Solar radius is a unit of distance used to express the size of stars 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]

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

695,700 kilometres (432,300 miles) is approximately 10 times the average radius of Jupiter, 109 times 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 rotation, which induces an oblateness in the order of 10 parts per million.^[2]

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 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).

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 ${\displaystyle R_{\odot }^{N))$) to be equal to exactly 695700 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).

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 45 R_☉)
• Parker Solar Probe (as close as 9 R_☉)