Skyglow

Skyglow is the diffuse brightening of the night sky over and around urban areas caused by artificial lighting. It is a major component of light pollution and arises when light from streetlights, signage, buildings, and vehicles is emitted upward or leaks into the surroundings and then scattered by molecules and aerosols in the atmosphere. The scattering processes, including Rayleigh and Mie scattering, tend to shift shorter wavelengths toward a visible glow, particularly in blue-rich lighting, contributing to a characteristic halo above populated regions.

The presence of skyglow reduces the contrast of celestial objects, making fainter stars and nebulae harder to observe and hindering astronomical research and recreational stargazing. Beyond astronomy, skyglow can affect ecosystems by altering nocturnal behavior, migration patterns, and reproductive cycles in wildlife, and it has been associated with potential circadian disruption in humans. It also represents energy waste and unnecessary carbon emissions from lighting that serves little practical purpose.

Measurement and mitigation: Sky brightness is commonly described in magnitudes per square arcsecond and can be assessed with all-sky imaging, photometers, or sky quality meters. Mitigation strategies focus on reducing upward light emission and improving lighting design. These include fully shielding fixtures, directing light downward, using lower color temperatures, dimming fixtures when possible, and implementing lighting ordinances or protective areas such as dark-sky reserves. International cooperation and public awareness efforts aim to preserve dark skies while meeting safety and outdoor lighting needs.