gegenschein

gegenschein

[gey-guhn-shahyn]
gegenschein or counterglow, a slight brightening of the night sky in the region of the zodiac directly opposite the sun, i.e., 180° from the sun. Discovered by the Danish astronomer Theodor Brersen in 1854, its nature and cause are still unknown. It may be caused by reflection of sunlight by small dust particles that lie in the plane of the solar system. It is possible that these particles are part of a tail of the earth, extending away from the sun like the tail of a comet. The gegenschein is one of several sky glows, as is the zodiacal light.
or counterglow

Oval patch of faint light exactly opposite the Sun in the night sky, so faint it can be seen only in the absence of moonlight, away from city lights, with the eyes adapted to darkness. It is lost in the light of the Milky Way except in February, March, April, August, September, and October. The gegenschein and the zodiacal light form the most notable parts of a band of very faint light along the ecliptic. Both phenomena are thought to be the result of the reflection of sunlight from interplanetary dust grains.

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Gegenschein (very roughy like GAY-guhn-shine, German for "counter shine"), is a faint brightening of the night sky in the region of the antisolar point.

Observation

The gegenschein is so faint that it cannot be seen if there is any moonlight or light pollution, or if it falls in the vicinity of the Milky Way. The gegenschein appears as a softly glowing oval region a few degrees wide and 10–15° in length, oriented along the plane of the ecliptic.

Explanation

Like the zodiacal light the gegenschein is sunlight reflected by interplanetary dust. Most of this dust is orbiting the sun in about the ecliptic plane (zodiac).

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 gegenschein is (relatively) enhanced because (a) each dust particle is seen in full phase, and (b) the backscattering geometry leads to constructive interference.

History

The gegenschein was first described by the French Jesuit astronomer and professor Esprit Pézenas (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. He was also the first to observe that the Zodiacal light can embrace the complete sky, because under favorable conditions, a feeble light bridge connecting the Zodiacal light and the gegenschein can be observed. Besides, Brorsen had already proposed the correct explanation for the gegenschein (interplanetary dust reflections).

See also

References

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