Gegenschein (German: [ˈɡeːɡənʃaɪn]; lit. "countershine") is a faintly bright spot in the night sky centered at the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon.
Like zodiacal light, gegenschein is sunlight scattered by interplanetary dust. Most of this dust orbits the Sun near the ecliptic plane, with a possible concentration of particles centered at the L2 point of the Earth–Sun system.
Gegenschein is distinguished from zodiacal light by its high angle of reflection of the incident sunlight on the dust particles. It forms a slightly brighter elliptical spot directly opposite the Sun within the dimmer band of zodiacal light. The intensity of the gegenschein is relatively enhanced because each dust particle is seen at 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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- Astrophotographs of the gegenschein
- "Zodiacal Light and the Gegenschein", an essay by J. E. Littleton
- NASA Astronomy Picture of the Day: The Gegenschein Over Chile (7 May 2008)
- Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. .