Measuring Anthropogenic Sky Glow Using a Natural Sky Brightness Model

Anthropogenic sky glow (a result of light pollution) combines with the natural background brightness of the night sky when viewed by an observer on the earth''s surface. In order to measure the anthropogenic component accurately, the natural component must be identified and subtracted. A model of the moonless natural sky brightness in the V-band was constructed from existing data on the Zodiacal Light, an airglow model based on the van Rhijn function, and a model of integrated starlight (including diffuse galactic light) constructed from images made with the same equipment used for sky brightness observations. The model also incorporates effective extinction by the atmosphere and is improved at high zenith angles (>80 degrees degrees) by the addition of atmospheric diffuse light. The model may be projected onto local horizon coordinates for a given observation at a resolution of 0.05 degrees degrees over the hemisphere of the sky, allowing it to be accurately registered with data images obtained from any site. Zodiacal Light and integrated starlight models compare favorably with observations from remote dark sky sites, matching within +/-+/- 8nL over 95% of the sky. The natural airglow may be only approximately modeled, errors of up to +/-+/- 25nL are seen when the airglow is rapidly changing or has considerable character (banding); +/-+/- 8nL precision may be expected under favorable conditions. When subtracted from all-sky brightness data images, the model significantly improves estimates of sky glow from anthropogenic sources, especially at sites that experience slight to moderate light pollution.