Solar radius

class="wikitable floatright" \|+ Conversion of nominal solar radius ! 1 {{solar radius}} = ! Units
{{val\|6.95700\|e=8}}	metres
695,700	kilometres
0.00465047	astronomical unit
432,288	miles
{{val\|7.35355\|e=-8}}	light-year
{{val\|2.25461\|e=-8}}	parsec
2.32061	light-seconds

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 equals 2/3:

$1\,R_{\odot} = 6.957\times 10^8 \hbox{ m}$

{{convert|695,700|km|mi|abbr=off}} is approximately 10 times the average radius of Jupiter, 109 times the radius of the Earth, and 1/215 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 rotation, which induces an oblateness in the order of 10 parts per million.{{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 }}

Measurements

File:Solar evolution (English).svg, radius and effective temperature compared to the present-day Sun. After Ribas (2009){{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 }}]]

The uncrewed 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}}.{{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 }}

Haberreiter, Schmutz & Kosovichev (2008){{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 }} 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 $R^{N}_{\odot}$ ) to be equal to exactly {{val|695700|u=km}}.{{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 }} 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{{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 }} 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}}{{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}}

UY Scuti

|909{{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}}

|{{val|632400000|fmt=commas}}

Betelgeuse

|764{{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}}

|{{val|531500000|fmt=commas}}

Antares A

|680{{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}}

|{{val|473076000|fmt=commas}}

Rigel A

|74.1{{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}}

|{{val|51550000|fmt=commas}}

Aldebaran

|45.1{{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}}

|{{val|31375000|fmt=commas}}

Arcturus

|25.4{{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}}

|{{val|17670000|fmt=commas}}

Pollux

|9.06{{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 }}

|{{val|6300000|fmt=commas}}

Sirius A

|1.711{{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}}

|{{val|1190350|fmt=commas}}

style="background:#faf86b;"

|Sun

|{{val|695700|fmt=commas}}

Proxima Centauri

|0.1542{{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}}

|{{val|107275|fmt=commas}}

Jupiter

|0.1028

|{{val|71492|fmt=commas}}{{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}}

Saturn

|0.0866

|{{val|60268|fmt=commas}}

Uranus

|0.03673

|{{val|25559|fmt=commas}}

Neptune

|0.03559

|{{val|24764|fmt=commas}}

Earth

|0.009168

|{{val|6378|fmt=commas}}

Venus

|0.00869

|{{val|6051.8|fmt=commas}}

Mars

|0.00488

|{{val|3396.19|fmt=commas}}

Mercury

|0.0035

|{{val|2440.53|fmt=commas}}

Moon

|0.0025

|{{val|1738.1|fmt=commas}}{{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}}

Pluto

|0.0017

|{{val|1188.3|fmt=commas}}

References

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}}

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Radius

Category:Stellar astronomy

Category:Units of length

Category:Radii