When the sun sets on the horizon, color is such that a green rim appears on the upper part of our star, with red counterbalancing on the bottom. If you catch a glimpse of this green top-glow right when the sun dips below the horizon, the green seems isolated, producing this "green flash."
Photog buffs ought to find this a pretty nifty challenge.
Vid Credit: Sixty Symbols - U. of Nottingham
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