I’ve always wondered why the Earth’s sky is blue and when looking up from the surface of the moon, why there is no sky. Simply put, the moon doesn’t have an atmosphere, so there is no sky to see. But what does this mean and how does it apply to the Earth’s sky and its perceived blueness?

The Earth has light from the sun shining on it whenever there is a blue sky. This causes the light, which when coming from the sun is the full spectrum of light, to hit the atmosphere before reaching us. When this happens, the blue light, which has a higher energy than the rest of the colors of the spectrum, dissipates throughout the sky and turns it blue. The atmosphere allows the rest of the colors to come through it and hit the surface of the earth. This is why when we look at a sunset from Earth, it appears red because its on the horizon which is the farthest away the sun can be during the day. This makes more red light pass through the atmosphere and causes less blue light to reach our eyes.

Sources:

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html