Editing Astronomical unit (section)

{{Short description|Mean distance between Earth and the Sun}}
{{About|the unit of length|constants|astronomical constant|units in astronomy|astronomical system of units|other uses of "AU"|AU (disambiguation){{!}}Au}}
{{Use dmy dates|date=September 2019}}
{{Use British English|date=October 2015}}
{{Infobox unit
| bgcolour =
| name = Astronomical unit
| image= File:Astronomical unit.png
| caption=The grey line indicates the Earth–Sun distance, which on average is about 1&nbsp;astronomical unit.
| standard = [[Astronomical system of units]]<br/>[[Non-SI units mentioned in the SI|(Accepted for use with the SI)]]
| quantity = [[length]]
| symbol = au or {{sc|au}} or AU
| units1 = [[metric system|metric]] ([[International_System_of_Units|SI]]) units
| inunits1 = {{val|1.495978707|e=11|ul=m}}
| units2 = [[Imperial units|imperial]]&nbsp;&&nbsp;[[United States customary units|US]]&nbsp;units
| inunits2 ={{val|9.2956|e=7|ul=mi}}
| units3 = [[Astronomical system of units|astronomical units]]
| inunits3 = {{val|4.8481|e=-6|ul=pc}}<br/>&nbsp;&nbsp;&nbsp;{{val|1.5813|e=-5|ul=ly}}<br/>&nbsp;&nbsp;&nbsp;{{val|215.03|ul=solar radius}}
}}

The '''astronomical unit''' (symbol: '''au'''<ref name="IAUresB2">{{cite conference |title=On the re-definition of the astronomical unit of length |id=Resolution&nbsp;B2 |conference=XXVIII&nbsp;General Assembly of International Astronomical Union |publisher=International Astronomical Union |place=Beijing, China |date=31 August 2012 | url = http://www.iau.org/static/resolutions/IAU2012_English.pdf|url-status=dead |archive-url=https://web.archive.org/web/20250305215811/http://www.iau.org/static/resolutions/IAU2012_English.pdf |archive-date=5 March 2025 |quote=... recommends ... 5. that the unique symbol "au" be used for the astronomical unit.}}</ref><ref name="mnras_style">{{cite web | url=http://www.oxfordjournals.org/our_journals/mnras/for_authors/#6.4%20Miscellaneous%20journal%20style | archive-url=https://web.archive.org/web/20121022064348/http://www.oxfordjournals.org/our_journals/mnras/for_authors/#6.4%20Miscellaneous%20journal%20style | url-status=dead | archive-date=22 October 2012 | title=Monthly Notices of the Royal Astronomical Society: Instructions for Authors | website=Oxford Journals | access-date=20 March 2015 | quote=The units of length/distance are Å, nm, μm, mm, cm, m, km, au, light-year, pc.}}</ref><ref name="AAS_style"/><ref>{{SIbrochure9th |page=145}}</ref> or '''AU''') is a [[unit of length]] defined to be exactly equal to {{val|149597870700|u=metres}}.<ref>{{cite conference |id=Resolution B2 |title=On the re-definition of the astronomical unit of length |publisher=International Astronomical Union |conference=XXVIII&nbsp;General Assembly of International Astronomical Union |place=Beijing |date=31 August 2012 |url=http://www.iau.org/static/resolutions/IAU2012_English.pdf |url-status=dead |archive-url=https://web.archive.org/web/20250305215811/http://www.iau.org/static/resolutions/IAU2012_English.pdf |archive-date=5 March 2025|quote=... recommends [adopted] that the astronomical unit be re-defined to be a conventional unit of length equal to exactly {{val|149,597,870,700}} metres, in agreement with the value adopted in IAU&nbsp;2009 Resolution&nbsp;B2}}</ref> Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's [[aphelion]] and [[perihelion]]), before its modern redefinition in 2012.

The astronomical unit is used primarily for measuring distances within the [[Solar System]] or around other stars. It is also a fundamental component in the definition of another unit of astronomical length, the [[parsec]].<ref name=au_parsec>{{cite journal |author1=Luque, B. |author2=Ballesteros, F.J. |year=2019 |title=Title: To the Sun and beyond |journal=[[Nature Physics]] |volume=15 |issue=12 |page=1302 |doi=10.1038/s41567-019-0685-3 |doi-access=free|bibcode=2019NatPh..15.1302L }}</ref> One au is approximately equivalent to 499 [[light-second]]s.