Table 1. Pristine sites with natural sky background brightness can be identified by measuring the sky background (astronomical unit mag/arcsec² in the visual range corresponds to a negative logarithmic scale of sky luminance) at modern astronomical observatories. Older observatories were built at dark sites close to cities. Today, these peri-urban sites are characterized by bright skies. Some observatories in the southwest of the USA were able to preserve relatively dark skies because of lighting ordinances (low pressure sodium lamps, full cut-off luminaires) in the nearby cities, resulting in a decrease of sky brightness in Flagstaff, Arizona since 1982 (Lockwood et al. 1990). The increase of the sky background depends on different model assumptions for several observatories (Garstang 1989). Indirect values can be derived from the increase of power consumption and lamp efficiency for street lighting.


Period Sky brightness (mag/arcsec²) Annual increase
(%)
Cause References
Pristine
Mt. Graham, AZ, USA 2000 – 2008 21.5 – 21.8 0 solar activity Pedani 2009
Hawaii, USA 1986 – 1996 21.3 – 21.9 0 solar activity Krisciunas 1997
Cerro Tololo/Chile 1992 – 2006 22.0 – 21.2 0 solar activity Krisciunas et al. 2007
Kitt Peak/Mt. Hopkins, AZ, USA 1988 – 1999 Δ 0.1 – 0.2 1 – 2 ordinances Massey and Foltz 2000
           
Urban
Flagstaff, AZ 1974 – 1982 Δ 0.4 5 Lockwood et al. 1990
IAO, India 2003 – 2007 21.3 – 21.1 5 Stalin et al. 2008
Mt. Wilson, USA 1973 – 1999 19.8 – 18.8 3.6 city lights Los Angeles Teare 2000
1900 – 2000 Δ 2.7 2.5 city lights Los Angeles Garstang 2004
Mt. Hamilton, CA, USA 1950 – 1975 21.9 – 20.6 5 city lights Walker 1973
Ekar, Asiago, Italy 1960 – 1995 21.6 – 20.2 10 city lights Cinzano 2000
G. Haro, Mexico 1994 – 1997 20.5 – 20.0 19 city lights Carrasco et al. 1998
NAO, Japan 1958/1978/1989 20.5/19.5/17.6 5 – 17 city lights Tokyo Isobe and Kosai 1998
           
Worldwide
Expert guess 3 Narisada and Schreuder 2004
           
Indirect (energy consumption)
UK Campaign to Protect Rural England (CPRE) 1993 – 2000 4 DMSP data CPRE 2003
UK McNeill 1984 – 1999 1.86x lmh 4.2 street light McNeill 1999
UK Fouquet/Pearson 1950 – 2000 5x lmh 3.3 overall energy consumption for light Fouquet and Pearson 2006
USA 1967 – 1970 20 energy + efficiency Riegel 1973
France 1990 – 2000 3 energy ADEME 2007
Germany 1950 – 1990 8 energy + efficiency Hänel 2001


Note: IAO India = Indian Astronomical Observatory, NAO Japan = National Astronomical Observatory of Japan