Asolar radius is a unit ofdistance, commonly understood as 695,700 km and expressed as, used mostly to express the size of anastronomical objects relative to that of theSun, or their distance from it. This length is also called thenominal solar radius. The sun's actualradius, 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'sphotosphere where theoptical depth equals 2/3.[1] One solar radius can be described as follows: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.
695,700 kilometres (432,300 miles) is approximately 10 timesthe average radius ofJupiter; 109 times the 6378 kmradius of theEarth at its equator; and or 0.0047 of anastronomical 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'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) to be equal toexactly695700 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).
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 as45 R☉ to the sun)
^Mamajek, E.E.; Prsa, A.; Torres, G.; et, al. (2015),IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties,arXiv:1510.07674,Bibcode:2015arXiv151007674M
^Meftah, M; Corbard, T; Hauchecorne, A.; Morand, F.; Ikhlef, R.; Chauvineau, B.; Renaud, C.; Sarkissian, A.; Damé, L. (2018), "Solar radius determined from PICARD/SODISM observationsand extremely weak wavelength dependence in the visibleand the near-infrared",Astronomy & Astrophysics,616: A64,Bibcode:2018A&A...616A..64M,doi:10.1051/0004-6361/201732159
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