The sky is blue because of a phenomenon called Rayleigh scattering. When sunlight passes through Earth's atmosphere, tiny air molecules scatter the shorter wavelengths of light, like blue and violet, more than the longer wavelengths like red or yellow. Our eyes are more sensitive to blue light, and since blue light is scattered in all directions, the sky appears blue during the day. At sunrise and sunset, the light travels through more atmosphere, scattering the blue light away and allowing the reds and oranges to dominate the sky color.
