Gegenschein (/ˈɡeɪɡənˌʃaɪn/;German:[ˈɡeːɡn̩ˌʃaɪn];lit. 'counter-shine') orcounterglow is a faintly bright spot in thenight sky centered at theantisolar point. Thebackscatter ofsunlight byinterplanetary dust causes thisoptical phenomenon, being azodiacal light and part of its zodiacal light band.
Likezodiacal light, gegenschein is sunlightscattered byinterplanetary dust. Most of this dust orbits the Sun near theecliptic plane, with a possible concentration of particles centered at theL2 point of the Earth–Sun system.[1]
Gegenschein is distinguished from zodiacal light by its highangle of reflection of the incident sunlight on the dust particles. It forms a slightly brighter elliptical spot of 8–10° across directly opposite the Sun within the dimmer band of zodiacal light and zodiac constellation.[2] The intensity of the gegenschein isrelatively enhanced because each dust particle is seen atfull phase,[3] having a difficult to measureapparent magnitude of +5 to +6, with a very lowsurface brightness in the +10 to +12 magnitude range.[4]
It is commonly stated that the gegenschein was first described by the FrenchJesuit astronomer and professorEsprit Pézenas [fr] (1692–1776) in 1730. Further observations were supposedly made by the German explorerAlexander von Humboldt during his South American journey from 1799 to 1803. It was Humboldt who first used the German termGegenschein.[5] However, research conducted in 2021 byTexas State University astronomer and professorDonald Olson discovered that the Danish astronomerTheodor Brorsen was actually the first person to observe and describe one in 1854, although Brorsen had thought that Pézenas had observed it first.[6] Olson believes what Pézenas actually observed was an auroral event, as he described the phenomenon as having a red glow; Olson found many other reports of auroral activity from around Europe and Asia on the same date Pézenas made his observation. Humboldt's report instead described glowing triangular patches on both the western and eastern horizons shortly after sunset, while true gegenschein is most visible near local midnight when it is highest in the sky.
Brorsen published the first thorough investigations of the gegenschein in 1854.[7] T. W. Backhouse discovered it independently in 1876, as didEdward Emerson Barnard in 1882.[8] In modern times, the gegenschein is not visible in most inhabited regions of the world due tolight pollution.
