Why is the sky blue and the sunset red? A light-path explanation

Sunlight contains more than one colour

The light we call white contains a range of visible wavelengths. The atmosphere redirects some of this light in different directions. Shorter wavelengths scatter more strongly, so blue light reaches an observer looking away from the Sun. NOAA’s explanation connects this scattering with the blue appearance of the daytime sky. [1]

Try phrasing the question as “Where did the light entering my eyes come from?” That distinguishes the bright solar disc from the much larger area of illuminated sky.

Why does sunset change the colour?

With the Sun low on the horizon, its light travels through more atmosphere before reaching you. More short-wavelength light is scattered out of the direct path, leaving a mixture richer in reds and yellows. [1]

The daytime blue sky and a red sunset are therefore different viewing situations within the same broad explanation. They do not require the Sun to become a different kind of light source every evening.

Three colourful scenes, three useful distinctions

The ocean and aurora mechanisms are described by NOAA and NASA respectively. [2] [3] The distinction is between redirecting existing light, selectively absorbing it and emitting light after excitation.

A simple observation without special equipment

Compare the colour of a clear patch of sky overhead with a patch near the horizon, keeping your gaze away from the Sun. Note the time and whether clouds are present. This is a way to record an observation, not to measure scattering or prove a theory from one photograph.

Then compare the questions addressed by our guides to ocean colour and the northern lights. Similar colours do not necessarily mean similar physics.