We have used the large database of photometric observations of the bright and dark portions of the face of the Moon from the Earthshine Project at Big Bear Solar Observatory to determine the surface brightness of the earthshine and its variations. Our purpose is to make these observations appropriate for the calibration of lunar flashes according to their magnitude. We have evaluated the daily, seasonal, and annual changes in magnitude for our entire data set and have also calibrated the surface brightness of the entire lunar geography for several lunar phases by means of the observation of lunar eclipses. We find variations between +12 and +17 mV arcsec-2 with hourly changes upward of the order 0.25 mV arcsec-2, which are uniquely due to the terrestrial meteorology. This rapid change in the terrestrial flux reaching the Moon is usually neglected when calibrating the magnitude of lunar impact events. We justify this using earthshine observations to determine the brightness for the day, time, and selenographic location of a given event in order to improve the accuracy of its brightness calibration up to 0.25 mag.