The sun near the horizon transforms from a brilliant white or yellow disk into a deep orange or red orb. This daily color shift, observable at both sunrise and sunset, is caused by the way Earth’s atmosphere interacts with sunlight, not by any change in the sun itself. The aesthetic change is a result of a predictable atmospheric science principle. This natural process involves light scattering, which is highly dependent on the sun’s angle and the distance the light must travel through the air.
How Light Travels Through the Atmosphere During the Day
During the midday hours, the sun appears bright white or slightly yellow because its light takes the most direct and shortest route through the atmosphere. When the sun is high overhead, the light rays travel perpendicular to Earth’s surface, minimizing the amount of air they must pass through. This short path length means that only a minimal amount of light is deflected or absorbed before reaching the ground. Consequently, all colors combine to make the sun appear white, or occasionally a pale yellow due to a slight amount of blue light scattering.
The Physics of Light Scattering
The mechanism responsible for altering the sun’s color is known as Rayleigh scattering, which explains how light interacts with the small nitrogen and oxygen molecules that compose the atmosphere. Sunlight is electromagnetic radiation made up of a spectrum of colors, each possessing a different wavelength; red light has the longest wavelength, while blue and violet light have the shortest. Rayleigh scattering dictates that shorter wavelengths, such as blue and violet light, are scattered much more easily and effectively by atmospheric gas molecules. Conversely, longer wavelengths, including red and orange light, are scattered far less efficiently as they pass through the air. This scattering process removes light from the direct path to the observer’s eye.
Why the Horizon View Filters Out Blue Light
The distinct red color seen at sunrise and sunset occurs because the sun is positioned near the horizon, forcing its light to travel through the greatest possible amount of atmosphere. When light enters the atmosphere at this low angle, it must pass through a path significantly longer than the midday path, maximizing the effect of Rayleigh scattering on the incoming solar radiation. Over this long distance, almost all of the short-wavelength blue and violet light is repeatedly scattered out of the direct line of sight by air molecules and dispersed across the sky. The cumulative removal of these shorter wavelengths leaves only the least-scattered, longer-wavelength light, predominantly red and orange, to continue its path toward the observer. The presence of larger particles like dust, smoke, or aerosols in the lower atmosphere can enhance this effect, leading to even more vibrant red colors.