green flash

A very fleeting - lasting barely one or two seconds - and difficult-to-observe atmospheric optical phenomenon, consisting of a flash of predominantly green, though sometimes bluish, color, that emerges from the upper edge of the solar, lunar, or planetary disk, right at the moment it disappears below the Earth's horizon or when it emerges above it. The optimal conditions for observing it are achieved if the air is very still, with hardly any atmospheric turbulence, and one is positioned in an elevated location, preferably facing a sea horizon.