Rayleigh scattering is responsible for the blueness of the clear sky. Air molecules scatter shorter-wavelength light (i.e. the blue part of the visible spectrum of colors) much more than the other colors. So, away from the sun the sky looks blue. However, if the sun is low above the horizon, a large part of the blue light will be scattered away out of the light rays from the sun which reach your eyes - so the sun (and any celestial body in general) appears more red.

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