Monitoring Long-Term Trends in the Anthropogenic Night Sky Brightness

Monitoring long-term trends in the evolution of the anthropogenic night sky brightness is a demanding task due to the high dynamic range of the artificial and natural light emissions and the high variability of the atmospheric conditions that determine the amount of light scattered in the direction of the observer. In this paper, we analyze the use of a statistical indicator, the m(FWHM), to assess the night sky brightness changes over periods of time larger than one year. The m(FWHM) is formally defined as the average value of the recorded magnitudes contained within the full width at half-maximum region of the histogram peak corresponding to the scattering of artificial light under clear skies in the conditions of a moonless astronomical night (sun below -18 degrees, and moon below -5 degrees). We apply this indicator to the measurements acquired by the 14 SQM detectors of the Galician Night Sky Brightness Monitoring Network during the period 2015-2018. Overall, the available data suggest that the zenithal readings in the Sky Quality Meter (SQM) device-specific photometric band tended to increase during this period of time at an average rate of +0.09 mag(SQM)/arcsec(2) per year.