Legend goes that just as the sun sets below the horizon, a flash of green light appears to kiss the day goodbye. Sailors say that when the flash appears, it means a soul has crossed over. Jules Verne wrote that those who witnessed it could no longer be deceived because he could read the thoughts of others. Admiral Byrd supposedly saw one that lasted 35 minutes. For a full record of sightings, see here. But is this really stuff of legend or a real phenomenon in the sky?
It’s real and happens in more places than you’d think. All it really takes is a cloudless night and a clear horizon.
So what’s the physics behind the green? Well, there’s a much longer explanation at Andrew Young’s website but the long and short of it is that as soon as the sun sinks below the horizon, the last colors of the sunset take different amounts of time to travel through space and reach your eyes. This is called atmospheric dispersion. The red wavelengths travel faster and therefore disappear first. Wavelengths after green are subject to atmospheric extinction, that is all of the wavelengths are directed away from the earth by the time the distance between you and the sun is reached.
The same happens in reverse at sunrise. If you are in the right place at the right time you could see two green flashes a day!
~Bluegrass Blue Crab