I've never seen a green flash: the flare of green light that suddenly appears at the top of the Sun moments before sunset. Pictures on the web abound, but seeing it for yourself is a rare event.
So I was astonished to see this amazing series of shots from photographer Gerhard Hüdepohl showing a green flash from the Moon!
That's pretty cool. This shot was taken on the Cerro Paranal mountain in Chile, where the European Southern Observatory keeps its 8-meter Very Large Telescope. Hüdepohl is one of several ESO Photo Ambassadors, people who take wonderful shots of the area and the night sky to help promote what ESO is doing.
And this one qualifies! The Earth's atmosphere acts like a lens, bending light. Near the horizon this bending effect can be quite large, distorting objects when they rise or set (I explain this in detail in this post showing a similar picture of the squished Moon). Different wavelengths of light are bent by different amounts, so in extreme cases the colors from an object can be separated out, like the way a prism separates colors*.
The green flash is an example of this. As the Sun sets, the top can appear to change color as the sunlight is bent by different amounts (want details?). It's common to see blue and red flashes, but green ones are rarer. And to be honest I've never even heard of this effect happening with the Moon, so it must be rare indeed. Hüdepohl was very lucky to be able to capture this... but as I've pointed out before, luck favors the well-prepared. The more pictures you take, and the more you look for opportunities, the more likely it is you'll catch something extraordinary.
Image credit: ESO/G.Hüdepohl
* Note that the Moon looks red; that's because of a different effect. The blue light gets absorbed by junk in the air like dust, while red light can pass through such dust more easily. At the horizon you're looking through more air, magnifying the effect, so the Sun and Moon (and even stars) look redder near the horizon.