The gegenschein is a German word for the opposition effect. Like dry heiligenschein, the gegenschein is caused by three-dimensional shadowing of an open structure, which shadows are absent if you look to the antisolar point, along the direction of the sunlight. In this case, the open structure is the vast cloud of billions of small dust particles floating in the inner solar system. The gegenschein is a small (typically 10x20 degrees) area of light centered on the opposition (antisolar) point, which is above the horizon during the night. It is very hard to detect, due to the low brightness. On a dark night at a location far from cities, however, it is unmistakable.
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