The observation of the sun appearing red or deep orange as it rests on the horizon is a common and striking visual event. This change in color from the bright white or yellow seen midday is not due to any change in the sun itself. Instead, the phenomenon is a direct consequence of the interaction between sunlight and the Earth’s atmosphere. The colors we perceive at sunrise or sunset are a predictable result of fundamental physics governing how light travels through a medium composed of gases and fine particles.
How the Atmosphere Filters Sunlight
The Earth’s atmosphere acts as a deep, variable filter for incoming solar radiation. This gaseous envelope, composed mostly of nitrogen and oxygen molecules, surrounds the planet. The amount of atmosphere sunlight must penetrate depends on the sun’s angle relative to the horizon.
When the sun is positioned directly overhead, its light travels through the minimum possible amount of the atmosphere. As the sun moves toward the horizon at dawn or dusk, the light rays strike the atmosphere at a shallow, grazing angle. This shallow angle significantly increases the distance the light must travel through the dense layers of air near the Earth’s surface.
At the horizon, the light’s path length through the atmosphere can be many times greater than the overhead path. This extended journey through a dense medium is the physical precondition necessary for the color shift to occur.
The Physics of Light Scattering
Sunlight, which appears white to the eye, is actually a spectrum containing all the colors of the rainbow, each corresponding to a different wavelength. Red light has the longest wavelength in the visible spectrum, while blue and violet light have the shortest. The atmosphere’s gas molecules interact with these wavelengths through a process known as Rayleigh scattering.
Rayleigh scattering describes how light is dispersed by particles much smaller than the light’s wavelength, such as nitrogen and oxygen molecules. The efficiency of this scattering depends strongly on the wavelength of the light. Shorter wavelengths, like blue and violet, are scattered in all directions far more effectively than the longer red and orange wavelengths.
This differential scattering explains why the sky appears blue during the day. When the sun is high, blue light is scattered across the entire sky, reaching our eyes from every direction. Longer wavelengths, including reds and yellows, pass more directly through the atmosphere with less scattering.
The Long Journey of Red Wavelengths
The appearance of the sun as a fiery red sphere at the horizon is the culmination of the increased path length and differential scattering. As the sun sinks toward the horizon, the light must traverse a vastly increased column of air, forcing it to encounter a greater number of scattering molecules.
The cumulative effect of this extended path is the near-total removal of the shorter, highly scattered wavelengths. Blue and violet light are scattered so thoroughly out of the direct line of sight that very little remains to reach the observer’s eye. What is left is primarily the light at the long-wavelength end of the spectrum: orange and red.
These longer wavelengths scatter minimally, allowing them to penetrate the thick atmospheric layer with greater success. Therefore, the light that finally reaches the observer’s eye has been depleted of its blue components, making the sun appear red. Airborne particulates like dust, smoke, or aerosols can further enhance this effect by increasing the total amount of scattering, leading to more saturated red and orange hues.