Solar radius

Conversion of nominal solar radius

1 Template:Solar radius =	Units
Template:Val	metres
695,700	kilometres
0.00465047	astronomical unit
432,288	miles
Template:Val	light-year
Template:Val	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:

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

Template:Convert 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.

Measurements

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

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 Template:Convert.

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

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 $R_{\odot }^N$) to be equal to exactly Template:Val. 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 ±Template:Val).

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 Template:Val)
• Parker Solar Probe (as close as Template:Val)

Radius of another objects relative to the Sun's radius

Name	Radius Template:Nowrap	Radius (kilometers)
Milky Way	Template:Val	Template:Val
UY Scuti	909	Template:Val
Betelgeuse	764	Template:Val
Antares A	680	Template:Val
Rigel A	74.1	Template:Val
Aldebaran	45.1	Template:Val
Arcturus	25.4	Template:Val
Pollux	9.06	Template:Val
Sirius A	1.711	Template:Val
Sun	1	Template:Val
Proxima Centauri	0.1542	Template:Val
Jupiter	0.1028	Template:Val
Saturn	0.0866	Template:Val
Uranus	0.03673	Template:Val
Neptune	0.03559	Template:Val
Earth	0.009168	Template:Val
Venus	0.00869	Template:Val
Mars	0.00488	Template:Val
Mercury	0.0035	Template:Val
Moon	0.0025	Template:Val
Pluto	0.0017	Template:Val

See also

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

External links

• Template:Cite journal
• Template:Cite journal