Influence of celestial light on lunar surface brightness determinations: Application to earthshine studies

We consider the influence of celestial-sphere brightness on determinations of terrestrial albedo from earthshine intensity measurements. In particular, the contributions from zodiacal light and starlight are considered. Methods: Using published data for the zodiacal light (ZL) and stellar brightness distribution across the sky, we calculate the expected contribution to the sky at the position of the Moon in typical earthshine observations, and the magnitude relative to typical earthshine intensities. We derive terrestrial albedo with and without the ZL correction in order to gauge the magnitude of the effect. Results: We find that celestial-sphere surface brightness can be so large that a considerable and unacceptable error level would have an impact on half of typical earthshine-based albedo-determinations if left unaccounted for. Considering the empirical uncertainty on ZL, we show that almost all our earthshine data can be used if a sky correction is made. In real observations we find up to a 1% effect on albedo results of correcting for the celestial brightness. Conclusions: Correction for ZL and starlight brightness is essential to earthshine measurements if climate-science relevant levels of terrestrial albedo accuracy are to be achieved, something that has not yet been realized. With ZL and starlight corrections the earthshine method can potentially yield accurate terrestrial albedo values