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Mr Blue Sky. Please Tell Us Why.
Science > Most astronomers - but not all - do their observing at night. It is then that the skies are see-through black and the objects that make up our starry heavens are at their best.
But once the sun comes out, observing time for those astronomers is over. It is then of course that our skies metamorphose from transparent and black into translucent and blue.
So why is the sky blue?
This question is a classic, but few people really know the answer. It's almost as if it were enough just to ask, but not to find out. Let's answer it now.
One reason for the blueness of the daytime skies that I have heard many times is that the sky is reflecting the oceans. Good guess, but if we took that to its logical end we'd have to ask why the sky isn't also brown and green and white, other colors which dominate the Earth's surface.
I'll bet we can find another, more logical answer. Let's try. The sky is blue during the day, so the sun must play some role. Here are another couple helpful clues: There is no blue sky on the Moon, ever. And have you noticed that when the Shuttle astronauts are working on things up there, they are being bathed in sunlight, but the "sky" all around them is pitch black? So the sun must have an accomplice; it cannot be working alone.
Immediate hypothesis? The sun and an atmosphere are needed to make a blue sky. The two work together somehow to bathe us in a sea of blue.
Consider this also. The Sun, as you know, bleeds out all the colors of the rainbow, from deep blue all the way over to the red end of the visible spectrum. Might there be something about the atmosphere that monkeys with just the blue parts of the spectrum sent down from the Sun? If you think so, you would be right.
The photons of light that make up the visible spectrum differ by wavelength. The blue end has the shorter wavelengths, the red end the longer.
The molecules of oxygen and nitrogen that make up our atmosphere are just the right size - that is, incredibly small - to interact with the short-wavelength blue guys. Those photons are absorbed by the air molecules and re-emitted in random directions all over the sky. This happens so much, they are scattered in so many directions, that everywhere we look we are getting hit by those redirected blue photons.
All of this works to give our inquisitive minds the impression that the blue photons are coming from everywhere, and ta-da! the sky is blue.
The other parts of the spectrum come on through with relative ease, but even some of these are challenged if there is a lot of atmosphere to go through. This of course is what happens at sunset.
As the sun sets, the light pouring from it has to travel through a lot more sky – and a lot more schmutz – to get to us. Much of the spectrum gets pealed away from the light stream leaving only the heartiest and longest of the wavelengths, those on the red end, to make it through, and giving us those spectacular sunsets.
You may have noticed that when the sky is smoky from a fire that the sun appears red. Same reason as above: the longer-wavelength red photons get though, the rest are scattered.
And now you know what happens to the stars when the sun is out. This whole effect – the scattering of the blue wavelengths - essentially fills the skies with that familiar blue glow, and the relatively dim stars beyond, still traveling over us, get blotted out. If you could suddenly take away our atmosphere during the day, the skies would be transparent and black again, except for the vicinity of the Great Star, our own Sun.
But then it would be kind of tough to breathe, our blood would begin to boil, and the thrill of seeing the stars during the day would soon be replaced by excruciating pain, followed by death less than a minute later.
I'll settle for studying the stars at night, and keeping the pretty blue skies for the daytime.
Posted by Mark Ritter at 2006.11.12 07:09 PM | Comments (0)
