A "Green Flash" from the Sun

Dalla Rubrica "NASA - Picture of the Day" del giorno 4 Gennaio 2011:"Many people think it is just a myth. Others think it is true but its cause isn't yet known. Adventurers pride themselves on having seen it.
What is it?
It's a "Green Flash" from the Sun. The truth is the Green Flash does exist and its cause is well understood.

Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green.
The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds. A Green Flash is also visible for a rising Sun, but takes better timing to spot.

A dramatic Green Flash, as well as an even more rare Blue Flash, was caught in the above photograph recently observed during a Sunset visible from Teide Observatory at Tenerife, Cannary Islands, Spain.
Obviously, the Sun does not partly turn either green or blue: the effect is caused by different layers of the Earth's Atmosphere, together acting like a prism".