Regular photometric observations of the moon's ashen light (earthshine) from the Big Bear Solar Observatory (BBSO) since December 1998 have quantified the earth's optical reflectance. We find large (~5%) daily variations in the reflectance due to large-scale weather changes on the other side of the globe. Separately, we find comparable hourly variations during the course of many nights as the earth's rotation changes that portion of the earth in view. Our data imply an average terrestrial albedo of 0.297¿0.005, which agrees with that from simulations based upon both changing snow and ice cover and satellite-derived cloud cover (0.296¿0.002). However, we find seasonal variations roughly twice those of the simulation, with the earth being brightest in the spring. Our results suggest that long-term earthshine observations are a useful monitor of the earth's albedo. Comparison with more limited earthshine observations during 1994--1995 show a marginally higher albedo then. ¿ 2001 American Geophysical Union