Forget the Green Lantern: here's the Green Flash

By Phil Plait | May 3, 2011 7:00 am

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.

Related posts:

From one moon to another
Squishy Moonrise seen from space
Ridiculously awesome pic of Discovery and the ISS taken from the ground
Moon doggies

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: green flash, Moon

Comments (31)

  1. I read some of the explanations, but wasn’t able to divine out exactly how long this phenomenon lasts as the earth rotates around. How specific in the green spectrum does the observation need to be in order to see that flash? I read that it takes abour 1.4 seconds for the region of the Sun (or moon in this case) to go through the entire spectrum of visible light. So is it just 1/6th of the spectrum, or an even more specific part of the green spectrum that gives us that effect?

    Just guesstimating, I am getting at best 0.2 seconds of green. That does take some planning (assuming all the atmospherics are right to start with!).

  2. What? No mention of the Atmospherics Optics website?

    But, cool nonetheless.

  3. One Eyed Jack

    I always assumed that the green “flash” was just the after image you would get from staring at an orange/red sun due to complementary colors.

    See what I get for assuming?

    I also assume that one day Eva Mendez will come to her senses and realize that I’m the only man for her.

    Keep your facts out of my assumptions. You’re crushing my world! 😉

  4. Trebuchet

    I’ve been fortunate enough to see several green flashes, and always make sure to look when I’m in the right place (you need a distant horizon, which is why so many green flashes are photographed over the sea) and the right conditions — if only I knew what the right conditions are! I was under the impression, however, that blue flashes were less common.

  5. That’s no Moon .. 😉

    .. Oh wait it is, our Moon, Luna to be precise.

    (Hey, somebody had to say it.)

    Yegods, that’s a remarkable image! 8)

    Never knew you could get a green flash from our Moon as well. 😉

  6. Bill Davidson

    I’ve seen a green flash while going out in the Gulf of Mexico to go fishing, but it was just at
    dawn. The fishing guide said he had seen it before and usually when the Gulf was calm.
    ( that day the gulf was like glass )
    He said it usually meant a great day fishing. I was spectical, but we caught so many snappers that we had to leave early. John McDonald ( mystery writer ) back in the early 60’s wrote a book whose title was “A Flash of Green”. In it he describes the flash very well.

  7. Lupine

    I thought it was caused by “The Flying Dutchman”.

  8. ntsc

    Admiral Dan Gallery mentioned in one of his books that the green flash occurs at sunrise as well as sunset.

    And John D. McDonald lived on a house above Midnight Pass on Siesta Key on the Gulf.

  9. James

    “Absorbed” -> “scattered”. The reddening effect doesn’t necessarily require dust, either.

  10. Dan

    I too have seen the green flash over the Gulf on a calm day. In addition to Bill’s comment about needing a distant horizon, I believe the amount of moisture in the air may bear on the phenomenon, the few times I have seen it, I recall a very light haze.

  11. dcsohl

    I used to work in a high-rise on the waterfront in Boston… for a period of time I was getting in to work every day before sunrise (in mid-winter), and set myself an alarm to go look for the green flash. In two months of daily (well, weekdaily) observations, I never saw it once. From a high-rise looking out over the ocean, so the horizon was very far away indeed.

    I must be cursed.

  12. Bill Davidson:

    I was spectical

    Nice typo, given the thread. :-)

  13. Daniel

    Cool! I have looked for green flashes many times but never seen one. This article helped me understand that I’ve probably been looking for the wrong thing, as I really expected a “flash” going across the sky.

  14. Chris Winter

    This is an impressive series of photographs — and getting TGF from reflected light makes it doubly so. (I expect the relative intensities of the different wavelengths would tell something scientifically useful about the lunar surface…)

    I too have looked for TGF but never seen it. My best chance was when I happened to be on top of that mountain east of Albuquerque just at sunset, a glass of scotch in my hand. I looked hard for TGF but saw only reds and oranges.

  15. Forget the Green Lantern?

    Hal Jordan died for your sins!

  16. Digital Atheist


    I’m with ya on this one. Forget GL? NEVER!!!!!!!!

  17. Darth Robo

    The Martians moved to the moon?!?


  18. Joseph G

    Chalk me up as a believer in the “distant horizon” camp.
    I live on the west coast and I’ve seen a number of green flashes, which makes sense – if you’re on a 40 foot cliff, and the sun is setting over the ocean, you’re seeing the sun through about the maximum thickness of atmosphere possible (without being on a plane or in orbit).

    @14 Chris Winter: I too have looked for TGF but never seen it. My best chance was when I happened to be on top of that mountain east of Albuquerque just at sunset, a glass of scotch in my hand. I looked hard for TGF but saw only reds and oranges.

    I’ll bet the sunset looks especially pretty through a half-empty glass of scotch. I’ll have to give that observation technique a try 😉

  19. I supposed it’s tautological that the moon is flashing us.

  20. Mike G

    You live in Boulder and have never seen the green flash?

    I saw it once on a west-facing beach in Kailua-Kona, Hawaii. It’s pretty fast — you have to be looking right at it, or you’ll miss it.

    You should have an easier time in the high mountains. You need a very distant horizon to the west, which should be possible on a 14er summit, on a clear day (yeah, I know….hens teeth and all that).

    I would think it would be well worth the trip up Mt. Evans.

  21. Michael Malomay

    I’ve seen TGF up in the Bering Sea in Alaska and also near Hawaii-very rare only a couple times in the 5 years I served on Coast Guard vessels.

  22. Mike Mullen

    I agree, forget The Green Lantern, Captain America: The First Avenger looks like a much better movie. :)

  23. I have seen many green flashes of different varieties (inferior mirage, mock mirage, elevated horizon and cloud top). I have also photographed every single one of those types. I could not see visually but have photos of blue and purple flashes as well (including one shot of the Sun setting behind a sloping mountain which captured green, blue and purple flashes behind different parts of the mountain).

    I am in Florida now and saw one Saturday night and one tonight. They are not nearly as rare or hard to photograph as people would have you think…just like anything else, a little practice goes a long way!

  24. Flat Earth

    On behalf of the Flat Earth Society, I wish to thank you for endorsing bendy light; a vital aspect of Flat Earth Theory.

    The Society shall immediately add you to our list of distinguished supporters, Mr. Plait.

  25. I’ve also seen the green flash of the sun over the Gulf of Mexico. What great photos, I never thought the moon would cause flashes!

  26. DennyMo

    I’ve seen TGF before, but this was the first I’d heard of other colors. Something new to investigate, cool. (Need to go back to Hawai’i to do more research…)

    24. Flat Earth Says:
    On behalf of the Flat Earth Society, I wish to thank you for endorsing bendy light; a vital aspect of Flat Earth Theory.

    I’d be curious to know how many of the posters there “believe” in FE, and how many are just entertaining themselves.

  27. David

    I’ve only seen one green flash, sailing between islands in Hawaii. I probably only saw it because I was looking for it, others missed it. It’s quite fast.

  28. I lived in La Paz, Baja California Sur (in Mexico) for a while (which, as an aside, has been described as “the city with 365 different sunsets per year”, really beautiful sunsets), and was able to see the green flash several times as the sun set over the ocean. You indeed have to look for it, since it happens very quickly.


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