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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It is distinguished from zodiacal light by its high angle of reflection of the incident sunlight on the dust particles. It forms a slightly more luminous, oval glow directly opposite the Sun within the band of luminous zodiacal light. The intensity of gegenschein is (relatively) enhanced because (a) each dust particle is seen in full phase, and (b) the backscattering geometry leads to constructive interference.
The Danish astronomer Theodor Brorsen published the first thorough investigations of the gegenschein in 1854. He was also the first to observe that the Zodiacal light can embrace the complete sky, because under favorable conditions, a feeble light bridge connecting the Zodiacal light and the gegenschein can be observed. Besides, Brorsen had already proposed the correct explanation for the gegenschein (interplanetary dust reflections).