The \(\boldsymbol{BVR_{c}I_{c}}\) Sky Brightness of the Caucasian Mountain Observatory of MSU

In this paper we analyse the measurements of the brightness of the night sky above the CMO SAI MSU in the visible and near-infrared range made in 2019–2014. In 2023–2024 the median zenith brightness of 1 arcsec\({}^{2}\) of the moonless night sky was \(21.31^{m}\) in the \(B\) band, \(20.63^{m}\) in the \(V\) band, \(20.15^{m}\) in the \(R_{c}\) band, and \(19.11^{m}\) in the \(I_{c}\) band. In 5 years the sky brightness had increased by \(0.7^{m}\) in \(B\) and \(V\), by \(0.45^{m}\) in \(R_{c}\), and \({\sim}0.1^{m}\) in \(I_{c}\). We found that the brightness growth is mostly (up to \({\sim}85\%\)) due to the increasing light pollution from nearby cities, while the remainder can be attributed to the increase of solar activity after the 2019 minimum. We discuss how the sky brightness is influenced by such factors as the airmass and the location of the Sun and the Moon in the sky. A qualitative analysis of the sky emission spectrum has demonstrated the growing role of LED lamps in light pollution. These changes in sky brightness which are only going to get harder favour observations that are less sensitive to the degree of light pollution—IR photometry and spectroscopy and high-resolution optical spectroscopy.