Why is the sky blue? The rays of the sun, in striking the earth's atmospheree, are scattered by the countless tiny specks that fill the air, the blue rays being scattered farther than the red. It is thus the blue rays which we see, and which lead us to believe that the sky is of that colour. If the air were free of all particles of dust, the sky would be quite dark, relieved only by the brightness of the sun and the moon and of the stars which would then always be visible.
(When I was a boy we had a little book from my dad's home called How much do you know? By a man called Harold Wheeler, it came out in the thirties and is a fascinating read. I saw an edition in a second hand shop this morning and bought it. This is the first question in the general knowledge section).