The airglow is a faint blue to green glow of the atmosphere, caused by chemical interactions of air molecules. Some air molecules and atoms become excited by ultraviolet sunlight during the day, and deexcite to a lower-energy state at night, emitting light. Airglow is one of the causes of the general skylight at night. Airglow is most evident near the horizon, since the optical path length through the atmosphere is larger when one looks low. There should not be any citylights or other light pollution: the airglow is a faint glow.
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