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, also called counterglow.
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.
It is commonly stated that the gegenschein was first described by the French Jesuit astronomer and professor Esprit Pézenas (1692–1776) in 1730. However, research conducted in 2021 by Texas State University astronomer and professor Donald Olson discovered that the Danish astronomer Theodor Brorsen was actually the first person to observe and describe one in 1854, although Brorsen had thought that Pézenas had observed it first. 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.
Brorsen published the first thorough investigations of the gegenschein in 1854, believing 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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