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Why is the Sky Blue?
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On a clear night we can see thousands of stars. On a clear day we see but one, and with that one plenty of blue sky surrounding it. What is it about this one star --- our sun --- that gives us that blue sky? And just why is the sky blue, not green or red?
It was in the 17th century that Isaac Newton discovered that when the sun's light is shone through a prism it separates into the red, orange, yellow, green, blue, and violet we have all seen in the rainbow. White light is really a collage of all those different colors.
But it was only in the late 1800's that people started figuring out what visible light's rainbow of colors had to do with the color of the sky. It was then that John Tyndall and others discovered that the reason the sky is blue is not due to reflections of the oceans or water in the sky or any optical illusion. It was the nitrogen and oxygen that make up our atmosphere --- those little critters we breathe in and out all day were the culprits! Here's how it works:
As the full visible spectrum of the sun's light, from blue all the way to red, floods through our atmosphere, some wavelengths are not allowed a free, unimpeded trip to the surface of the earth. The photons of light with shorter wavelengths, the blue end of the spectrum, get scattered by the tiny air molecules. By being scattered and re-scattered all over hither and yon, those bluer, shorter-wavelengthed photons of light can randomly hit your eye from just about anywhere up there, like the silver ball in a pinball machine gradually making it to the bottom. Looking in about any direction we are likely to get hit with stray photons of the sun's scattered blue light. Thus, a blue sky.
This effect can be demonstrated at home. (Attention Homeschoolers!)
Fill an aquarium with water, put a few spoonfuls of milk into it, then
stir it all up. Shine a flashlight through the milky water from one side
of the aquarium to the other. You'll notice the water takes on a bluish
cast. That's the bluer part of the flashlight's spectrum being scattered
about.
This phenomenon can explain a lot of seemingly mysterious occurrences.
Astronauts as they lift off up into orbit see the sky outside change from light blue to darker blue to black in just minutes. They rise from lots of atmosphere to no atmosphere, from much scattered light to none. Once they are above the nitrogen and oxygen, they get the full dose of the sun's full spectrum, losing the blue sky altogether. But this allows them to see the rest of the stars even when the sun is present. If we had no atmosphere down here we could see the stars during the day. But, of course, Earth would then be a tough place to raise a family.
You'll have noticed that all the photographs from the Moon have a deep black "sky" even when the Sun is up and bright. That is because there is no atmosphere on the Moon to scatter the light.
Other phenomena explained by the scattering are: the blue haze over some mountains, the blue cast of cigarette smoke in a harsh light, the bluish "haze" in images of the Pleiades star cluster and other starry locales. The list is huge.
There are images to help you "see" this effect at firstlightastro.com.
Until next time, clear blue skies!
(Phoro Courtesy Astronomy Picture of the Day, G. Greaney)
Posted by Administrator at 2002.02. 2 03:14 PM | Comments (0)
