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"Skies so blue"

Written by Paul Sutter on Monday, 31 October 2016. Posted in From The Desk of...The Chief Scientist

There's a saying in physics: "Simple doesn't mean easy."

Chris Hurtubise wrote on the board outside my office a nice little question: why is the sky blue? Like the water boiling question from a few weeks ago, simple questions like this often deceptively hide complex answers.

Here's a snappy reply: the sky is blue because it's a blue thing. All sorts of things are all sorts of colors, and air happens to be blue. Just not...very blue. It's mostly clear, but look through enough of it and you can see its blueness.

But what about sunsets? If you look through a whole bunch of air, it turns pink or red.

Hmmm. Simple, but not easy.

It turns out that not all kinds of light bounce off air molecules in the same way. When reddish light hits air, it tends to just keep going in the same direction. When blueish light hits air, however, it tends to ricochet off into a random direction. This is called Rayleigh Scattering and it's really fun.

So white light (a combination of all the colors) from the sun hits our atmosphere, and the blue light scatters around while the red doesn't. So we see a blue sky and a yellow (white minus blue) sun.

During sunsets and sunrises, when we're looking right at the sun through lots of air, all the blue light has scattered off to the sides, and even some of the yellows, leaving a lovely red tint.

About the Author

Paul Sutter

Paul Sutter

Paul Sutter is COSI's Chief Scientist. He is an astrophysicist and offers a wealth of knowledge about our universe. In addition to his COSI position, Paul Sutter is a Cosmological Researcher and Community Outreach Coordinator at The Ohio State University's Center for Cosmology and AstroParticle Physics (CCAPP).