From AMS Glossary
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A diffuse bright region surrounding the shadow an observer's head casts on a irregular surface.
It is most apparent when the sun is low in the sky and when the surface is dew-covered. The explanation of the heiligenschein varies depending upon whether it is seen over a dry or a dew- covered surface. When an observer's shadow is cast on a dry, irregular surface (such as gravel or vegetation), each irregularity near the antisolar point covers its own shadow. In other directions, the average brightness results from a mixture of sunlit and shaded surfaces. The lower the sun in the sky, the longer the shadows and so the greater the contrast with the brighter region near the antisolar point. While evident over virtually any irregular surface, the ready appearance of the heiligenschein in sunlit wooded areas when seen from an airplane has spawned the epithet, the "hot spot in the forest." Observations of the "hot spot in the forest." are undoubtedly all the more striking when the plane is high enough that its own shadow (the umbra portion) has vanished. The presence of dew on some species of grass greatly enhances the heiligenschein. Dewdrops held off the surface of the leaf by small hairs focus sunlight on the leaf where it is diffusely reflected. The drop, acting in a manner similar to the lens in a lighthouse, then collects a large fraction of this diffusely reflected light that would have otherwise gone in other directions and sends it back toward the source and the observer. The heiligenschein is occasionally called Cellini's halo, after Benvenuto Cellini who described its behavior in his memoirs of 1562. He even pointed out that "it appears to the greatest advantage when the grass is moist with dew," but felt its appearance bespoke "the wondrous ways of [God's] providence toward me." Shades of this interpretation are also found in the name heiligenschein: It is German for "the light of the holy one."