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The gegenschein is a German word for the opposition effect. Like dry heiligenschein, the gegenschein is caused by three-dimensional shadowing of an open structure, which shadows are absent if you look to the antisolar point, along the direction of the sunlight. In this case, the open structure is the vast cloud of billions of small dust particles floating in the inner solar system. The gegenschein is a small (typically 10x20 degrees) area of light centered on the opposition (antisolar) point, which is above the horizon during the night. It is very hard to detect, due to the low brightness. On a dark night at a location far from cities, however, it is unmistakable.

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Observing tips

If you want to observe the opposition light yourself, you should first find a location that is at least 100km (70 mi) from the nearest major city, and preferably at a high altitude (over 1000m or 3000ft above sea level would be good).

The night should be moonless, and the bright planets Jupiter, Saturn and Mars should not be too close to the antisolar point. This point in the sky is the point on the ecliptic just opposite the sun. The best times to observe the opposition light are around local midnight, around October (otherwise many stars and the Milky Way interfere somewhat).

You may need to adjust your eyes to the dark for at least 20 minutes (shorter if the location is truly dark). Scan around the antisolar point; you should be able to see the sky brightening.

Good luck!