The sun often appears as a vivid orange or red sphere during sunrise and sunset, a striking contrast to its usual yellow or white appearance during the day. This daily spectacle prompts curiosity about its cause. While the sun itself radiates white light, its apparent color shifts due to its interaction with Earth’s atmosphere.
The Nature of Sunlight and Earth’s Atmosphere
Sunlight, which appears white, is a composite of all colors within the visible light spectrum. Each color possesses a different wavelength, with violet and blue light having shorter wavelengths, and red and orange light having longer wavelengths.
Earth’s atmosphere surrounds the planet, composed primarily of gases like nitrogen and oxygen. It also contains small amounts of other gases and tiny particles, such as dust and water vapor. As sunlight enters this layer, it interacts with these molecules and particles.
The Science of Light Scattering
The phenomenon responsible for the sun’s changing colors is known as Rayleigh scattering. This process occurs when light encounters particles much smaller than its wavelength, such as individual gas molecules in the atmosphere. Shorter wavelengths of light, like blue and violet, are scattered much more efficiently by these tiny atmospheric particles.
Because blue light is scattered widely across the sky, it makes the daytime sky appear blue. Conversely, longer wavelengths, such as red, orange, and yellow light, are scattered much less by these small particles. This is why the sun typically appears yellowish during midday, as some blue light is scattered away, leaving more of the yellow and red light to pass through.
Why Sunrise and Sunset Are Different
During sunrise and sunset, the sun appears more intensely orange or red because sunlight travels through a greater thickness of Earth’s atmosphere. When the sun is low on the horizon, its light rays take a longer, more angled path compared to midday. This extended path means sunlight encounters a larger volume of atmospheric molecules and particles.
As light traverses this increased distance, a greater proportion of shorter-wavelength blue and green light is scattered away. This leaves predominantly the longer-wavelength colors—yellow, orange, and red—to continue their path directly towards the observer. The more blue and green light that is scattered, the richer and deeper the orange and red hues of the sun become.
Other Influences on Solar Color
Other elements within the atmosphere can further influence the sun’s color. Aerosols, which are fine solid or liquid particles suspended in the air, play a role. These can include natural components like dust from deserts, sea spray, or smoke from wildfires. Human activities also contribute through pollution, such as smog from vehicles and factories.
These larger particles can scatter light differently than the smaller gas molecules. The presence of a greater concentration of these aerosols can intensify the scattering of light, contributing to more dramatic reds and oranges in the sky. For instance, smoke from wildfires can make sunsets particularly vivid due to the increased particle count. However, excessive pollution can also lead to hazy or muted displays rather than enhanced colors.