Solar radius

 ( Radius ) 

+ Conversion of nominal solar radius ! 1 = ! Units

astronomical unit

light-year

parsec

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:

$$1\backslash ,R\_\{\backslash odot\}\; =\; 6.957\backslash times\; 10^8\; \backslash hbox\{\; m\}$$

is approximately 10 times [[the average radius|Jupiter radius]] of [[Jupiter]], 109 times [[the radius|Earth 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|stellar rotation]], which induces an [[oblateness|flattening]] in the order of 10 parts per million. The '''solar diameter''' is double the solar radius.
     

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Measurements 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 .

Haberreiter, Schmutz & Kosovichev (2008) determined the radius corresponding to the solar photosphere to be . This new value is consistent with helioseismic estimates; the same study showed that previous estimates using inflection point methods had been overestimated by approximately .

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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 $R^\{N\}\_\{\backslash odot\}$) to be equal to exactly . 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 only known to about an accuracy of ±).

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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 )
• Parker Solar Probe (as close as )

+Radius of another objects relative to the Sun's radius !Name !Radius !Radius (kilometers)
Milky Way
UY Scuti	909
Betelgeuse	764
Antares A	680
Rigel A	74.1
Aldebaran	45.1
Arcturus	25.4
Pollux	9.06
Sirius A	1.711
Sun	1
Proxima Centauri	0.1542
Jupiter	0.1028
Saturn	0.0866
Uranus	0.03673
Neptune	0.03559
Earth	0.009168
Venus	0.00869
Mars	0.00488
Mercury	0.0035
Moon	0.0025
Pluto	0.0017

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See also

• Astronomical unit
• Earth radius
• Jupiter radius
• List of largest stars
• Orders of magnitude (length)
• Solar luminosity
• Solar mass
• Solar parallax

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External links

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