The phenomenon responsible for the reddish appearance of the Sun during sunrise and sunset is the scattering of sunlight by particles in the Earth's atmosphere, primarily Rayleigh scattering. When the Sun is near the horizon, sunlight passes through a much longer path in the atmosphere. Shorter wavelengths of light (blue, violet) are scattered away more effectively than longer wavelengths (red, orange). As a result, the light that reaches our eyes after travelling through this longer path is enriched with longer wavelengths, making the Sun and the sky around it appear reddish.