Opposition surge

The opposition effect is an optical phenomenon in the form of an apparent brightening of surfaces on the counter- point of a light source. Located as the sun in the back of an observer, the effect can be observed as a relatively bright area around the sun counter- point to the floor.

In contrast to the halo of the opposition effect occurs on dry surfaces. The reason is that surface structures, such as blades of grass, grains of sand, leaves, etc. to the equivalent point light source in front of the observer hide their own shadows.

In astronomy, the effect plays a role. Celestial bodies that have no atmosphere, appear significantly brighter when they exactly are in opposition to the Sun from the observer. The full moon, for example, is not twice, but more than ten times as bright as the crescent - for twice as large illuminated area is still in the full moon phase increased by the opposition effect surface brightness of the moon. The effect was first observed by Tom Gehrels in 1956 to an asteroid. Also in the rings of Saturn, he could be demonstrated.