  The largest natural source of light at night is the Moon, and it is the major
contributor to the astronomical sky background. Being able to accurately
predict the sky background, including scattered moonlight is important for
scheduling astronomical observations. We have developed an improved scattered
moonlight model, in which the components are computed with a better physical
understanding as opposed to the simple empirical fit in the frequently used
photometric model of Krisciunas & Schaefer (1991). Our spectroscopic model can
better trace the spectral trends of scattered moonlight for any position of the
Moon and target observation. This is the first scattered moonlight model that
we know of which is this physical and versatile. We have incorporated an
observed solar spectrum, accurate lunar albedo fit, and elaborate scattering
and absorption calculations that include scattering off of molecules and
aerosols. It was designed for Cerro Paranal, but can be modified for any
location with known atmospheric properties. Throughout the optical range, the
uncertainty is less than 20%. This advanced scattered moonlight model can
predict the amount of scattered moonlight for any given geometry of the Moon
and target, and lunar phase for the entire optical spectrum.
