Quantifying circalunar periodicity, long-term trends and seasonal variations in the night sky brightness based on the Austrian SQM network

Based on the paper by Posch, Binder & Puschnig (JQSRT 211, 144), we extend the study of the night sky brightness (henceforth: NSB) at 26 locations in Eastern Austria in time and complement it by a new method to quantify the amplitude of the circalunar periodicity. Using a novel weighting technique for SQM measurements, we evaluate the long-term development of the night sky brightness between 2012-18 for one of our sites, the Vienna University Observatory. Additionally, we present new spectra of Vienna's skyglow, obtained with a 0.8m reflector, and compare them to the spectra obtained five years earlier (i.e. after versus before the installation of thousands of LEDs in the city). The large dynamical range of NSB levels found at our 26 SQM stations further allows us to study and quantify the impact of light pollution on the amplitude of the circalunar periodicity. We present first results from our Fast Fourier Transform based approach.For two selected regions where the measured NSB values are close to those expected for unpolluted skies [Losenstein Hohe Dirn, Feuerkogel NP Attersee-Traunsee], we compare the results of our SQM measurements to those obtained with a calibrated all-sky-camera and discuss the implications for two planned Dark Sky Parks.We show suitable ways to plot and analyze huge long-term NSB datasets, such as mean-NSB histograms, circalunar, annual ("hourglass") and cumulative ("jellyfish") plots. Based on the hourglass plots, we find a strong circalunar periodicity of the NSB in small towns and villages (< 5.000 inhabitants), with amplitudes of up to 5 magnitudes. Using the jellyfish plots, on the other hand, we demonstrate that the examined city skies brighten by up to 3 magnitudes under cloudy conditions, which strongly dominate in those cumulative data representations. Nocturnal gradients of the NSB of 0.0-0.14 mag_SQM/arcsec²/h are found