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Q & A: Why is the sky blue, but the sun and moon aren't?

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Most recent answer: 05/25/2015
Q:
After I have seen the college of Walter Lewin about why the skies are blue and the clouds are white, I was left with the question: Why are the sun, moon and stars white? After the explanation of Walter Lewin I would suggest the sun, moon and stars would be blue, because of the lightscattering. Could someone explain to me why the sun is white, even if the skies are blue? and why are the moon and stars white, even if you take the scattering of light into account?
- G. Kisteman (age 19)
Zwolle, Overijssel, Netherlands
A:

As you learned, the sky is blue because the atmosphere scatters shorter-wavelength blue light more than longer-wavelength red light. That scattered light is what you see when you look at the sky, so it appears blue.

When you look at the moon or the sun, you're seeing the light that wasn't scattered by the atmosphere. This scattering only affects a small portion of the total light, so it normally doesn't have much effect on the apparent color of the sun or the moon. But if the sun is low in the sky, like at sunrise or sunset, its light travels through so much atmosphere that a significant portion of blue light is scattered away—making the sun appear red.

You can read more about scattering in the atmosphere (and see a cool picture) in .

Rebecca H. 


(published on 05/25/2015)

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