Supposedly, this is seen on a clear day when the sun sets. If you do see it, it'll be when the sun it just disappearing, when you see its last sliver of light disappear. You see a small green flash of light. When I saw it, it wasn't a flash that filled the sky, it was just a little sliver of green that slightly extended the size of the sliver of yellow sun that was disappearing.

Explanatory excerpt from the NASA site, listed on the bottom: The Sun itself does not turn partly green, the effect is caused by layers of the Earth's atmosphere acting like a prism.

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  • the main url to the above site - < >
  • nasa's picture/brief description - < >