Solar radius is a unit ofdistance used to express the size of objects inastronomy relative to theSun. The solarradius is usually defined as the radius to the layer in theSun'sphotosphere where theoptical depth equals 2/3:^[1]

$${\displaystyle 1R_{\odot }=6.957\times {10}^8\text{ m}}$$

695,700 kilometres (432,300 miles) is approximately 10 timesthe average radius ofJupiter, 109 timesthe radius of theEarth, and 1/215 of anastronomical 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'srotation, which induces anoblateness in the order of 10 parts per million.^[2]

The uncrewedSOHO 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 696,342 ± 65 kilometres (432,687 ± 40 miles).^[4]

Haberreiter, Schmutz & Kosovichev (2008)^[1] determined the radius corresponding to the solar photosphere to be 695,660 ± 140 kilometres (432,263 ± 87 miles). This new value is consistent with helioseismic estimates; the same study showed that previous estimates using inflection point methods had been overestimated by approximately 300 km (190 mi).

In 2015, theInternational Astronomical Union passed Resolution B3, which defined a set of nominal conversion constants for stellar and planetaryastronomy. Resolution B3 defined thenominal solar radius (symbol${\displaystyle R_{\odot }^N}$ ) to be equal toexactly695700 km.^[5] 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^[6] only known to about an accuracy of ±100–200 km).

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 as45 R_☉)
• Parker Solar Probe (as close as9 R_☉)

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

• S. C. Tripathy; H. M. Antia (1999). "Influence of surface layers on the seismic estimate of the solar radius".Solar Physics.186 (1/2):1–11.Bibcode:1999SoPh..186....1T.doi:10.1023/A:1005116830445.S2CID 118037693.
• T. M. Brown;J. Christensen-Dalsgaard (1998). "Accurate Determination of the Solar Photospheric Radius".Astrophysical Journal Letters.500 (2): L195.arXiv:astro-ph/9803131.Bibcode:1998ApJ...500L.195B.doi:10.1086/311416.S2CID 13875360.