Green Flash on a Red Moon
April 28, 2011 8:19 AM   Subscribe

The green flash isn't quite the light show that some might imagine, but is still impressive. But sunsets aren't alone in producing the green flash - the flash can also appear above the moon. Up on Cerro Paranal in northern Chile, ESO Photo Ambassador Gerhard Hüdepohl has captured a very clear example of the a green flash above the moon. posted by filthy light thief (18 comments total) 10 users marked this as a favorite
Awesome -- I had no idea this existed. Well stolen, light thief!
posted by The Bellman at 8:26 AM on April 28, 2011 [1 favorite]

what is this i don't even
posted by thsmchnekllsfascists at 8:42 AM on April 28, 2011

I'd thought that the green flash was an effect of temporal backwards masking. When the last bit of red disappears at sunset, a green afterimage might be briefly seen. When the first red appears at sunrise, the "afterimage" might extend in the other direction, coloring what has already been seen, but is still being labeled and processed in context. That green flash would be a purely perceptual phenomenon, "which no artist could ever obtain on his palette."

I wouldn't be too surprised if some of that is actually occurring, in addition to the refractions.
posted by StickyCarpet at 8:45 AM on April 28, 2011

posted by Pastor of Muppets at 9:15 AM on April 28, 2011

very cool.
posted by puny human at 9:25 AM on April 28, 2011

After years of looking for them, I finally managed to sort of catch one on film earlier this year.
posted by jquinby at 9:45 AM on April 28, 2011 [1 favorite]

Eponysterical? Discuss.
posted by oulipian at 9:53 AM on April 28, 2011

The Green Flash is also known for its killer omelettes.
posted by shakespeherian at 9:54 AM on April 28, 2011

I knew about the flash, but I never even considered the moon. That's awesome.

I'd thought that the green flash was an effect of temporal backwards masking.

Unrelated to the Green Flash. Temporal backwards masking is an effect of Superman flying around the Earth.
posted by DU at 9:57 AM on April 28, 2011

A couple years ago I was driving down the Nacimiento-Fergusson Road, the only inland route to Big Sur. The sun was about to set and it looked beautiful so I pulled off at a turnout to take in the view. A backpacker who had set up camp was nearby, strumming a guitar. Just as the sun was about to go under the horizon, I looked down to fiddle with my camera. I heard the backpacker guy say "hey, did you see that green flash?" I was so pissed. I've always wanted to see one.
posted by zsazsa at 10:11 AM on April 28, 2011 [1 favorite]

The Green Flash is also known for its killer omelettes.

Hey, I've eaten there! (but not the omelette)
They have photos of green flash sunsets all around the place.
posted by rocket88 at 11:14 AM on April 28, 2011

I'd thought that the green flash was an effect of temporal backwards masking.

I would be a little surprised if that had much of an effect; the sun is moving so slowly through our visual field.
posted by Jpfed at 11:48 AM on April 28, 2011

I'd thought that the green flash was an effect of temporal backwards masking.

Yeah, but the photos...
posted by ryanrs at 12:17 PM on April 28, 2011

Obligatory mention of Green Flash Brewing Company out of San Diego County. Their West Coast IPA is hands down one of the most delightful examples of the style.
posted by dhammond at 1:16 PM on April 28, 2011

So cool, thank you for posting this.
posted by theora55 at 1:22 PM on April 28, 2011

I had a fun opportunity to demonstrate the green flash to a group of friends & random spectators at Venice Beach. When I realized the sun was only a minute or so from setting, I began explaining the green flash to my friends. They were mostly incredulous, but intrigued. Some random bystanders were also listening in, clearly with some skepticism. When the flash finally did appear, general amazement and people asking "how did you know that was gonna happen?" ensued. A great moment!
posted by ShutterBun at 1:53 PM on April 28, 2011 [3 favorites]

No one ever seems to talk about this happening at sunrise, but only at sunset despite the apparent time symmetry of the two.

This got me pondering what important difference to conditions for a green flash there could be between sunrise and sunset, and the only thing I could come up with was that on a clear day the sun has been heating up the surface of the land or water before a sunset, but not before a sunrise.

Right at sunset, though, there are lots of shadows, and the sun has been at a very low angle for a while, so you could expect fairly strong radiative cooling of the ground to have already taken place, and that could make for a layer of cool air next to the ground and warmer air above. An inversion, in other words, since air is normally cooler as you go up, and the Wikipedia link does mention inversions as an important factor in some kinds of green flashes.

But I think any dependence on inversions raises the possibility that another phenomenon besides refraction is important to the green flash: total internal reflection.

Total internal reflection toward the ground can take place from the interface between the cold air next to the ground and warm air above as the Sun dips below the horizon because denser cold air will have a higher index of refraction than warm air. TIR is responsible for the mirage you often see when driving on a hot asphalt road, only there the cooler layer is above the warmer and so is the object reflected.

Total, as opposed to partial, internal reflection takes place only for angles above a certain critical angle. For angles less than that, some light is transmitted through the layer. The sine of the critical angle is equal to the ratio of the indicies of refraction on either side of the interface. In the case of hot and cold air, the indices are very close to 1, therefore the critical angle is almost 90 degrees, measured from the normal to the interface. That is, it is almost right on the horizon.

I think the green flash, however, is not from the total internal reflection after the critical angle has been reached, but is instead from the light that is transmitted through the layer just before the critical angle is reached. That light skims just along the upper surface of the interface, and it's green because the transmission of light near the critical angle is very sharply dependent on the angle, and the Sun in green light has a slight angular displacement from the Sun in red light because of differing atmospheric dispersions of the two colors.

That the light of the green flash comes from above the inversion layer is consistent with one of the conditions for a kind of inversion-dependent green flash I linked about above:

You must be above the inversion layer to see a pure M-Mir flash.

The FPP's green flash seems to be of this variety.
posted by jamjam at 4:09 PM on April 28, 2011

I had heard the term 'Green Flash' growing up, but was never quite sure what it was. Just that it was rare, and that I should look for it at sunset. Many years ago I was driving in the country in NC, the sun had just then set -- it was the middle of summer and night comes on slow there -- when, for some inexplicable reason, the entire sky suddenly turned a deep solid green, like tourmaline, or the color of fine jade. I thought to myself, that's it, that's the green flash. I pulled my car over in a field in awe just to look at it. Such a deep emerald color covering the entire dome of sky. It lasted for twenty, maybe thirty minutes, then merged with the night. Years later I saw a photo of an actual green flash on the sun, and realized I had been mistaken, but honestly, I would rather see the sky turn into a translucent, diaphonous emerald the way it did that evening than to see the actual thing.
posted by puny human at 4:43 PM on April 28, 2011

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