A “red sun” describes an optical phenomenon where the sun appears distinctly red or orange, often seen when it is low on the horizon during sunrise or sunset. This effect transforms the sun’s usual yellow or white appearance into a vibrant red or amber. It results from how sunlight interacts with Earth’s atmosphere, primarily involving the scattering of light.
Understanding Light and Atmospheric Scattering
The colors we perceive in the sky and from the sun are a consequence of sunlight interacting with atmospheric particles. Sunlight, which appears white to our eyes, is actually composed of various colors, each with a different wavelength. When sunlight enters Earth’s atmosphere, it encounters tiny gas molecules, predominantly nitrogen and oxygen.
These small molecules scatter shorter wavelengths of light, such as blue and violet, more efficiently than longer wavelengths like red and orange. This process, known as Rayleigh scattering, is why the sky appears blue during the day; blue light is scattered across the sky and reaches our eyes from all directions. When the sun is high in the sky, its light travels through less atmosphere, allowing more of the full spectrum to reach us, making it appear yellow or white.
At sunrise or sunset, sunlight must travel through a much greater thickness of the atmosphere. During this longer journey, most shorter-wavelength blue and violet light is scattered away by the atmospheric molecules and particles. This leaves the longer-wavelength red, orange, and yellow light to pass through unimpeded, resulting in the sun’s reddish appearance.
Environmental Factors Affecting Sun Color
Beyond the natural effect of the sun’s angle, various particles suspended in the atmosphere can intensify the sun’s red hue. Larger particles, such as dust, smoke, and water droplets, scatter light differently than the smaller gas molecules responsible for Rayleigh scattering. When present, these larger particles can scatter all wavelengths of light more uniformly, but they also absorb some blue and green light, allowing more red and orange light to dominate.
For example, smoke from wildfires can fill the air with soot and ash particles, leading to a pronounced red or even blood-orange sun. Volcanic eruptions also inject vast amounts of ash and sulfate particles into the atmosphere, creating conditions for vivid red sunsets. Similarly, large dust plumes can cause the sun to appear intensely red by filtering out shorter wavelengths.
Safe Observation of a Red Sun
Observing a red sun requires prioritizing eye safety. Even when the sun appears dimmer or redder, its harmful ultraviolet (UV) and infrared (IR) radiation are still present and can cause severe eye damage. Looking directly at the sun, even briefly, can lead to conditions such as solar retinopathy or photokeratitis. These injuries can result in temporary vision loss, blurred vision, or even permanent blind spots.
It is never safe to look directly at the sun without specialized protection. Regular sunglasses, no matter how dark, do not offer adequate protection against the sun’s intense UV and IR rays. For safe observation, use certified solar filters or indirect viewing techniques like pinhole projectors.