The Energy Flux of Direct Solar Radiation The Sun can be considered as a spherical source of black-body radiation. In this case the energy flux is given by [11]: where B is a geometric factor and Ts is the temperature of the Sim's photosphere. However, there are two ways of using the general formula (1). In a first approximation, the Sun can be considered as a source of isotropic radiation. In this case the following equations apply: Here Q is the solid angle subtended by the Sun when viewed from the receiving surface, while 0 is the angle between the normal of the receiving surface and the direction to the center of the solar disc. Delta (5) is the half angle of the cone subtended by the Sun (figure 1). However, it is known that the solar brightness falls considerably with the distance from the center of the disc. The effect, which is referred to as "darkening to the limb” is a consequence of the fact that the Sun is a non-isotropic source of radiation. In a recent paper [12 ] we proposed a simple formula for the solar energy flux which, on one hand, includes the limb darkening effect and, on the other one, is valid at any distance from the Sun. In this second case we have:
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