What is “green flash” at sunset all about?

Dear Cecil:

Do you have any info on the so-called "green flash"? It's not a superhero, but rather an optical phenomenon involving a burst of pure green light that occurs just as the sun rises or sets over the ocean. I've seen it several times but my friends won't believe me, saying it's just delayed mescaline aftereffects. Set these unbelievers straight.

Cecil replies:

Adds a sort of postmodern element to the process of scientific discovery, doesn’t it? “I believe I’ve discovered a new perturbation in the space-time continuum! However, it could just be the drugs.” Don’t worry, though — there really is such a thing as green flash. Usually it’s a thin green band or splotch visible for a split second at or near the top edge of the sun as it sinks beyond the horizon. You can see it at sunrise too. Sometimes it lasts longer; sometimes it’s blue or violet or turns from green to blue. To see it you need a clearly delineated horizon and a haze-free sky. The ocean (or any large body of water) will do fine, as will a desert or mountain.

Most people have never seen green flash and think it’s a myth, ascribing it to retinal fatigue on the part of the observer or other causes. One reason they’re so adamant is that green flash is impossible to photograph with an ordinary camera — the image is too small too register. But researchers managed it in the 1950’s using telescopes. (For some of their handiwork, see the January, 1960 Scientific American.)

Green flash is caused by atmospheric refraction — that is, the bending of sunlight as it passes through the air so that it splits into a rainbow of colors. Refraction causes the solar disk to be surrounded by ghost images like a cheap TV, with a violet/blue/green “shadow” above and a red/orange/yellow one below. None of this is visible except at sunrise and sunset, when refraction hits the max and the sun’s light is so reduced that the ghosts don’t wash out. The red ghost disappears below the horizon, the orange and yellow ones are absorbed by the atmosphere, the blue and violet ones scatter (usually), and what’s left is green. Count yourself lucky if you’ve seen it; you’re one of a privileged few.

Send questions to Cecil via cecil@straightdope.com.

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