gegenschein or counterglow, a slight brightening of the night sky in the region of the zodiac directly opposite the sun, i.e., 180° from the sun. Discovered by the Danish astronomer Theodor Brersen in 1854, its nature and cause are still unknown. It may be caused by reflection of sunlight by small dust particles that lie in the plane of the solar system. It is possible that these particles are part of a tail of the earth, extending away from the sun like the tail of a comet. The gegenschein is one of several sky glows, as is the zodiacal light.

or counterglow

Oval patch of faint light exactly opposite the Sun in the night sky, so faint it can be seen only in the absence of moonlight, away from city lights, with the eyes adapted to darkness. It is lost in the light of the Milky Way except in February, March, April, August, September, and October. The gegenschein and the zodiacal light form the most notable parts of a band of very faint light along the ecliptic. Both phenomena are thought to be the result of the reflection of sunlight from interplanetary dust grains.

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