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   Opposition effect mechanism 


       Grey objects casting black shadows onto a light surface.
Simulations at three different magnifications. The objects could be lunar soil grains, earthly rocks, sand or fir trees.


When looking direction away from the sun (the centre of the simulations) shadows are hidden by the objects themselves.  Away from the antisolar point they become increasingly visible.  

The antisolar region is brighter because it contains less shadow and more sunlit surfaces than its surroundings.

The lower simulations resemble the opposition effect as seen from an aircraft over forests, deserts and even cornfields. 

The effect is sometimes visible from the ground - for example, it can be seen on rough road surfaces or in the dry powdery sand above the high water line of a beach.

Antisolar brightenings have other causes.  Retro-reflection might also occur from isolated crystals within rocks and there is an effect called coherent backscattereing.   On Earth, where there is nearly always plenty of water droplets around, the heiligenschein and glories produce their effects and these are sometimes hard to separate from those made purely by shadow hiding.