Science & TechFact
The sky is blue because air loves to scatter blue.
Air molecules are far smaller than light waves and scatter short wavelengths much more than long ones, proportional to the fourth power. Blue at 450 nanometres scatters nearly six times more than red at 700. That scattered blue reaches you from every direction. At sunset, light travels a long path; blue is long gone and red remains.
Useful: Without an atmosphere the daytime sky would be black with visible stars, just like on the Moon.
Source: en.wikipedia.org