The solar radius R - half the diameter of the sun - is used in astronomy as a unit of measurement to indicate the size of other celestial bodies, especially stars . It is 6.96342 · 10 8  m = 696,342 km ± 65 km or 109 times the mean earth radius . The earth - moon system provides a clear comparison : the distance between the earth and the moon is on average 384,400 km or 55 percent of the solar radius. If the sun were instead of the earth, the lunar orbit would run entirely within the sun - a little further outside than half the solar sphere measures.

As a result of its rotation , the sun is slightly flattened ( f = (8.3 ± 1.9) · 10 −6 ), which could only be demonstrated in the 2000s.

## Measurement methods

Different methods are used to measure the sun's radius, with which consequently different radius terms are associated.

From the angle measurements between the two edges of the sun, the diameter of the sun disk results , from which the sun size itself results by multiplying it by the distance between earth and sun (mean value is an astronomical unit ). Determining this distance, however, has been a problem for centuries.

The easiest way to get the apparent diameter of the sun is by measuring the time, analogous to a star passage , whereby a telescope with a sun filter and a thread net is required. The duration of the sunset also gives good results and was probably already used by Babylonian priest astronomers and in ancient Greece to determine the sun's diameter (see Aristarchus of Samos , who first estimated the size of the sun to be ten times that of the earth).

Direct measurements with optical micrometers became possible from around 1750, in the 19th century the Fraunhofer heliometer was developed to search for the suspected flattening of the sun .

With the help of helioseismological measurements of the f-modes of surface waves of the sun, a value of around 695.8 µm was determined. Photoelectric measurements and their comparison with models of the limb darkening function of the sun resulted in a value of approx. 695.5 · 10 6  m for the mean radius near the equator.

Further methods are the measurement of the transit time of Mercury or the optical determination of the angular dimensions of the sun. The angular dimension of the sun is about 16 ' when viewed from the earth , the entire diameter of the solar disk is 32' or 0.53 °. Because of the somewhat elliptical orbit of the earth , however, the value fluctuates by 1.7% in both directions: in the perihelion of the earth's orbit (beginning of January) it is 32'32 ", in aphelion (beginning of July) it is only 31'28". We owe the rare but impressive phenomena of total solar eclipses to the fact that the apparent moon diameter has a similar, but more strongly fluctuating angle (29′10 ″ –33′30 ″).

In astro-geodetic measurements of the direction to the sun - for example in solar azimuths - the center of the solar disk should be aimed at. As this special instruments such as the Roelofs- sun prism would require you aim in practice the right and left edge of the sun , and averages the two measurements. The sun's radius, taken from ephemeris , divided by the sine of the zenith distance can serve as a control .

## literature

• TM Brown, J. Christensen-Dalsgaard: Accurate Determination of the Solar Photospheric Radius , Astrophys. J. Lett. 500, 1998, p. L195, arxiv : astro-ph / 9803131v1 .
• Precisely measured sun - article on scienceticker.info from March 27, 2012

## Individual evidence

1. Marcelo Emilio et al .: Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits. In: Astrophysical Journal Vol. 750, No. 2, bibcode : 2012ApJ ... 750..135E , doi : 10.1088 / 0004-637X / 750/2/135
2. Jean-Pierre Rozelot: What is Coming: Issues Raised from observation of the shape of the Sun . In: JP Rozelot, Coralie Neiner (ed.): The Rotation of Sun and Stars , Springer, 2009, ISBN 978-3-540-87830-8 , limited preview in Google Book Search