The Chapel Hill News

February 15, 2004

Ask the Postdoc


By Phillip Manning

Being temporarily between low-paying jobs, the Postdoc has agreed to answer science questions from those of you who want to better understand everyday events and gadgets. We’ll start with a question I received several months ago.

Q: I was in Key West last winter and heard much talk about a mysterious “green flash” that appeared at sunset. Several times I went to the waterfront at Mallory Square to watch the sun go down. I saw the Iguana Man and a scantily clad female sword swallower but no green flash. Does the green flash exist? If so, what causes it?
-Milton Gillespie, Orlando, Florida

Yes, Milton, the green flash does exist, and the Postdoc has seen it. The green flash is a rare optical event that usually occurs at sunset. When the top of the sun drops below a low, clearly defined horizon (most often a large body of water), a bright green band, lasting only a second or two, sometimes appears where sea meets sky. Although the green flash presumably has been occurring for millennia, few people noticed until 1882, when Jules Verne called attention to the phenomenon in a romantic novel, “Le Rayon vert” (or “The Green Ray”). After that, sightings were a dime a dozen.

The renowned British scientist Lord Kelvin (1824-1907) reported seeing a blue flash at sunrise in the Alps. More reports came in, and books were written and Ph.D.'s awarded for speculations about the source of the green flash (no doubt contributing to the surplus of Ph.D.'s that pushed the Postdoc into his current penurious condition). Even so, many scientists believed that it was an optical illusion brought on by staring too long at the setting sun while sipping tropical drinks in places like Key West. Then, in 1954, a Vatican priest took color photographs of a green flash as the sun was setting over the Mediterranean Sea. Father D.J.K. O’Connell published the pictures in the January 1960 issue of Scientific American. The green flash was certainly real and on the cover of a magazine. Soon afterward, physicists explained the phenomenon.

The green flash is caused by the refraction (or bending) of the sun’s rays by the atmosphere. Refraction is what causes a bar of soap on the bottom of a filled bathtub to appear to be in a different place than it actually is. But air doesn’t bend all light the same amount; it bends blue (or lower wavelength) light more than red (or higher wavelength) light. As the sun sets, light is bent toward the Earth. And because blue light is bent the most, the last light one sees as the sun sinks below the horizon should be blue. So why does one not see a blue flash?

The molecules, aerosols, and dust particles that make up our planet’s atmosphere scatter blue light the most, which is why the sky is blue. And when the sun is low in the sky, its rays must pass through more of the Earth’s atmosphere. Consequently, most of the blue is scattered away. And because the color next to blue in the visible spectrum is green, the last thing one sees at sunset is a green flash. Except, of course, the last thing one usually sees on the horizon at sunset in Key West is . . . nothing. Which leads to a final question: Why is the green flash so rare?

Compared to, say, astronomy, the study of the atmosphere is an inexact science. Astronomers know the precise time a solar eclipse will occur centuries in advance, but no one can predict what the weather will be on that day. And because the green flash is a product of the way the sun’s rays interact with the atmosphere, the reason it occurs or doesn’t occur is somewhat speculative. However, all scientists agree that clouds, high humidity, and atmospheric haziness can hide the green flash. A temperature inversion can have the same effect, too. Still, the Postdoc has watched for the flash on clear, cloudless, low humidity days and seen nothing.

Which brings us back to Key West and the one green flash the Postdoc has seen. As the sun dipped below the Caribbean, a bright emerald-green line formed along the top of the sea, about as long as the sun’s diameter and lasting about one second. In addition to confirming the green flash, the Postdoc can also attest to the existence of the Iguana Man and the female sword swallower — from whom he cadged a few rum punches one night at Sloppy Joe’s Saloon after they worked the sunset crowd at Mallory Square.
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Consider this explanation an introduction to the green flash. For more information, go to the web site of Andrew T. Young at San Diego State University; http://mintaka.sdsu.edu/GF/. Also, it is dangerous to stare at the setting sun, so avert your eyes until the last moment before sunset.
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Send questions for the Postdoc to pvmanning@mindspring.com or call the newspaper at (919) 932-2019.