Why does the sky appear blue from the earth?
When sunlight enters the atmosphere, fine air particles scatter blue colour more than the others. The scattered blue light enters our eyes.
The main phenomenon behind the colour of the sky is due to scattering. When sunlight passes through the atmosphere, the fine particles in the air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark.