Date: April 27, 2010
Title: The Green Flash
Podcaster: Rob Sparks
Organization: Half Astro – http://halfasstro.wordpress.com
Description: Today I am going to explore the topic of green flashes. I’ll talk about how to see green flashes, what causes them, and give some tips on how to photograph them. Pictures of green flashes and diagrams helping to explain their formation can be found on my blog, halfastro.wordpress.com under today’s date.
Bio: Rob Sparks is a Science Education Specialist at the National Optical Astronomy Observatory. A lifelong astronomy enthusiast, he earned a B.A. in physics at Grinnell College and his M.S. at Michigan State University. He taught high school physics, math and astronomy for 11 years at schools on St. Croix, Florida and Wisconsin. He spent the 2001-2002 school year working on the Sloan Digital Sky Survey as a recipient of the Fermilab Teacher Fellowship. He spent the summer of 2003 at the National Radio Astronomy Observatory as part of the Research Experience for Teachers. He has been working as a NASA Astrophysics Ambassador since 2002. He was a member of the Galileoscope Working Group for the International Year of Astronomy.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Ben Lillie in memory of Frederick M. Richards.
Transcript:
Welcome to today’s episode of the 365 Days of Astronomy podcast. This is Rob Sparks of the Half-Astrophysicist Blog. Today I am going to explore the topic of green flashes. I’ll talk about how to see green flashes, what causes them, and give some tips on how to photograph them. Pictures of green flashes and diagrams helping to explain their formation can be found on my blog, halfastro.wordpress.com under today’s date.
First off, green flash can be a bit of a misnomer. The flashes seen at sunset can be yellow, blue, green or even violet, but green is the most commonly seen color.
The green flash is a brief flash of (usually) green light from the Sun that can be seen just as the Sun sets. Green flashes can be seen at dawn as well, but are much more difficult to spot simply due to the fact that you cannot see the Sun prior to the green flash so you don’t know exactly where and when to look!
A very unrealistic green flash was featured in the most recent Pirates of the Caribbean movie. The bright and dramatic flash depicted in the movie leads to unrealistic expectations of what you can expect to see at sunset or sunrise. Oh, the movie also erroneously states that green flashes occur only at sunset!
So how do you see a green flash? Many people believe green flashes are only visible from some exotic location or over water. While I have seen several green flashes over the ocean, I have also seen them from Kitt Peak, various mountains in Arizona, and Wilson Hall at Fermilab (looking west…no water there!) The important thing is to have a nice clear western horizon. A flat horizon is nice, but not necessary. If you are looking toward hills, it helps if you are on a higher hill or mountain so you are looking slightly down at the horizon. A slight elevation will improve your odds of seeing a green flash, even if you just are on the second or third story of a building.
Next, check the weather. Green flashes are most common when the sky is nice and clear. Haze near the horizon can easily block out the green flash.
Now you are all set to watch sunset. Even when the Sun is close to the horizon, you don’t want to stare at it! The green flash usually occurs near the very end of sunset so you don’t need to look until the Sun is about halfway set (or more).
Now you need to know a little about green flashes. There is more than one type of green flash. If you see the indentations on both sides of the disc of the Sun, watch them closely. The indentations will frequently move to the top of the Sun and a small piece of the Sun will pinch off and turn green. This is a mock mirage flash and often happens a couple minutes before Sunset. You can sometimes see multiple mock mirage flashes in one Sunset. Check out my website for a photo of two mock mirage flashes in progress at the same time.
If you don’t see a mock mirage flash, keep watching. Just as the very last bit of the Sun goes below the horizon, you can see the inferior mirage flash. This flash is only about 1/10th the size of the Sun’s disc, so you have to look very close to see it. The inferior mirage flash is the most common type seen.
There are a couple of other types of green flashes, but the inferior mirage flash and mock mirage flash account for 99% of all reports, so we will leave those alone for now.
Now we will move onto the causes of a green flash. Green flashes are real phenomena and not just optical artifacts from staring at the Sun too much! Let’s review a little optics here. When light is traveling in a vacuum, its speed is 3*10^8m/s. When it enters Earth’s atmosphere, light slows down a little bit. Not only does the light slow down, it also changes direction slightly through a process called refraction. Refraction is the change in direction of a light wave due to a change in its speed. You may have observed refraction by placing a pencil in a clear glass of water and noticing that the pencil appears bent at the air-water boundary.
But not all colors of light bend the same amount. Short wavelength blue light bends more than long wavelength red light. Although you need a telescope to see it (with a proper filter or using solar projection) the top rim of the Sun will have a blue and green arc around it and the bottom rim of the Sun will have a red and orange arc. Earth’s atmosphere is acting like a prism and breaking up the Sun’s light into different colors.
We used the McMath-Pierce Solar Telescope at Kitt Peak to watch Sunset using solar projection. I have pictures of this where you can see the colored arcs around the Sun on my blog.
Another effect you will notice is that the Sun is no longer round, but gets compressed vertically. That is, the Sun gets short and fat. Refraction bends sunlight upwards as it enters the atmosphere. Since light from the bottom part of the Sun passes through a more dense layer of atmosphere, it gets bent more than light from the top part of the Sun. This effect compresses the Sun vertically so it looks abnormally short and fat.
Now things get tricky. Green flashes are actually mirages. Note I called the two types a mock-mirage flash and an inferior mirage flash. The inferior mirage flash is caused by a layer of hot air just above the surface, similar to the “wet road” mirage we see on hot days. The mock-mirage flash is caused by a layer of cool air (called a temperature inversion) below eye level of the observer. The green flash we see is really a mirage!
For a more complete explanation of the exact optical phenomena involved in green flashes, see Andrew Young’s excellent green flash page at http://mintaka.sdsu.edu/GF/ .
Green flashes can be photographed with digital cameras. I recommend a digital single lens reflex camera with at least a 150mm zoom lens, preferably longer (I use a Canon Digital Rebel XTi with a 55-250mm image stabilized zoom lens which is the equivalent of a 400mm zoom lens on a 35mm camera). The camera should be mounted on a tripod. Do not look through the viewfinder to locate the Sun. Instead, hold your hand behind the viewfinder and move the camera. You will see a light on your hand when the Sun is in the field of view.
The Sun is very bright so you will need a short exposure and use a low ISO setting (100 or even 50 if your camera has it). Your camera’s light meter is not a good indication of the proper exposure time as it usually goes how bright the sky appears. If you go by it, the Sun and potential green flash will be overexposed and washed out. I usually start underexposing by about two stops. Take a test exposure and see how it looks. Remember, your subject is the Sun, not the landscape!
The other thing you need to be aware of is that the lighting levels change very quickly as the Sun sets. Take test shots as the Sun goes down and quickly adjust your exposure times as necessary. You need to know your camera controls well so you can change your settings quickly…this is not the time to learn!
If you see the tell tale “pinching” moving vertically up the face of the Sun, get ready for a mock-mirage flash. I set my camera to burst mode and let it rip. I have even caught more than one green flash in a single picture using this technique.
The inferior mirage flash occurs just as the last bit of the Sun disappears. I am always ready to take another burst of exposures at this point (with the proper exposure settings of course!)
Make sure you have a LOT of room on your memory card and a fully charged battery. Between test exposures and bursts trying to capture green flashes, I can easily take 100 exposures or more in one sunset trying to capture the elusive green flash.
Don’t be discouraged if you have difficulty getting green flash images. Green flashes don’t occur every sunset and even when they do, there are so many variables that even professional photographers have a difficult time getting good images. Keep trying! And remember when you look at my photos, there are many dozens of photos that show nothing for every one that captures a green flash!
For images of green flashes I have captured and more info, see my entry for today on my blog at http://halfastro.wordpress.com.
Thanks for listening to today’s episode of the 365 Days of Astronomy podcast.
End of podcast:
365 Days of Astronomy
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