Why is the sky blue?

When sunlight enters Earth's atmosphere it collides with gas molecules — mainly nitrogen and oxygen. Shorter blue wavelengths (~475 nm) are scattered roughly 5.5× more than longer red wavelengths (~650 nm), a phenomenon called Rayleigh scattering. This scattered blue light reaches our eyes from all directions across the sky. At sunrise and sunset, light travels through more atmosphere, scattering away blues and leaving warm reds and oranges. On Mars, the thin CO₂ atmosphere produces a butterscotch sky during the day and a blue-tinted sunset — the exact inverse of Earth.