Zenitshimmljusstyrkan

Zenitshimmljusstyrkan, often translated as zenith sky brightness, refers to the apparent brightness of the sky when viewed directly overhead, at the zenith. This measurement is a crucial parameter in understanding and quantifying light pollution. Light pollution, the excessive or misdirected artificial light, scatters in the atmosphere, making the night sky appear brighter than it naturally would. Measuring zenith sky brightness helps astronomers and environmental scientists assess the degree of skyglow in a particular location.

The measurement is typically expressed in magnitudes per square arcsecond. A darker sky will have a lower numerical value, indicating less brightness. For instance, a truly dark, pristine night sky might have a zenith sky brightness of around 21-22 magnitudes per square arcsecond. In contrast, brightly lit urban areas can have zenith sky brightness values as low as 10-15 magnitudes per square arcsecond, making astronomical observations extremely difficult and obscuring fainter celestial objects.

Various instruments, such as sky quality meters, are used to measure zenith sky brightness. These devices quantify the amount of light entering the sensor and convert it into the standardized magnitude scale. Understanding zenith sky brightness is vital for setting effective light pollution control policies, protecting nocturnal ecosystems, and preserving the aesthetic and scientific value of the night sky. It provides a quantitative basis for comparing the "darkness" of different locations.