gegenschein [key] 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, it is caused by reflection of sunlight by interplanetary dust particles that lie in the plane of the solar system. The brightening is relatively faint and is obscured by moonlight and, in the weeks on either side of the solstices, by the Milky Way. The gegenschein is one of several sky glows, as are zodiacal light and airglow.
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