Circalunar variations of the night sky brightness -- an FFT perspective on the impact of light pollution

TL;DR

This study uses FFT analysis of two-year sky brightness data from 23 photometers in Austria to quantify how artificial light pollution diminishes the natural circalunar cycle's detectability in night sky brightness.

Contribution

It provides a quantitative assessment of light pollution's impact on circalunar periodicity detection using frequency analysis across multiple sites.

Findings

Light pollution reduces the circalunar signal amplitude significantly.

Natural lunar rhythms are detectable only in protected national parks.

Urban sites show seasonal variations linked to increased winter skyglow.

Abstract

Circa-monthly activity conducted by moonlight is observed in many species on Earth. Given the vast amount of artificial light at night (ALAN) that pollutes large areas around the globe, the synchronization to the circalunar cycle is often strongly perturbed. Using two-year data from a network of 23 photometers (Sky Quality Meters; SQM) in Austria (latitude ~48{\deg}), we quantify how light pollution impacts the recognition of the circalunar periodicity. We do so via frequency analysis of nightly mean sky brightnesses using Fast Fourier Transforms. A very tight linear relation between the mean zenithal night sky brightness (NSB) given in mag$_{SQM}$ and the amplitude of the circalunar signal is found, indicating that for sites with a mean zenithal NSB brighter than 16.5 mag$_{SQM}$ the lunar rhythm practically vanishes. This finding implies that the circalunar rhythm is still detectable (within the broad bandpass of the SQM) at most places around the globe, but its amplitude against the light polluted sky is strongly reduced. We find that the circalunar contrast in zenith is reduced compared to ALAN-free sites by factors of 1/9 in the state capital of Linz (~200,000 inhabitants) and 1/3 in small towns, e.g. Freistadt and Mattighofen, with less than 10,000 inhabitants. Only two of our sites, both situated in national parks (Bodinggraben and Z\"oblboden), show natural circalunar amplitudes. At our urban sites we further detect a strong seasonal signal that is linked to the amplification of anthropogenic skyglow during the winter months due to climatological conditions.