Light pollution is the ambient artificial light that interferes with the natural darkness of the night sky. This interference obscures the faint light from distant celestial objects. Imagining how the sky would look without light pollution means envisioning a return to a pristine night. The dramatic visual difference between typical urban skies and the world’s few remaining dark sites reveals an astonishing level of detail and structure hidden from most people today.
How Artificial Light Scatters
The widespread brightening of the night sky, known as skyglow, is caused by atmospheric scattering. Artificial light emitted upwards from sources like streetlights interacts with molecules and particles suspended in the atmosphere. This interaction diffuses the light, causing it to spread out and form a luminous dome over populated areas.
A significant portion of this effect is explained by Rayleigh scattering, where light hits particles smaller than its wavelength, primarily air molecules. Shorter, bluer wavelengths are scattered far more effectively than redder wavelengths. This is why blue-rich artificial sources contribute disproportionately to skyglow. Larger particles, such as dust and aerosols, also scatter light through Mie scattering, which contributes to the overall brightness and gives the sky a hazy, uniform tint.
The Unveiling of Celestial Detail
Without skyglow, the night sky transforms. The density of visible stars increases dramatically, revealing thousands of fainter stars and changing familiar constellation patterns. Under the darkest conditions, the naked-eye limiting magnitude—the faintest stars an observer can see—can extend to magnitude 7.6 to 8.0, compared to magnitude 5.0 or 6.0 in suburban skies.
The Milky Way galaxy undergoes the most profound change, appearing as a vast, structured band of light rather than a faint smudge. Its core in Sagittarius and Scorpius is so bright it can cast discernible shadows on the ground. Intricate dust lanes and nebulae within the galaxy become clearly visible, giving the Milky Way the appearance of a luminous, veined structure stretching from horizon to horizon.
The high contrast of a dark sky allows the human eye to perceive subtle colors in astronomical objects. Distant galaxies like the Triangulum Galaxy (M33) become naked-eye objects, and globular clusters appear as distinct, fuzzy spheres. A truly dark sky also reveals natural phenomena often mistaken for light pollution, such as the faint, reddish-green glow of airglow caused by chemical reactions high in the atmosphere. The Zodiacal Light, a cone of sunlight scattered by interplanetary dust, becomes visible, often appearing yellowish and bright enough to cast weak shadows just before dawn or after dusk.
The Scale of Lost Visibility
The extent of lost visibility due to light pollution is quantified by the Bortle Dark-Sky Scale, a nine-level system for comparing night sky darkness. This scale ranges from Class 1, representing the most pristine dark-sky sites, to Class 9, which describes the heavily light-polluted skies of an inner city.
A Class 9 sky is so bright that only the moon, major planets, and a few dozen of the brightest stars are visible, and the sky background appears as a gray or orange haze. In contrast, a Bortle Class 1 sky is the ideal state, where the Milky Way is fully structured and airglow is readily apparent. Class 9 skies only allow stars brighter than magnitude 4.5 to be seen, while Class 1 allows for stars up to magnitude 8.0. Truly pristine Bortle 1 skies are extremely rare, but designated International Dark Sky Places work to preserve these remaining pockets of darkness.