Estimating the horizontal irradiance from measurements of the zenith night-sky radiance is a useful operation for basic and applied studies in observatory site assessment, atmospheric optics, and environmental sciences. The ratio between these two quantities, also known as the Posch ratio, has previously been studied for some canonical cases and reported for a few observational sites. In this work we (a) generalize the Posch ratio concept, extending it to any pair of radiance-related linear indicators, (b) describe its main algebraic properties, and (c) provide analytical expressions and numerical evaluations for its three basic night-time components (moonlight, starlight and other astrophysical light sources, and artificial light). We show that the horizontal irradiance (or any other linear radiance indicator) is generally correlated with the zenith radiance, enabling its estimation from zenith measurements if some a priori information on the atmospheric state is available.

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