Gegenschein (; ; ) or counterglow is a faintly bright spot in the

night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include ...

centered at the

antisolar point The antisolar point is the abstract point on the celestial sphere directly opposite the Sun from an observer's perspective. This means that the antisolar point lies above the horizon when the Sun is below it, and vice versa. On a sunny day, t ...

. The

backscatter In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, a ...

sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...

interplanetary dust The interplanetary dust cloud, or zodiacal cloud (as the source of the zodiacal light), consists of cosmic dust (small particles floating in outer space) that pervades the space between planets within planetary systems, such as the Solar System. ...

causes this

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the sun or moon wit ...

Explanation

zodiacal light The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction in ...

, gegenschein is sunlight scattered by

. Most of this dust orbits the Sun near the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...

plane, with a possible concentration of particles centered at the point of the Earth–Sun system. Gegenschein is distinguished from zodiacal light by its high

angle of reflection Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The ' ...

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. The intensity of the gegenschein is relatively enhanced because each dust particle is seen at full phase, having a difficult to measure

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

of +5 to +6, with a very low

surface brightness In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on its ...

in the +10 to +12 magnitude range.

History

It is commonly stated that the gegenschein was first described by the French

Jesuit , image = Ihs-logo.svg , image_size = 175px , caption = ChristogramOfficial seal of the Jesuits , abbreviation = SJ , nickname = Jesuits , formation = , founders ...

astronomer and professor (1692–1776) in 1730. Further observations were supposedly made by the German explorer

Alexander von Humboldt Friedrich Wilhelm Heinrich Alexander von Humboldt (14 September 17696 May 1859) was a German polymath, geographer, naturalist, explorer, and proponent of Romantic philosophy and science. He was the younger brother of the Prussian minister, p ...

during his South American journey from 1799 to 1803. It was Humboldt who first used the German term ''Gegenschein''. However, research conducted in 2021 by

Texas State University Texas State University is a public research university in San Marcos, Texas. Since its establishment in 1899, the university has grown to the second largest university in the Greater Austin metropolitan area and the fifth largest university ...

astronomer and professor Donald Olson discovered that the Danish astronomer

Theodor Brorsen Theodor Johan Christian Ambders Brorsen (29 July 1819 – 31 March 1895) was a Danish astronomer. He is best known for his discovery of five comets, including the lost periodic comet, 5D/Brorsen, and the periodic comet 23P/Brorsen-Metcalf. ...

was actually the first person to observe and describe one in 1854, although Brorsen had thought that Pézenas had observed it first. 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. T. W. Backhouse discovered it independently in 1876, as did

Edward Emerson Barnard Edward Emerson Barnard (December 16, 1857 – February 6, 1923) was an American astronomer. He was commonly known as E. E. Barnard, and was recognized as a gifted observational astronomer. He is best known for his discovery of the high proper mo ...

in 1882. In modern times, the gegenschein is not visible in most inhabited regions of the world due to

light pollution Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...

References

External links

Gegenschein page on EarthSky.org
taken from Stromboli volcano
"Zodiacal Light and the Gegenschein", an essay by J. E. Littleton
* *{{Cite EB1911, wstitle=Gegenschein Observational astronomy Optical phenomena de:Gegenschein

Explanation

History

See also

References

External links