Four years ago I learned about the green flash.  Apparently as the Sun descends below the horizon, the last rays bend through the atmosphere and there is a quick flash of green light.  Some people have told me that it covers the whole sky, others have said that the Sun itself flashes green for a second.  Well, I still have not seen it for myself, but here is a picture of what I would have seen had I seen it in person.  Via APOD.