Solar radius

A solar radius is a unit of distance, commonly understood as 695,700 km and expressed as ${\displaystyle R_{\odot }}$, used mostly to express the size of an astronomical objects relative to that of the Sun, or their distance from it. This length is also called the nominal solar radius. The sun's actual radius, from which the unit of measurement is derived, is usually calculated as the radius from the sun's center out to the layer in the Sun's photosphere where the optical depth equals 2/3.^[1] One solar radius can be described as follows:$${\displaystyle 1\,R_{\odot }=6.957\times 10^{8}{\hbox{ m}}}$$This is an approximation: both because such distance is difficult to measure and can be measured in various ways, and because the sun is not a perfectly spherical object itself, and thus the actual radius varies depending on the point(s) measured and modality of measurement employed.

Conversion of nominal solar radius

1 R☉ =	Units
6.95700×108	metres
695,700	kilometres
0.00465047	astronomical unit
432,288	miles
7.35355×10−8	light-year
2.25461×10−8	parsec
2.32061	light-seconds

695,700 kilometres (432,300 miles) is approximately 10 times the average radius of Jupiter; 109 times the 6378 km radius of the Earth at its equator; and ${\textstyle {1 \over 215}}$ or 0.0047 of an astronomical unit, the approximate average distance between Earth and the Sun. The solar radius to the sun's poles 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]

The solar diameter is double the solar radius.

Measurements

Evolution of the solar luminosity, radius and effective temperature compared to the present-day Sun. After Ribas (2009)^[3]

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 140 km uncertainty band 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 units of distance measurement are common especially when describing the paths of spacecraft moving close to the sun. Two such spacecraft in the 2010s include:

• Solar Orbiter (which flew as close as 45 R_☉ to the sun)
• Parker Solar Probe (which flew as close as 9 R_☉ to the sun)

Radius of other objects relative to the Sun's radius

Name	Radius (Solar radius)	Radius (kilometers)
Milky Way	5.94×1011	4.134×1017[7]
UY Scuti	909[8]	632,400,000
Betelgeuse	764[9]	531,500,000
Antares A	680[10]	473,076,000
Rigel A	74.1[11]	51,550,000
Aldebaran	45.1[12]	31,375,000
Arcturus	25.4[13]	17,670,000
Pollux	9.06[14]	6,300,000
Sirius A	1.711[15]	1,190,350
Sun	1	695,700
Proxima Centauri	0.1542[16]	107,275
Jupiter	0.1028	71,492[17]
Saturn	0.0866	60,268[17]
Uranus	0.03673	25,559[17]
Neptune	0.03559	24,764[17]
Earth	0.009168	6,378[17]
Venus	0.00869	6,051.8[17]
Mars	0.00488	3,396.19[17]
Mercury	0.0035	2,440.53[17]
Moon	0.0025	1,738.1[18]
Pluto	0.0017	1,188.3[17]

See also

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