The scientific principle explaining why the sky is blue is Rayleigh scattering. This phenomenon occurs when sunlight passes through the Earth's atmosphere, and the shorter blue wavelengths of light scatter more than the longer red wavelengths. As sunlight interacts with air molecules, the blue light is dispersed in all directions, making the sky appear predominantly blue during the day. This scattering is also the reason for other atmospheric phenomena, such as the reddish hues seen during sunrise and sunset, when the light passes through more atmosphere and longer wavelengths are scattered less.