Solar radius

The solar radius R ☉ - half the diameter of the sun - is used in astronomy as a unit to specify the size of other celestial bodies, in particular stars. He is 6.96342 · 108 m = 696,342 km ± 65 km or the 109 times the mean radius of the Earth. A clear comparison provides the Earth-Moon System: The Earth-Moon distance is on average 384.4 thousand kilometers or 55 percent of the solar radius. Would the sun instead of the earth, the moon's orbit would run completely inside the sun - something outside measures more than half the solar orb.

Due to the rotation of the Sun is slightly flattened (f = ( 8.3 ± 1.9 ) · 10-6).

Measurement methods

There are several methods for measuring the solar radius, which assists various radius terms are therefore linked.

From the angular measurements between two sun edges gives the diameter of the solar disk, from which the sun size is determined by multiplying itself with Earth-Sun distance (average is one astronomical unit ). The determination of this distance, however, was for centuries a problem.

The apparent solar diameter is erthält easiest by timing similar to a star passage, a telescope with sun filters and reticle is needed. The duration of the sunset are already good results and was probably already of Babylonian priests and astronomers in ancient Greece (see Aristarchus of Samos, the size of the sun for the first time estimated as ten times the Earth ).

Direct measurements with optical micrometers were possible from about 1750, in the 19th century, the Fraunhofer'sche Heliometer was to search for the suspected Sonnenabplattung developed.

With the help of helioseismologischen measurements of the f - modes of surface waves of the sun, a value of approximately 695.8 mm was determined. Photoelectric measurements and their comparison with models of limb - darkening function of the sun gave a value of about 695.5 x 106 m for the mean radius near the equator.

Other methods are the measurement of the transit time of Mercury or the optical determination of the angular size of the sun. The angular size of the Sun is from the Earth as seen from about 16 ', the whole diameter of the solar disk that is 32' or 0.53 °. However, due to the slightly elliptical orbit of the earth, the value varies by 1.7 % in both directions: at perihelion of the Earth's orbit ( beginning of January ) there are 32'32 ", at aphelion (early July ) but only 31'28 ". The rare but impressive phenomena of total solar eclipses we owe the fact that we have the moon randomly appears under a similar, though somewhat varying angle.

In astro- geodetic measurements of direction to the sun - such as solar azimuths - would be the center of the solar disk to target. Since this would require special instruments such as the Roelofs - sun prism, it aims in fact to the right and left edge of the sun and averages the two measurements. As a control can serve the ephemeris taken from the solar radius, divided by the sine of the zenith distance.