Geometry of the scattering in the Earth’s atmosphere (cf. Wolstencroft & van Breda 1967; Noll et al. 2012). Point M, where the moonlight enters the top of the atmosphere, is not in the same plane as the other points. The azimuth of M as seen from S is A_M (not shown). S and T are at an azimuth A₀ (also not shown) for an observer at O. Single scattering is shown at point S along paths s₁ and s₂. Double scattering occurs at point D or off the ground G (Sect. 2.4).