Green flash
Q: Why does the sun turn green just before it sets? [Not just the sun but
also the moon and Venus - AH] J.R., Albuquerque, NM
 A
rare look at the green flash as the sun sets on Mount Wilson, California.
Copyright 1994. Lu Rarogiewicz. Used with permission.
A: The sun turns green just before it sets because the atmosphere acts as a
weak prism, bending the light, say David Lynch and William Livingston in their
outstanding book, "Color and Light in Nature". See graphic. The prism bends the
blue light the most and the red light the least and all other colors, somewhere
in between. So the apparent sun you see at sunrise or sunset is a vertical
stack, a continuum, of sun images: each showing its own color and at its own
location in the vertical array.
Graphic: Courtesy of USATODAY.com
Since the atmosphere prism bends the blue light the most, the blue image of
the sun appears highest.
So, why doesn't the top of the sun look blue instead of green? Because
there's more than one phenomenon we're coping with. Blue light also interacts
more with air molecules than the other colors do and the interaction scatters
the blue light over the sky (which, by the way, is the reason the sky is blue).
More green light than blue gets through the atmosphere and that's why we see a
green flash. In extremely clear air, the "green" flash is occasionally blue,
says Lu Rarogiewicz, weatherman on Mount Wilson and retired astronomer.
"Contrary to popular belief," say Lynch and Livingston, " the green flash is
quite common, especially over water." The difficulty is in observing it.
"There's a restaurant in San Diego named the Green Flash with a western view
of the horizon over the bay," says Rarogiewicz, "A few times a year folks in the
restaurant will see the green flash and they applaud spontaneously. 'WOW! Look
at that! I don't believe that!'"
Further Surfing:
An introduction to green flashes by
Andrew T. Young of San Diego State University
(Answered December 2000; updated Oct. 17, 2007)
Click for printer version.
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