Editing Unit of length

{{Short description|Reference value of length}}
{{Use American English|date=December 2023}}
{{More citations needed|date=October 2012}}

[[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>

== History ==
{{excerpt|History of measurement#Units of length}}

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

===SI===
{{main article|International System of Units}}
{{See also|Orders of magnitude (length)}}
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 [[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
|1 [[femtometer]]
|-
| [[ångström]] 
| Å
| 100 [[picometer]]s
|-
| micron
| μm
| 1 [[Micrometre|micrometer]]
|-
| [[Norwegian/Swedish mil]] or [[myriameter]]
| 
| 10,000 meters
|-
| [[x unit]] 
| xu
| 0.1 picometer
|}

==Imperial/U.S.==
{{main article|Imperial units|United States customary units|English units#length}}
The basic unit of length in the imperial and U.S. customary systems is the [[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 (unit of length)|thou]] or mil ({{frac|1000}} of an inch)
* [[inch]] ({{val|25.4|u=mm}})
* [[foot (unit)|foot]] (12&nbsp;inches, 0.3048 m)
* [[yard]] (3&nbsp;feet, 0.9144 m)
* (terrestrial) [[mile]] (5280&nbsp;feet, or 1760 yards 1609.344 m) 
* (land) [[League (unit)|league]] {{convert|3|mile|m}}

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

==Aviation==
[[Pilot (aeronautics)|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]]

[[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}})

===Australian building trades===
The Australian building trades adopted the [[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 [[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|Astronomical system of units}}
[[Astronomy|Astronomical]] measure uses:
*[[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>
* [[Lunar distance (astronomy)|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.
*[[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.
*[[light-year]] ly&nbsp;≈&nbsp;{{val|9460730472580.8|u=km}}. The distance that light travels in a [[vacuum]] in one [[Julian year (astronomy)|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>
*[[parsec]] pc&nbsp;≈&nbsp;{{val|30856775814671.9|u=km}} or about {{val|3.26156|u=ly}}
*[[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]] 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 [[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 ([[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==
{{See also|English units#Length|label 1=English units of length}}
[[Archaism|Archaic]] units of distance include:
*[[cana (unit)|cana]]
*[[cubit]]
*[[Rope (unit)|rope]]
*[[league (unit)|league]]
*[[Li (unit)|li]] (China)
*[[Pace (unit)|pace]] (the "double pace" of about 5 feet used in [[Ancient Rome]])
*[[verst]] (Russia)

==Informal==
{{See also|List of humorous units of measurement|List of unusual units of measurement}}
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:
*[[Double-decker bus]] (9.5–11 meters in length)
*[[American football]] field (100 yards in length)
*Thickness of a human [[hair]] (around 80 micrometers)

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

{{Systems of measurement}}
{{Units of length used in Astronomy}}
{{Authority control}}

[[Category:Units of length| ]]