Night sky brightness measurement, quality assessment and monitoring

Ground-based optical astronomy necessarily involves sensing the light of astronomical objects along with the contributions of many natural sources ranging from the Earth's atmosphere to cosmological light. In addition, astronomers have long contended with artificial light pollution, which further adds to the 'background' against which astronomical objects are seen. Understanding the brightness of the night sky is therefore fundamental to astronomy. The last comprehensive review of this subject was nearly a half-century ago, and we have learned much about both the natural and artificial night sky since. This Review considers which influences determine the total optical brightness of the night sky, the means by which this brightness is measured and how night sky quality is assessed and monitored in the long term. Light pollution is a growing problem affecting the night sky. This Review takes a comprehensive look at the factors contributing to elevated brightness at night, the measurement techniques and the ongoing assessment of sky quality.