Welcome to the newest edition of The Why. “Why does the sun flash green?” you ask? Good question. (Besides, it beats answering the question: “Why do I have to change my underwear every day when no one sees it? Really? No one ever sees it? I feel sad for you. Besides, didn’t your mommy ever tell you that you could get in an accident? )
When the sun sets the sky is lit up in a rainbow of colors. Even your rascally writer has had moments in the barely-recallable past where he has watched a sunset (and the following sunrise in fact) with a lovely lady in his arms. (Mind you, that was when his muse was on an extended vacation but you get the point.)
According to M. M. Dworetsky of the University College London’s Department of Physics and Astronomy in London, England, the “setting Sun’s ‘disk’ is made up of light of all colors.” As you may recall from early editions of this series, the colors of light have to do with wavelengths. Having not been out from under his laptop to share a sunset with anyone lately though, it seemed like a good idea to consult another source to explain why the sun turns green.
Dworetsky continues: “Green and blue light (shorter wavelengths) are refracted by air slightly more than red light, so the disk actually consists of a flattened red disk, with a yellow disk slightly above it, a green disk above that, and blue and violet disks at the top.”
Robbie Gonzalez, contributor to i09, adds that when the sun actually sets, the different “bands of color dip below the horizon in (a specific) order, with red disappearing first and violet last.”
OK, but why hasn’t anyone ever seen a blue, or violet or green sunset? Gonzalez confirms that Andrew T. Young of the San Diego State University in San Diego, California notes that it has to do with “an effect known as atmospheric extinction.”
“Both air molecules and aerosol particles scatter the shortest wavelengths most strongly (which is why the sky is blue: the strongly-scattered blue light goes in all directions, so we see it when we look anywhere in the sky). At the horizon, the path length through the air is very long, and the shortest wavelengths are almost completely removed.”
“Scattering by molecules alone is not quite enough to make the shortest wavelengths invisible; so if the air is very clear, violet is the last color seen. But usually there is enough haze in the air that violet, and even blue, is completely removed, so that green is the last color seen at sunset, or the first at sunrise.”
Gonzales agrees that when the atmospheric extinction and atmospheric dispersion work together with an unobstructed, clear “view of a distant, low-lying horizon, conditions are prime for observing a green flash”. This is confirmed to be the simplest, most common explanation.
Why does the sun flash green? Now you know.
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