|Unit of length}}