English: Ratio of solar flux above the atmosphere to flux from a black body at 5775 K, with a size in the sky of 0.00000679 steradians,which is the average size of the sun. The sun emits the amount of light that would be emitted by a black body at 5775 with that size in the sky, but the spectrum is actually quite different. The black-body spectrum in watts per square metre per nanometre is given by ${\displaystyle \frac{{\lambda }^{-5}}{\exp (\frac{14387777}{\lambda T})-1}\times 1.191\times 10^{20}\times (1391400/2/149597871{)}^2\pi }$ where ${\displaystyle \lambda }$ is the wavelength in nanometres and ${\displaystyle T}$ is the temperature in Kelvin (5775). The solar data are from The sun’s total and spectral irradiance for solar energy applications and solar radiation models by Christian Gueymard (available from academia.edu). In the extreme ultraviolet and x-ray range (to the left of this graph) the solar radiation is much higher than the 5775K black-body radiation, and is variable. Interestingly, the solar spectrum does match a black-body spectrum quite well in the wavelength range 20-250 microns, but for a temperature of about 4000 K (that is, proportional to but weaker than for 5775 K, given the size of the sun in the sky).