Scattering of direct solar radiation by the earth’s atmosphere under twilight conditions occurs in a certain effective layer, whose height is a function of the sinking of the sun. This was first demonstrated by V. G. Fesenkov  and later examined in detail by N. M. Shtaude . The thickness of the layer that determines the main flow of scattered radiation is not great, so in the first approximation the layer is often replaced by a single ray (half-line), which N. M. Shtaude called the “twilight ray.” This twilight ray determines the mean height of the scattering layer for a given solar zenith distance. In Fig. 1, the twilight ray is shown by line SM, which is at distance R + ho from the center of the earth (R is the radius of the earth). When observations are made in direction BM, the height of the twilight layer in this direction is the difference h = OM −R, i.e., the height above the earth’s surface of the point of intersection of the line of sight BM and the twilight ray SM.
KeywordsScattered Radiation Monochromatic Radiation Direct Solar Radiation Solar Radiation Flux Effective Layer
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