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About "Ask A Scientist!"

On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Published: 1 May, 2003

What is a Green Flash?

If you watch the sun set from a place that has a good view of the distant horizon, like a mountain top or a place with a western view of the ocean, you will sometimes see a small flash of green light from the top edge of the sun at the moment that it disappears from view. This "green flash" is rare because it results from a rare combination of effects in the earth's atmosphere and in our eyes.

One effect, a mirage, creates distorted images of distant objects because of differences in temperature and thickness of the air. The air in our atmosphere also acts like a lens, bending rays of light coming from the sun over the horizon, an effect called atmospheric refraction. Refraction briefly delays our view of the sunset and it does this more for green, blue, and violet light than for red, yellow, and orange light. So the top edge of the sun, as it disappears at sunset, sometimes looks green, blue, or violet (or a mixture of these colors).

But a third effect, scattering by water vapor and dust particles, often filters out the blue and violet light leaving only green. Finally, our eyes become less sensitive to red and yellow when we stare at a bright light, so watching the sun set makes us more aware of green light.

WARNING: You should never look directly at the sun except briefly, just before it sets; and you should NEVER look at the sun or sunset through binoculars or a telescope!

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