One important issue with observing artificial night lighting from space that needs to be addressed is a phenomenon known as skyglow. Even in its pristine state the night sky is not completely dark. Some light comes from the stars, some from sunlight scattered by space dust in the plane of the solar system, and some from atmospheric gases subject to radiation and particle fluxes mostly from the sun (Clark 2008). This is called natural skyglow. Light emitted from human settlements in the atmosphere is refracted or scattered by air and water molecules and suspended particles (atmospheric aerosol) caused by dust, pollen, salt from sea spray, and waste products from industry (House of Commons 2003). Artificially illuminating the sky over great distances this is called artificial skyglow. In particular in the field of astronomy skyglow obscuring the night sky is an issue of utmost importance with extensive scientific research being conducted in recent years. Baddiley’s guide ‘Towards Understanding Skyglow’ (2007a) lists different sources contributing to skyglow in urban and rural areas. Furthermore a mathematical model of skyglow is presented considering different skyglow mechanisms (i.e. directly radiated light above the horizontal; reflected light from the road, ground and other surfaces; light scattered by air molecules; light scattered by aerosols) and different types of luminaires. According to Clark (2008) the total artificial light flux emitted by a city tends to be proportional to the product of two quantities, (1) the
number of light sources and (2) their mean output of light. Related to a growing economy and urban population growth typically both of these quantities increase over time. Considering artificial skyglow entails that the DMSP satellite sensors record much larger areas than just the immediate location of the lighting sources. Using satellite observed nighttime lights for delineating urban areas (Small et al. 2005) and approximating impervious surfaces (Elvidge et al. 2007) requires eliminating skyglow from the data, i.e. by applying thresholds to the digital number values. When dealing with ecological issues skyglow is a significant factor of light pollution as already very low light intensities alter the natural environment. Following recent approaches of modeling ecological impact of artificial night lighting (Aubrecht et al. 2008a) for the present analysis skyglow is thus not modeled out but rather considered as important contribution.