sky?

[ETMON]

why does the sky look blue?

[sft42]

It's the refraction of the sun's light as it comes through the earth's outer atmosphere......On the few other planets and moons in the solar system that have atmospheres the "light" on these planets is a different shade because of the different gases...green on some and red on others. 

[Clanad]

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white...

[Ray The Grey]

Because that is how our visual sensors pick it up and how our brain deciphers it. The correct philosophical answer is because that is how we see it. It is subjective just because you perceive that shade to be blue and see it so - I may not. I may have a diffferent frame of reference and I might see it as you would define red (for example).

I just wanted to add another dimension to the purely scientific answers (which appear correct to me)