  Artificial skyglow is dynamic due to changing atmospheric conditions and the
switching on and off of artificial lights throughout the night. Street lights
as well as the ornamental illumination of historical sites and buildings are
sometimes switched off at a certain time to save energy. Ornamental lights in
particular are often directed upwards, and can therefore have a major
contribution towards brightening of the night sky. Here we use differential
photometry to investigate the change in night sky brightness and illuminance
during an automated regular switch-off of ornamental light in the town of
Balaguer and an organized switch-off of all public lights in the village of
\`Ager, both near Montsec Astronomical Park in Spain. The sites were observed
during two nights with clear and cloudy conditions using a DSLR camera and a
fisheye lens. A time series of images makes it possible to track changes in
lighting conditions and sky brightness simultaneously. During the clear night,
the ornamental lights in Balaguer contribute over 20% of the skyglow at zenith
at the observational site. Furthermore, we are able to track very small changes
in the ground illuminance on a cloudy night near \`Ager.
