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This is because the solar constant is evaluated at a fixed distance of 1 Astronomical Unit (au) while the solar irradiance will be affected by the eccentricity of the Earth's orbit. Its distance to the Sun varies annually between 147.1·10 6 km at perihelion and 152.1·10 6 km at aphelion .
The solar constant is a conventional measure of mean TSI at a distance of one astronomical unit (AU). Direct normal irradiance (DNI), or beam radiation, is measured at the surface of the Earth at a given location with a surface element perpendicular to the Sun direction.
Solar radiation pressure on objects near the Earth may be calculated using the Sun's irradiance at 1 AU, known as the solar constant, or G SC, whose value is set at 1361 W/m 2 as of 2011. [17] All stars have a spectral energy distribution that depends on their surface temperature. The distribution is approximately that of black-body radiation.
The solar constant is equal to approximately 1,368 W/m 2 (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, at or near Earth's orbit). [99] Sunlight on the surface of Earth is attenuated by Earth's atmosphere , so that less power arrives at the surface (closer to 1,000 W/m 2 ) in clear conditions when ...
Radiative forcing is defined in the IPCC Sixth Assessment Report as follows: "The change in the net, downward minus upward, radiative flux (expressed in W/m 2) due to a change in an external driver of climate change, such as a change in the concentration of carbon dioxide (CO 2), the concentration of volcanic aerosols or the output of the Sun." [3]: 2245
the right hand side represents the total outgoing longwave power (in Watts) from Earth, calculated from the Stefan–Boltzmann law. The constant parameters include S is the solar constant – the incoming solar radiation per unit area—about 1367 W·m −2; r is Earth's radius—approximately 6.371×10 6 m; π is the mathematical constant (3. ...
Of the ~340 W/m 2 of solar radiation received by the Earth, an average of ~77 W/m 2 is reflected back to space by clouds and the atmosphere and ~23 W/m 2 is reflected by the surface albedo, leaving ~240 W/m 2 of solar energy input to the Earth's energy budget. This amount is called the absorbed solar radiation (ASR).
The earth's atmosphere absorbs a considerable amount of the ultraviolet light. The resulting spectrum at the Earth's surface has fewer photons, but they are of lower energy on average, so the number of photons, above the bandgap, per unit of sunlight energy is greater than in space. This means that solar cells are more efficient at AM1 than AM0.