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counterglow,a slight brightening of the night sky in the region of the zodiaczodiac
[Gr. zoion=animal], in astronomy, zone of the sky that includes about 8° on either side of the ecliptic. The apparent paths of the sun, the moon, and the major planets all fall within this zone.
..... Click the link for more information. directly opposite the sun, i.e., 180° from the sun. Discovered by the Danish astronomer Theodor Brersen in 1854, it is caused by reflection of sunlight by interplanetary dust particles that lie in the plane of the solar system. The brightening is relatively faint and is obscured by moonlight and, in the weeks on either side of the solstices, by the Milky Way. The gegenschein is one of several sky glows, as are zodiacal lightzodiacal light
or zodiacal band,
a faint band of light sometimes seen in the western sky just after sunset in the spring, extending up from the horizon at the point where the sun has just set, or in the eastern sky just before sunrise in the autumn.
..... Click the link for more information. and airglowairglow,
faint diffuse illumination of the sky originating in the upper atmosphere. Although it occurs at all times of day throughout the upper atmosphere, it is most typically visible to an observer on earth on dark nights, above the horizon.
..... Click the link for more information. .
gegenschein(gay -gĕn-shÿn) (counterglow) A very faint patch of light that can be seen on a clear moonless night on the ecliptic at a point 180° from the Sun's position at the time of observation. It is part of the zodiacal light.
(counterglow), a faintly glowing, low-contrast, diffuse spot located in the night sky in the area opposite the sun. The gegenschein is joined to the cones of the zodiacal light by what is called the zodiacal band, a very faint luminescence that extends along the ecliptic in the form of a band about 10° wide. The brightness of the gegenschein exceeds the brightness of the background of the night sky by just 10–15 percent, so it can only be seen on dark, moonless nights when the atmosphere is very clear and the area of the sky opposite the sun is far from the horizon and the Milky Way (in the spring and autumn).
The gegenschein was first observed by A. Humboldt in the period 1799–1803. It is now studied by photometric and spectographic methods, and important results have been obtained from observations made by satellites and space probes. Its diameter is about 20°, with brightness decreasing from the center toward the periphery. Fraunhofer lines found in the solar spectrum have been detected in the gegenschein spectrum. It has been established that the gegenschein is caused by the scattering of solar light by dust particles in interplanetary space, but the spatial distribution of the mass of dust that causes the gegenschein has been studied very little.
Various hypotheses have been proposed to explain the nature of the gegenschein. The most important are the hypothesis of a circumsolar dust cloud; the hypothesis of the accumulation of dust particles at the antisolar libration point of the sun-earth system—a point that is located at a distance of 1.5 million km from the earth; and the hypothesis that the earth has tails of dust and gas, similar to the tails of comets. Measurements taken from the American Pioneer 10 spacecraft when it was 5–8 million km from the earth and 1.011 astronomical units from the sun provide evidence, however, that the gegenschein is unrelated to the earth but is caused by the scattering of solar light by dust particles located outside the earth’s orbit in interplanetary space.
N. B. DIVARI