2000PASP..112..566M

Recent (1998-1999) absolute spectrophotometry of the night sky over two southern Arizona astronomical sites, Kitt Peak and Mount Hopkins, is compared to similar data obtained in 1988 at each site. The current zenith sky brightness in the range ∼3700-6700 Å is essentially identical at the two sites and is as dark now as Palomar Observatory was in the early 1970s, when it was generally considered a premier dark observing site. Converted to broadband measurements, our spectrophotometry is equivalent to B=22.63, V=21.45 mag.arcsec^–2, for the zenith night sky. The contribution of high-pressure sodium street lights to broadband V is about 0.2 mag.arcsec^–2, comparable to the strong airglow O I λ5577 line. During the period from 1988 to 1998-1999, the zenith sky brightness increased only modestly, with the largest changes being seen for Kitt Peak, where the zenith sky has brightened by ~0.1-0.2 mag.arcsec^–2 in the blue-optical region. For Kitt Peak we also have both 1988 and 1999 observations at modestly large zenith distances (ZD~60°). In the directions away from Tucson, the sky has brightened by ~0.35 mag.arcsec^–2 over the intervening decade. Toward Tucson the change has been larger, approximately 0.5 mag.arcsec^–2. In most directions the increase in the sky brightness has lagged behind the fractional increase in population growth, which we attribute to good outdoor lighting ordinances, a fact which is further reflected in the decrease in Hg emission. However, our results emphasize the need for diligent attention as developments creep closer to our observing sites.