A photometric mapping of the night sky brightness of the Maltese islands

Over the years, the Maltese Islands have seen a marked rise in the prevalence of artificial lighting at night. The most evident type of light pollution arising from this evolution in anthropogenic night-time lighting is artificial skyglow via partial back-scattering in the atmosphere, leading to an increase in the Night Sky Brightness (NSB). The importance of understanding and quantifying the geographical distribution of the NSB is underscored by the adverse impact of light pollution on various spheres, from astronomical observation to ecology and human health. For the first time, we present a detailed map of the NSB over the Maltese archipelago carried out with Unihedron Sky Quality Meters. We show that the vast majority of the area of the Maltese Islands is heavily light polluted, with 87% of the area registering a NSB < 20.39 mag(SQM)/arcsec(2) (Sortie Class 5 or higher) and 37.3% < 19.09 mag(SQM)/arcsec(2) (Bortle Class 6 or higher), with the Milky Way being visible for only 12.8% of the area (adopting a visibility threshold > 20.4 - 21.29 mag(SQM)/arcsec(2); Bortle Class 4). Coastal Dark Sky Heritage Areas on the island of Gozo retain generally darker skies than the rest of the islands, but light pollution originating further inland is encroaching upon and adversely affecting these sites. The methodology presented in this study can be adopted for continued future studies in Malta as well as for other regions.