skyeye
Airglow, Gegenschein, and Milky Way

Explanation: As far as the eye could see, it was a dark night at Las Campanas Observatory
in the southern Atacama desert of Chile. But near local midnight on April 11, this mosaic
of 3 minute long exposures revealed a green, unusually intense, atmospheric airglow
stretching over thin clouds. Unlike aurorae powered by collisions with energetic charged
particles and seen at high latitudes, the airglow is due to chemiluminescence, the production
of light in a chemical reaction, and found around the globe. The chemical energy is provided
by the Sun's extreme ultraviolet radiation. Like aurorae, the greenish hue of this airglow
does originate at altitudes of 100 kilometers or so dominated by emission from excited oxygen
atoms. The gegenschein, sunlight reflected by dust along the solar system's ecliptic plane
was still visible on that night, a faint bluish cloud just right of picture center. At the
far right, the Milky Way seems to rise from the mountain top perch of the Magellan telescopes.
Left are the OGLE project and du Pont telescope domes.