  Anthropogenic skyglow dominates views of the natural night sky in most urban
settings, and the associated emission of artificial light at night (ALAN) into
the environment of cities involves a number of known and suspected negative
externalities. One approach to lowering consumption of ALAN in cities is
dimming or extinguishing publicly owned outdoor lighting during overnight
hours; however, there are few reports in the literature about the efficacy of
these programs. Here we report the results of one of the largest municipal
lighting dimming experiments to date, involving $\sim$20,000 roadway luminaires
owned and operated by the City of Tucson, Arizona, U.S. We analyzed both
single-channel and spatially resolved ground-based measurements of broadband
night sky radiance obtained during the tests, determining that the zenith sky
brightness during the tests decreased by ($-5.4\pm0.9$)% near the city center
and ($-3.6\pm0.9$)% at an adjacent suburban location on nights when the output
of the street lighting system was dimmed from 90% of its full power draw to 30%
after local midnight. Modeling these changes with a radiative transfer code
yields results suggesting that street lights account for about ($14\pm1$)% of
light emissions resulting in skyglow seen over the city. A separate derivation
from first principles implies that street lighting contributes only 2-3% of
light seen at the zenith over Tucson. We discuss this inconsistency and suggest
routes for future work.
