Editing Unit of length (section)

==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" |
|}