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Solar radius is a unit of distance used to express the size of stars in astronomy. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3:
The solar radius is approximately 695,700 kilometres (432,300 miles), which is about 10 times the average radius of Jupiter, about 109 times the radius of the Earth, and 1/215th of an astronomical unit, the distance of the Earth from the Sun. It varies slightly from pole to equator due to its rotation, which induces an oblateness in the order of 10 parts per million. (See 1 gigametre for similar distances.)
The unmanned 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).
Haberreiter, Schmutz & Kosovichev (2008) 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 ) to be equal to exactly 695,700 km. The nominal values were 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 the ±100–200 km accuracy).
Solar radii as a unit are popular when talking about spacecraft moving close to the sun. Two spacecraft in the 2010s.
- NASA RHESSI oblateness measurements 2012
- Emilio, Marcelo; Kuhn, Jeff R.; Bush, Rock I.; Scholl, Isabelle F., "Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits", The Astrophysical Journal, 750: 135, arXiv: , Bibcode:2012ApJ...750..135E, doi:10.1088/0004-637X/750/2/135
- Haberreiter, M; Schmutz, W; Kosovichev, A.G., "Solving the Discrepancy between the Seismic and Photospheric Solar Radius", Astrophysical Journal, 675 (1): L53–L56, arXiv: , Bibcode:2008ApJ...675L..53H, doi:10.1086/529492
- Mamajek, E.E.; Prsa, A.; Torres, G.; et, al., IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties, arXiv: , Bibcode:2015arXiv151007674M
- 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.