A truly dark night sky is increasingly rare, especially near populated areas. The brightness overhead is a mixture of light from human activities and light from natural cosmic and atmospheric sources. Understanding why the night sky is not uniformly black involves looking closely at how light behaves after it leaves its source.
The Primary Culprit: Artificial Skyglow
Skyglow is the diffuse luminance of the night sky caused by artificial light sources scattered by the atmosphere. This form of light pollution is the collective result of countless lights from streetlights, commercial advertising, security fixtures, and industrial facilities.
The problem stems from inefficient or unshielded lighting designs that direct a large percentage of light horizontally or upward. The combined upward emission from thousands of sources travels into the atmosphere. This artificial light at night (ALAN) interacts with air molecules and suspended particles, creating a pervasive brightness visible far outside urban centers. The intensity of this glow is significantly higher in urban and industrial areas compared to rural regions.
How Ground Light Becomes Skyglow
The physical mechanism that transforms localized ground light into a widespread skyglow is atmospheric scattering. This process occurs when photons of light collide with particles suspended in the air, causing the light to be redirected away from its original path. The atmosphere contains two main types of scattering agents: tiny molecules and larger aerosols.
Scattering by gas molecules, such as nitrogen and oxygen, is described by Rayleigh scattering, which preferentially affects shorter, bluer wavelengths of light. Scattering by larger particles like dust, smoke, and water vapor is described by Mie scattering, which affects all wavelengths more uniformly. Both mechanisms work to diffuse the upward-traveling artificial light, spreading it across the sky and redirecting a portion of it back toward the ground.
The presence of aerosols, pollutants, or water droplets significantly enhances this scattering effect. When light hits these particles, the glow is amplified and scattered over a much wider area. This makes the night sky appear brighter than it would be on a clear, dry night.
Natural Illumination Sources
The night sky is not absolutely dark, due to several natural sources of illumination. The most prominent natural contributor to night brightness is the Moon, which reflects sunlight toward Earth. A full Moon can provide hundreds of times more illumination than a moonless night, dramatically affecting visibility and rendering fainter celestial objects invisible to the naked eye.
Integrated starlight is the cumulative light from billions of distant stars and the diffuse glow of the Milky Way galaxy. While the light from any single star is faint, the sheer number of stars adds up to a measurable background brightness. This starlight is also scattered by the Earth’s atmosphere, contributing to the overall faint background glow.
Airglow is a faint emission of light by the Earth’s upper atmosphere. This light is generated when atoms and molecules, energized by the Sun during the day, recombine at night, releasing energy in the form of photons. Airglow is always present and ensures that the night sky never reaches a state of true, absolute blackness.