Several atmospheric effects are formed by light effects that occur while looking directly away from the sun. The particular optics that form these effects ensure that they are always centered around the head of the observer's cast shadow.

They are the heiligenschein, the dewbow, the glory, and the opposition effect.

The opposition effect is a brightening of the area around the head of the observer's shadow. When reflecting off of many column-shaped objects, such as plant stalks, the effect will continue in vertical columns above and below the main focus.

Two factors account for this effect. The first has nothing to do with refraction or diffusion, just proper alignment. With the sun directly behind you, the shadows of any objects along the sun-eye axis pass out of sight—hidden behind themselves. (A charming extension of this idea is that the sun may simply not know about shadows at all. I imagine the moon gets frustrated trying to explain it.)

The term originated with planetary observations. Ancient astronomers noticed that Mars and the moon at opposition appear brighter than could be accounted for by extrapolating the luminance of their partial phases. The answer is that the surfaces themselves appeared brighter as the shadows of their rocks and craters became hidden.

The second factor is that if there are clear or translucent particles in the surface, the physical effect will be enhanced by coherent backscattering as occurs in the heiligenschein. In fact the opposition effect is also known as the "dry heiligenschein".

Most irregular surfaces will display this affect at the antisolar point. The astronauts saw it in the regolithic dust around their shadows on the moon, farmers observe it in dry wheat fields when the sun is low in the sky, and pilots often see it amongst flora below. You can even find it in gravel on roads or in dry powdery sand on the beach if you're paying attention.


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