gegenschein, oval patch of faint luminosity exactly opposite to the Sun in the night sky. The patch of light is so faint it can be seen only in the absence of moonlight, away from city lights, and with the eyes adapted to darkness. The gegenschein is lost in the light of the Milky Way in the summer and winter. The best observing periods are February, March, April, August, September, and October. The gegenschein and the zodiacal light, a similar phenomenon not restricted to a position opposite the Sun, form the most notable parts of a band of very faint light along the ecliptic. The spectrum of the gegenschein is similar to that of the Sun, and it is generally believed that it is the result of the reflection of sunlight from meteoric material or other dust in the Earth’s orbital plane.