You Asked:
Why is the sky blue? — Thomas, Tangerang, Indonesia

Marshall Answered:
If you were to stand on the moon and look at the sky, it is always black and filled with stars. It doesn’t matter whether it is “day” (the sun is visible) or night (the sun is not visible) – the sky is always black.

On earth the sky is black at night, just like it is on the moon. But during the day something very odd happens. The sky turns bright blue. The reason for the blueness is the earth’s thick atmosphere, and the types of gases in that atmosphere.

The gases interact with the light coming from sun, but interact differently depending on the color of the light. Light at the red end of the spectrum tends to pass straight through the atmosphere. Light at the blue/violet end of the spectrum tends to get scattered by the atmosphere (a process called Rayleigh scattering). So, if you want to think of it this way, the red light from the sun falls straight to the ground, while the blue and violet light gets scattered in all directions. You are seeing that scattered blue and violet light when you look at the sky.

So why is the sky blue rather than violet? This page has a helpful explanation. It has to do with the way that the color sensing cells in our eyes work.

See also this very nice desciption: