Paul, what happens is that the last bit of sun visible, just before it drops below the horizon completely, turns emerald green for a fraction of a second. It's the visible surface of the sun that changes color, not a ray extending above it, or anything like that. The cause is atmospheric refraction. The line-of-sight from observer to the setting sun passes through a lot more air than if you were looking up, and it acts like a prism. As the sun drops, the solar spectrum essentially passes vertically, in the opposite direction of the sun's motion. The sun is moving "down", and the spectrum moves "up". When the green portion passes your eye, you see the sun as green, since for a brief moment you are only getting the green portion of the spectrum. Some have reported jumping up at the moment of green flash, to get a second, or prolonged flash, as they follow that portion of the spectrum upward for a fraction of a second. Much more rare is the "blue flash", which follows the green portion of the spectrum. I have never seen it. It has been reported that Venus also exhibits a green flash, though I have never seen it do so myself. Remember, it's only the last, teeny piece of visible sun that flashes green, just before it's gone completely. Timing is critical. DO NOT stare at the sun without proper filtration! The green flash is seen with no filtration. Chuck --- Paul Witte <questort@attbi.com> wrote:

I'd never even heard of this before. How far above the horizon does the light extend?

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