Gegenschein (German pronunciation: [ˈɡeːɡənʃaɪn] German for "countershine") is a faintly bright spot in the night sky, around the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon.
Like the zodiacal light, the gegenschein is sunlight scattered by interplanetary dust. Most of this dust is orbiting the Sun in about the ecliptic plane, with a possible concentration of particles at the L2 Earth–Sun Lagrangian point.
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 the gegenschein is relatively enhanced, because each dust particle is seen in full phase.
The gegenschein was first described by the French Jesuit astronomer and professor Esprit Pezenas (1692–1776) in 1730. Further observations were made by the German explorer Alexander von Humboldt during his South American journey from 1799 to 1803. It was also Humboldt who gave the phenomenon its German name Gegenschein.
The Danish astronomer Theodor Brorsen published the first thorough investigations of the gegenschein in 1854, stating that Pezenas was the first to see it. T. W. Backhouse discovered it independently in 1876, as did Edward Emerson Barnard in 1882. In modern times, the gegenschein is not visible in most inhabited regions of the world due to light pollution.
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- Levasseur-Regourd, Anny-Chantal; Hiroichi Hasegawa (1991). Origin and Evolution of Interplanetary Dust. International Astronomical Union Colloquium. p. 159. ISBN 0-7923-1365-8.
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