The night sky, even in remote locations, is never truly pitch black. This faint, persistent glow is a measurable phenomenon known as Night Sky Brightness (NSB). It results from a complex interplay of light sources both within and far beyond our atmosphere, setting a natural limit on how dark the sky can become for an observer on the ground. Distant celestial objects, chemical reactions in the upper atmosphere, and human-made light sources all contribute to this background illumination.
Light from Distant Cosmic Sources
The night sky’s natural light includes illumination originating from outside Earth’s atmosphere. This collective light is dominated by integrated starlight from billions of unresolved stars and galaxies. The hazy band of the Milky Way is the visible manifestation of the combined light of countless distant stars, creating a diffuse glow across the sky.
Another significant extraterrestrial source is the Zodiacal Light, a faint, pyramid-shaped glow visible along the ecliptic plane. This light is sunlight scattered by microscopic dust particles orbiting the Sun within the solar system’s plane. The Zodiacal Light is a permanent feature that can be brighter than all other integrated starlight combined in certain parts of the sky. The Moon’s reflected sunlight is also a major cosmic contributor, easily overwhelming all other sources when it is above the horizon.
The Phenomenon of Airglow
A major part of the night sky’s illumination is generated in Earth’s upper atmosphere through airglow, or nightglow. This faint, self-luminous emission is caused by chemical reactions occurring at altitudes between 50 and 300 kilometers. During the day, solar ultraviolet radiation energizes and splits atmospheric molecules, such as oxygen (O2) and nitrogen (N2), into individual atoms.
After sunset, these separated atoms and molecules begin to recombine, releasing the stored energy as photons—light—in a process known as chemiluminescence. For example, the recombination of oxygen atoms often produces a distinct green emission at a wavelength of 557.7 nanometers, concentrated around 97 kilometers in altitude. The hydroxyl (OH) radical is another significant contributor, emitting light primarily in the red and infrared parts of the spectrum. Airglow is present constantly and its collective brightness can often surpass the light from all distant stars on a moonless night.
How Artificial Light Contributes to Night Sky Brightness
For most of the world’s population, the dominant factor in nighttime light is human-made illumination, collectively termed light pollution. This manifests as sky glow, the visible dome of light that hangs over urban and suburban areas. Sky glow is created when light emitted or reflected from ground sources travels upward and is scattered back down toward the observer by the atmosphere.
The scattering process is governed by two primary mechanisms: Rayleigh scattering from tiny air molecules and Mie scattering from larger aerosols and dust particles. Because shorter, blue wavelengths scatter more efficiently than longer, red wavelengths, modern light sources with a high blue content, such as Light Emitting Diodes (LEDs), contribute disproportionately to sky glow. This scattering effect allows light from a city to brighten the sky for hundreds of kilometers around the source. Unshielded or poorly directed outdoor lighting sends photons directly into the upper atmosphere where they are scattered, creating a diffuse veil that alters the nighttime environment.
Quantifying and Monitoring Nighttime Light
Standardized methods are used to quantify the mix of natural and artificial light sources. The brightness of the night sky is measured in astronomical units called magnitudes per square arcsecond (mag arcsec^-2). This is a logarithmic scale where a higher number indicates a darker sky; for example, a pristine sky might measure around 21.8 mag arcsec^-2, while a city center sky could be below 18.0 mag arcsec^-2.
Visual Assessment
One common classification method for visual assessment is the Bortle Dark-Sky Scale, a nine-level system that ranks the quality of the night sky from Class 1 (excellent, truly dark) to Class 9 (inner-city sky).
Instrumental Measurement
Instrumental measurements are often taken using devices like the Sky Quality Meter (SQM), a handheld photometer that measures the zenith brightness in mag arcsec^-2. Beyond ground-based measurements, Earth-orbiting satellites track the upward-directed radiance of artificial light globally, providing data that helps map the extent and intensity of sky glow and monitor changes over time.