Active Planet

The earth appear blue because of the way sunlight is selectively scattered as it goes through our atmosphere. It turns out that the SIZE of oxygen molecules and nitrogen molecules are almost the same and they are comparable in size to the wavelength of blue light. In simple terms, this makes it so waves of blue light effectively run into the oxygen and nitrogen molecules in the atmosphere and are then bounced off in all directions. Scientists call this scattering. The wavelengths of red light are somewhat bigger, and they tend to be able to get past the oxygen and nitrogen molecules without being entirely blocked. This allows the red light part of sunlight to more easily pass through the atmosphere, while blue light doesn't get very far before being scattered. When you're on the ground looking up at the sky, you are looking at an area where sunlight has already passed by (cross-ways to you). You see a good amount of scattered blue light and much less scattered red light. So the sky looks blue to you.