Extraterrestrial Irradiance
This function computes the extraterrestrial irradiance outside of the earth’s atmosphere, based a Fourier series representation of the position of the sun (earth-sun distance).
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Reference
Bird, R., Hulstrom, R., A Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces, 1981
NREL SERI/TP-642-761
Spencer, J. W., Fourier Series Representation of the Position of the Sun, Vol. 2, 1971, p. 172.