Why is the sky blue?

Dear Cecil:

Why is the sky blue?

Cecil replies:

Ah, jeez, Bobberoonie, can’t you ask your old man anything? It’s because sunlight striking air molecules is scattered in all directions. For abstruse technical reasons no Texan can hope to understand, the blue portion of sunlight is scattered with greater intensity than other wavelengths. Since scattered light is the only reason the sky is illuminated at all (the airless lunar sky, as you no doubt recall, is black), what you see is blue. Similarly, when sunlight takes a very long path through the atmosphere, as at sunset, so much blue is diverted out of the direct path that you get a shift to the other end of the spectrum and the normally yellow-white sun appears red.

Send questions to Cecil via cecil@straightdope.com.

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