The Half-Astrophysicist Blog

My First Green Flash Pictures

I have wanted to get a picture of a green flash for a while and finally got it!  I was on a cruise this week and managed to get the image near sunset on December 16th.

Many of you have probably heard of the legendary green flash and wondered how it works.  Andrew Young at San Diego State University has a page with lots of info and pictures, but I will break it down for you here.  Sunlight is composed of all visible colors.  Earth’s atmosphere acts like a prism and bends shorter wavelength light (violet, blue, green) more than the long wavelength light (yellow, orange red).  When viewed near the horizon, the top of the Sun contains the blues and greens and the bottom of the Sun contains the oranges and reds.

As the Sun sets, it passes through layers of airs with different temperatures.  Different temperatures will bend the light different amounts and can make the Sun appear very distorted.  You can see this pretty dramatically below (as always, click any image to embiggen).

Note how the Sun is distinctly not round and it appears a little piece of it is about to break off at the top.  That piece will break off and form a little flash of light.  Actually, I got two of them.  Both of the flashes started out yellow and turned green as the got farther from the Sun.  Yes, you can get more than one green flash from a Sunset.  This sunset had at least three that I managed to record.

I put those four images together in a short animation which is on Youtube.  I don’t think the colors were preserved nicely in the video conversion process, but you can see the progression of the flash nicely.

And then there was a little break when until the last little bit of the Sun disappeared below the horizon and then I got on more.

You might think that the flash should be blue or purple, but the atmosphere scatters short wavelength light so effectively that most of the time the last bit you can see is green.  However, people have seen and photographed blue flashes as well.

If you have never seen a green flash and don’t live near the water, don’t despair. You don’t need water to see them (although it is a good place).  I have seen them many times from Kitt Peak and even from Fermilab in Illinois (looking west there is no water for a long way!)  Part of the key is to get above ground.  You get a much better view if you are up even a few stories.  Look closely as the effect is subtle.

If you want to try and photograph them, you won’t have much luck with a point and shoot digital camera.  You need to be able to play with exposure times and have a pretty good zoom lens.  I used a Canon Digital Rebel XTi with a 55-250mm image stabilized zoom lens (cranked all the way to 250mm!)  Anything less than 150mm and you are probably dead in the water.  You also have to keep adjusting exposure times if you are shooting in manual mode like I was.  The lighting changes very rapidly in the minutes leading up to sunset.

You can see green flashes at sunrise as well, but it is more difficult since you have to be looking right when the Sun comes above the horizon and the timing of that can be difficult.

Now that I got my first one, I am looking forward to trying some more.

December 18, 2009 - Posted by | Astrophotography, Observing

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