Sky glow is the brightening of the night sky over cities, reducing visibility of stars and contributing to light pollution. 

Light pollution, especially in the form of increased sky brightness, most visibly affects the transformation of how we perceive the visual appearance of the night landscape. We usually perceive the landscape during the day, when the overall impression also includes the sky above it and the phenomena associated with it (sunrise and sunset, weather). Likewise, at night the landscape includes the night sky, whose elements consist of astronomical objects such as the Moon, stars, the band of our galaxy known as the Milky Way, planets and zodiacal light, as well as atmospheric phenomena such as auroras, noctilucent clouds and airglow. For many thousands of years, people have used the starry sky for spatial and temporal orientation, as a source of information about the weather; it has been closely connected with religious and mythological narratives and has also served as a source of inspiration for artists and scientists. Light pollution results in the loss of visibility of this part of the landscape—while under natural skies it is possible to see several thousand stars with the naked eye, in most parts of the Czech Republic only hundreds of stars are visible, and in the most severely affected areas, such as the centres of large cities and agglomerations, only a few or a few dozen stars can be seen. The night sky is considered by UNESCO to be part of the world’s heritage, and in order to preserve and promote the starry sky, so-called dark-sky areas and parks have been established in recent years. These are often part of other protected areas, such as national parks. In the Czech Republic, there are three dark-sky areas (the Jizera Mountains, the Beskydy Mountains and the Manětín area). The cultural significance of the starry sky for humans is practically incomparable to its importance for most wild species that are described as “nocturnal.” For them, natural darkness is a necessary condition for survival and for the long-term prospects of their species. From the position of a diurnal creature, humans can hardly imagine everything that night provides for animals active in what we call darkness. Between dusk and dawn, various phases of the night occur, which individual species use, for example, to obtain food and carry out other activities under conditions in which they are as protected as possible from predators higher up the food chain. By introducing artificial light sources into the night landscape, it commonly happens—due to attraction to light, for example—that insects are displaced and concentrated at times and in places where this is not natural, which inevitably affects birds, amphibians, as well as plants, and on a broader scale, the entire ecosystem.