Is the night sky darkest in the direction opposite the Sun? No. In fact,
a rarely discernible faint glow known as the gegenschein
(German for "counter glow") can be seen 180 degrees around from the Sun in an
extremely dark sky. The
gegenschein is sunlight back-scattered off small interplanetary dust particles. These dust particles are millimeter
sized splinters from asteroids and orbit in the ecliptic plane of the planets. Pictured above from last
year is one of the more spectacular pictures of the gegenschein yet taken.
Here a deep exposure of an extremely dark sky over Las Campanas
Observatory in Chile shows
the gegenschein so clearly that even a surrounding glow is visible. Notable background objects include the
Andromeda galaxy, the Pleiades star cluster, the California Nebula, the belt of
Orion just below the Orion Nebula and inside Barnard's Loop, and bright stars Sirius and Betelgeuse. The
gegenschein is distinguished from zodiacal light
near the Sun by the high angle of reflection. During the day, a phenomenon
similar to the
gegenschein called the glory can be seen in
reflecting air or clouds opposite the Sun from an
airplane.
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