Gegenschein

gegenschein (gā`gənshīn') 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.

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gegenschein

 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 [′gāg·ən‚shīn]

(astronomy)

A round or elongated, faint, ill-defined spot of light in the sky at a point 180° from the sun. Also known as counterglow; zodiacal counterglow.

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Gegenschein 

(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