View source for Solar radius

{{short description|Unit of measurement}}
{| class="wikitable floatright"
|+ Conversion of nominal solar radius
! 1 {{solar radius}} =
! Units
|-
| {{val|6.95700|e=8}} || [[metre]]s
|-
| 695,700 || [[kilometre]]s
|- 
| 0.00465047 || [[astronomical unit]]
|-
| 432,288 || [[mile]]s
|-
| {{val|7.35355|e=-8}} || [[light-year]]
|-
| {{val|2.25461|e=-8}} || [[parsec]]
|-
| 2.32061 || [[light-second]]s
|}

'''Solar radius''' is a unit of [[distance]] used to express the size of objects in [[astronomy]] relative to the [[Sun]]. The solar [[radius]] is usually defined as the radius to the layer in the [[Sun]]'s [[photosphere]] where the [[Optical_depth_(astrophysics)|optical depth]] equals 2/3:<ref name='Hab2008' />

<math display="block">1\,R_{\odot} = 6.957\times 10^8 \hbox{ m} </math>

{{convert|695,700|km|mi|abbr=off}} is approximately 10 times [[Jupiter radius|the average radius]] of [[Jupiter]], 109 times [[Earth radius|the radius]] of the [[Earth]], and 1/215th of an [[astronomical unit]], the approximate distance between Earth and the Sun. The solar radius to either pole and that to the equator differ slightly due to the Sun's [[stellar rotation|rotation]], which induces an [[flattening|oblateness]] in the order of 10 parts per million.<ref>{{Cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2008/02oct_oblatesun/ |title=NASA RHESSI oblateness measurements 2012 |access-date=2017-07-12 |archive-date=2018-09-17 |archive-url=https://web.archive.org/web/20180917185431/https://science.nasa.gov/science-news/science-at-nasa/2008/02oct_oblatesun/ |url-status=dead }}</ref>

==Measurements==
[[File:Solar evolution (English).svg|right|thumb|upright=1.4|Evolution of the [[solar luminosity]], radius and [[effective temperature]] compared to the present-day Sun. After Ribas (2009)<ref>{{cite journal|last=Ribas|first=Ignasi|title=The Sun and Stars as the Primary Energy Input in Planetary Atmospheres|journal=Proceedings of the International Astronomical Union|volume=5|issue=S264 [Solar and Stellar Variability: Impact on Earth and Planets]| pages=3–18|date=August 2009|doi= 10.1017/S1743921309992298|bibcode= 2010IAUS..264....3R|arxiv= 0911.4872| s2cid=119107400| url=https://www.cambridge.org/core/services/aop-cambridge-core/content/view/293C0314C44A1A4AAF8175A3C288B50B/S1743921309992298a.pdf/sun_and_stars_as_the_primary_energy_input_in_planetary_atmospheres.pdf }}</ref>]]
The uncrewed [[Solar and Heliospheric Observatory|SOHO]] spacecraft was used to measure the radius of the Sun by timing transits of Mercury across the surface during 2003 and 2006. The result was a measured radius of {{Convert|696342|+/-|65|km|mi|abbr=off}}.<ref name=arxiv1203_4898>{{citation | first1=Marcelo | last1=Emilio | first2=Jeff R. | last2=Kuhn | first3=Rock I. | last3=Bush | first4=Isabelle F. | last4=Scholl | title=Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits  | arxiv=1203.4898 |bibcode = 2012ApJ...750..135E |doi = 10.1088/0004-637X/750/2/135 | volume=750 | issue=2 | journal=The Astrophysical Journal | pages=135| year=2012 | s2cid=119255559 }}</ref>

Haberreiter, Schmutz & Kosovichev (2008)<ref name=Hab2008>{{citation | first1 = M | last1=Haberreiter | first2=W | last2=Schmutz | first3=A.G. | last3=Kosovichev | title=Solving the Discrepancy between the Seismic and Photospheric Solar Radius | journal=Astrophysical Journal | volume=675 | issue=1 | pages=L53–L56 | doi=10.1086/529492 | bibcode=2008ApJ...675L..53H| arxiv=0711.2392 | year=2008 | s2cid=14584860 }}</ref> determined the radius corresponding to the solar photosphere to be {{Convert|695660|+/-|140|km|mi|abbr=off}}. This new value is consistent with helioseismic estimates; the same study showed that previous estimates using inflection point methods had been overestimated by approximately {{cvt|300|km|mi}}.

== Nominal solar radius ==

In 2015, the [[International Astronomical Union]] passed Resolution B3, which defined a set of nominal conversion constants for stellar and planetary [[astronomy]]. Resolution B3 defined the ''nominal solar radius'' (symbol <math>R^{N}_{\odot}</math>) to be equal to ''exactly'' {{val|695700|u=km}}.<ref name=IAU2015resB3>{{citation | first1=E.E. | last1=Mamajek | first2=A. | last2=Prsa | first3=G. | last3=Torres | first4=al. | last4=et | title=IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties | arxiv=1510.07674 |bibcode = 2015arXiv151007674M | year=2015 }}</ref> The nominal value, which is the rounded value, within the uncertainty, given by Haberreiter, Schmutz & Kosovichev (2008), was adopted to help astronomers avoid confusion when quoting stellar radii in units of the Sun's radius, even when future observations will likely refine the Sun's actual photospheric radius (which is currently<ref name=Mef2018>{{citation | first1 = M | last1=Meftah | first2=T | last2=Corbard | first3=A. | last3=Hauchecorne | first4=F. | last4=Morand | first5=R. | last5=Ikhlef | first6=B. | last6=Chauvineau | first7=C. | last7=Renaud | first8=A. | last8=Sarkissian | first9=L. | last9=Damé | title=Solar radius determined from PICARD/SODISM observationsand extremely weak wavelength dependence in the visibleand the near-infrared | journal=Astronomy & Astrophysics | volume=616 | pages=A64 | doi=10.1051/0004-6361/201732159 | bibcode=2018A&A...616A..64M | year=2018 | doi-access=free }}</ref> only known to about an accuracy of ±{{val|100|-|200|u=km}}).

==Examples==
Solar radii as a unit are common when describing spacecraft moving close to the sun. Two spacecraft in the 2010s include:

*[[Solar Orbiter]] (as close as {{val|45|u=solar radius}})
*[[Parker Solar Probe]] (as close as {{val|9|u=solar radius}})
{|class="wikitable"
|+Radius of another objects relative to the Sun's radius
!Name
!Radius {{nowrap|(Solar radius)}}
!Radius ([[kilometers]])
|-
|[[Milky Way]]
|{{val|5.94|e=11}}
|{{val|4.134|e=17}}<ref>{{Cite journal |last1=Goodwin |first1=S. P. |last2=Gribbin |first2=J. |last3=Hendry |first3=M. A. |date=1998-08-01 |title=The relative size of the Milky Way |url=https://ui.adsabs.harvard.edu/abs/1998Obs...118..201G |journal=The Observatory |volume=118 |pages=201–208 |bibcode=1998Obs...118..201G |issn=0029-7704}}</ref>
|-
|[[UY Scuti]]
|909<ref>{{Cite journal |last1=Healy |first1=Sarah |last2=Horiuchi |first2=Shunsaku |last3=Molla |first3=Marta Colomer |last4=Milisavljevic |first4=Dan |last5=Tseng |first5=Jeff |last6=Bergin |first6=Faith |last7=Weil |first7=Kathryn |last8=Tanaka |first8=Masaomi |date=2024-03-23 |title=Red Supergiant Candidates for Multimessenger Monitoring of the Next Galactic Supernova |journal=Monthly Notices of the Royal Astronomical Society |volume=529 |issue=4 |pages=3630–3650 |doi=10.1093/mnras/stae738 |doi-access=free |arxiv=2307.08785 |bibcode=2024MNRAS.529.3630H |issn=0035-8711}}</ref>
|{{val|632400000|fmt=commas}}
|-
|[[Betelgeuse]]
|764<ref>{{Cite journal |last1=Joyce |first1=Meridith |last2=Leung |first2=Shing-Chi |last3=Molnár |first3=László |last4=Ireland |first4=Michael |last5=Kobayashi |first5=Chiaki |last6=Nomoto |first6=Ken'ichi |date=2020-10-01 |title=Standing on the Shoulders of Giants: New Mass and Distance Estimates for Betelgeuse through Combined Evolutionary, Asteroseismic, and Hydrodynamic Simulations with MESA |journal=The Astrophysical Journal |volume=902 |issue=1 |pages=63 |doi=10.3847/1538-4357/abb8db |doi-access=free |arxiv=2006.09837 |bibcode=2020ApJ...902...63J |issn=0004-637X}}</ref>
|{{val|531500000|fmt=commas}}
|-
|[[Antares]] A
|680<ref>{{Cite journal |last1=Ohnaka |first1=K. |last2=Hofmann |first2=K. -H. |last3=Schertl |first3=D. |last4=Weigelt |first4=G. |last5=Baffa |first5=C. |last6=Chelli |first6=A. |last7=Petrov |first7=R. |last8=Robbe-Dubois |first8=S. |date=2013-07-01 |title=High spectral resolution imaging of the dynamical atmosphere of the red supergiant Antares in the CO first overtone lines with VLTI/AMBER |url=https://ui.adsabs.harvard.edu/abs/2013A&A...555A..24O |journal=Astronomy and Astrophysics |volume=555 |pages=A24 |doi=10.1051/0004-6361/201321063 |arxiv=1304.4800 |bibcode=2013A&A...555A..24O |issn=0004-6361}}</ref>
|{{val|473076000|fmt=commas}}
|-
|[[Rigel]] A
|74.1<ref>{{Cite journal |last1=Baines |first1=Ellyn K. |last2=Armstrong |first2=J. Thomas |last3=Schmitt |first3=Henrique R. |last4=Zavala |first4=R. T. |last5=Benson |first5=James A. |last6=Hutter |first6=Donald J. |last7=Tycner |first7=Christopher |last8=van Belle |first8=Gerard T. |date=2018-01-01 |title=Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer |journal=The Astronomical Journal |volume=155 |issue=1 |pages=30 |doi=10.3847/1538-3881/aa9d8b |doi-access=free |arxiv=1712.08109 |bibcode=2018AJ....155...30B |issn=0004-6256}}</ref>
|{{val|51550000|fmt=commas}}
|-
|[[Aldebaran]]
|45.1<ref>{{Cite journal |last1=Hatzes |first1=A. P. |last2=Cochran |first2=W. D. |last3=Endl |first3=M. |last4=Guenther |first4=E. W. |last5=MacQueen |first5=P. |last6=Hartmann |first6=M. |last7=Zechmeister |first7=M. |last8=Han |first8=I. |last9=Lee |first9=B. -C. |last10=Walker |first10=G. A. H. |last11=Yang |first11=S. |last12=Larson |first12=A. M. |last13=Kim |first13=K. -M. |last14=Mkrtichian |first14=D. E. |last15=Döllinger |first15=M. |date=2015-08-01 |title=Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity |url=https://ui.adsabs.harvard.edu/abs/2015A&A...580A..31H |journal=Astronomy and Astrophysics |volume=580 |pages=A31 |doi=10.1051/0004-6361/201425519 |arxiv=1505.03454 |bibcode=2015A&A...580A..31H |issn=0004-6361}}</ref>
|{{val|31375000|fmt=commas}}
|-
|[[Arcturus]]
|25.4<ref>{{Cite journal |last1=Ramírez |first1=I. |last2=Allende Prieto |first2=C. |date=2011-12-01 |title=Fundamental Parameters and Chemical Composition of Arcturus |url=https://ui.adsabs.harvard.edu/abs/2011ApJ...743..135R |journal=The Astrophysical Journal |volume=743 |issue=2 |pages=135 |doi=10.1088/0004-637X/743/2/135 |arxiv=1109.4425 |bibcode=2011ApJ...743..135R |issn=0004-637X}}</ref>
|{{val|17670000|fmt=commas}}
|-
|[[Pollux (star)|Pollux]]
|9.06<ref>{{Cite journal |last1=Baines |first1=Ellyn K. |last2=Armstrong |first2=J. Thomas |last3=Schmitt |first3=Henrique R. |last4=Zavala |first4=R. T. |last5=Benson |first5=James A. |last6=Hutter |first6=Donald J. |last7=Tycner |first7=Christopher |last8=Belle |first8=Gerard T. van |date=2017-12-21 |title=Fundamental Parameters of 87 Stars from the Navy Precision Optical Interferometer |journal=The Astronomical Journal |volume=155 |issue=1 |pages=30 |doi=10.3847/1538-3881/aa9d8b |doi-access=free |issn=0004-6256|arxiv=1712.08109 |bibcode=2018AJ....155...30B }}</ref>
|{{val|6300000|fmt=commas}}
|-
|[[Sirius]] A
|1.711<ref>{{Cite journal |last1=Liebert |first1=James |last2=Young |first2=Patrick A. |last3=Arnett |first3=David |last4=Holberg |first4=J. B. |last5=Williams |first5=Kurtis A. |date=2005-09-01 |title=The Age and Progenitor Mass of Sirius B |url=https://ui.adsabs.harvard.edu/abs/2005ApJ...630L..69L |journal=The Astrophysical Journal |volume=630 |issue=1 |pages=L69–L72 |doi=10.1086/462419 |arxiv=astro-ph/0507523 |bibcode=2005ApJ...630L..69L |issn=0004-637X}}</ref>
|{{val|1190350|fmt=commas}}
|-style="background:#faf86b;"
|[[Sun]]
|1
|{{val|695700|fmt=commas}}
|-
|[[Proxima Centauri]]
|0.1542<ref>{{Cite journal |last1=Kervella |first1=P. |last2=Thévenin |first2=F. |last3=Lovis |first3=C. |date=February 2017 |title=Proxima's orbit around Alpha Centauri |journal=Astronomy & Astrophysics |volume=598 |pages=L7 |doi=10.1051/0004-6361/201629930 |arxiv=1611.03495 |bibcode=2017A&A...598L...7K |issn=0004-6361}}</ref>
|{{val|107275|fmt=commas}}
|-
|[[Jupiter]]
|0.1028
|{{val|71492|fmt=commas}}<ref name=":0">{{Cite web |title=Planetary Physical Parameters |url=https://ssd.jpl.nasa.gov/planets/phys_par.html |access-date=January 24, 2024 |website=Jet Propulsion Laboratory}}</ref>
|-
|[[Saturn]]
|0.0866
|{{val|60268|fmt=commas}}<ref name=":0" />
|-
|[[Uranus]]
|0.03673
|{{val|25559|fmt=commas}}<ref name=":0" />
|-
|[[Neptune]]
|0.03559
|{{val|24764|fmt=commas}}<ref name=":0" />
|-
|[[Earth]]
|0.009168
|{{val|6378|fmt=commas}}<ref name=":0" />
|-
|[[Venus]]
|0.00869
|{{val|6051.8|fmt=commas}}<ref name=":0" />
|-
|[[Mars]]
|0.00488
|{{val|3396.19|fmt=commas}}<ref name=":0" />
|-
|[[Mercury (planet)|Mercury]]
|0.0035
|{{val|2440.53|fmt=commas}}<ref name=":0" />
|-
|[[Moon]]
|0.0025
|{{val|1738.1|fmt=commas}}<ref>{{Cite web |title=Moon Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html |access-date=January 24, 2024 |website=nssdc.gsfc.nasa.gov}}</ref>
|-
|[[Pluto]]
|0.0017
|{{val|1188.3|fmt=commas}}<ref name=":0" />
|}

== See also ==
* [[Astronomical unit]]
* [[Earth radius]]
* [[Jupiter radius]]
* [[List of largest stars]]
* [[Orders of magnitude (length)]]
* [[Solar luminosity]]
* [[Solar mass]]
* [[Solar parallax]]

== References ==
{{Reflist}}

== External links ==

*{{cite journal
| author=S. C. Tripathy| author2=H. M. Antia
| title=Influence of surface layers on the seismic estimate of the solar radius
| journal=Solar Physics
| date=1999
| volume=186
| issue=1/2
| pages=1–11
| bibcode=1999SoPh..186....1T | doi=10.1023/A:1005116830445
| s2cid=118037693
}}
*{{cite journal
| author=T. M. Brown| author2=J. Christensen-Dalsgaard| author2-link=Jørgen Christensen-Dalsgaard
| title=Accurate Determination of the Solar Photospheric Radius
| journal=Astrophysical Journal Letters
| date=1998
| volume=500
| issue=2
| pages=L195
| bibcode=1998ApJ...500L.195B | doi=10.1086/311416
|arxiv = astro-ph/9803131 | s2cid=13875360}}

{{Units of length used in Astronomy}}
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[[Category:Sun|Radius]]
[[Category:Stellar astronomy]]
[[Category:Units of length]]
[[Category:Radii]]