Hello marc and welcome back to JustAnswer.

Actually, the sky isn't blue at all -- the atmosphere of the Earth is colorless. What makes it look blue is the effect the atmosphere has on the sunlight passing through it to the surface.

Sunlight, of course, contains the full spectrum of colors. Molecules of water and dust in the air scatter the blue in sunlight more than they scatter red or other colors. As a result, on a clear day, the sky looks blue because we see more of that blue light that is being scattered than any other color.

As the Sun sets, however, light coming to our eyes passes through a thicker layer of atmosphere, the light is filtered more and the blue light is scattered out and away from our sight and we see more red and orange colors. Remember that old sailor's saying: "Red sky at night, sailor's delight. Red sky in morning, sailor's warning!"

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