the degree of cloudiness does not follow any strict periodicity. Therefore, the mean NSB measured in Vienna seems to vary in a stochastic way. The circalunar rhythm is barely recognizable here.

The lower half of Fig. \[fig:lightcurve\] also shows a circannual periodicity of the NSB close to full moon: winter nights with the full moon at higher elevations are significantly brighter (by $\approx$ 1.5) than summer nights.

![Mean daily NSB within astronomical twilight. Distribution over one year (date format: yyyy/mm/dd) obtained at both sites IfA (top) and LFOA (bottom). At LFOA the circalunar rhythm dominates the variations, whereas in Vienna the natural variation is barely recognizable. The LFOA data also show a circannual period depending on the maximum elevation of the full moon (see dashed lines).[]{data-label="fig:lightcurve"}](Figure3a.eps "fig:"){width="11.5cm"} ![Mean daily NSB within astronomical twilight. Distribution over one year (date format: yyyy/mm/dd) obtained at both sites IfA (top) and LFOA (bottom). At LFOA the circalunar rhythm dominates the variations, whereas in Vienna the natural variation is barely recognizable. The LFOA data also show a circannual period depending on the maximum elevation of the full moon (see dashed lines).[]{data-label="fig:lightcurve"}](Figure3b.eps "fig:"){width="11.5cm"}

Range of NSB values under urban conditions
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Figures \[fig:lightcurve\] (upper part) and 