Why Is the Sky Blue?

Earth's sky appears blue because when sunlight hits the atmosphere, blue photons scatter much more than other colors due to their frequency being closest to the resonant frequency of nitrogen and oxygen molecules, causing blue light to disperse throughout the atmosphere. The sky isn't violet (which scatters even more) because human eyes are poor at detecting violet wavelengths. This same scattering principle explains why sunsets appear red—sunlight travels through 40 times more atmosphere at low angles, allowing most blue light to scatter away, leaving primarily red wavelengths that barely scatter.