Solar constant
As solar constant E0 is the long-standing averaged extraterrestrial solar irradiance ( intensity) is called, striking the earth at mean Earth-Sun distance without the influence of the atmosphere perpendicular to the beam direction. The term " constant" is used conventionally, although it is not a natural constant.
Definition and seasonal variation
The mean value for the solar constant was set in 1982 by the World Meteorological Organization in Geneva to:
Due to the orbital eccentricity of the distance of the Earth to the Sun varies periodically between 147.1 years and 152.1 · 106 km x 106 km. With him the irradiance varies on earth between 1325 W / m and 1420 W / m². In the perihelion value is thus above about 3.4% and about 3.3 % at aphelion below the annual average.
At the bottom 54% of the incident solar energy arrive, so an average of about 740 W / m².
Fluctuations and long-term increase
The radiation power of the sun itself is almost constant. Also, the eleven-year sunspot cycle causes only changes - both in the visible spectrum as well as in the total radiation - less than 0.1 %.
In the UV range below 170 nm, the radiation may vary by a factor of 2. In the X-ray range between 0.2 and 3 nm, the radiation power can change by up to two orders of magnitude.
In solar flares in X-rays between 0.1 and 0.8 nm also changes by more than five orders of magnitude (A1 to > X17 as on November 4, 2003) are possible.
Medium-term disturbances of the Earth's orbit, also influence the irradiance on the Earth are described by the Milankovitch cycles.
In the long term takes the radiation power of the sun due to the natural development as a main sequence star by about one percent every 100 million years to. Shortly after its creation was its luminosity is only about 70 % of today's value. Although this development in historical periods is irrelevant for the climate change, it must be taken into account in the assessment of the climate in the earlier history of the earth.
Angular dependence
The power per square meter always refers to a surface which is perpendicular to the radiation. When the light is not perpendicular to the irradiated surface of the radiation power relative to the irradiated area:
Other facts
Continuously irradiating the radiation power on the earth of the sun can be calculated as the product of the solar constant at the surface of the Erdkontur. The Erdkontur is approximately a circle with radius of the earth. The whole of the earth radiation applied power of the sun is thus about 174 PW. For comparison, the world energy demand was 140 PWh per year in 2010. So the sun radiates more energy in one hour just to Earth than the current annual world energy demand is.
The Earth's atmosphere and its climate influence the global radiation on the earth's surface. The geometric influence describes the air mass ( air mass ).
To exclude the influence of atmosphere, the solar constant can be made in space since 1978 measurements. The SOHO satellite was launched in 1995 leads continuous observations of the Sun by the radiometer Virgo. The measurements are coordinated by the Royal Meteorological Institute of Belgium.
Radiant power of the Sun
From the solar constant, the radiant power Φ of the sun can be calculated by multiplying the surface area A of the said envelope sphere around the Sun, which = the radius of the middle Erdabstands r 149.6 · 109 m has:
The magnitude of the radiation output of the sun can be estimated with the Stefan- Boltzmann law, and thus inversely estimate the solar constant.