  The present work provides the results of the first six years of operation of
the systematic night-sky monitoring at ESO-Paranal (Chile). The UBVRI night-sky
brightness was estimated on about 10,000 VLT-FORS1 archival images, obtained on
more than 650 separate nights, distributed over 6 years and covering the
descent from maximum to minimum of sunspot cycle n.23. Additionally, a set of
about 1,000 low resolution, optical night-sky spectra have been extracted and
analyzed. The unprecedented database discussed in this paper has led to the
detection of a clear seasonal variation of the broad band night sky brightness
in the VRI passbands, similar to the well known semi-annual oscillation of the
NaI D doublet. The spectroscopic data demonstrate that this seasonality is
common to all spectral features, with the remarkable exception of the OH
rotational-vibrational bands. A clear dependency on the solar activity is
detected in all passbands and it is particularly pronounced in the U band,
where the sky brightness decreased by about 0.6 mag arcsec-2 from maximum to
minimum of solar cycle n.23. No correlation is found between solar activity and
the intensity of the NaI D doublet and the OH bands. A strong correlation
between the intensity of NI 5200A and [OI]6300,6364A is reported here for the
first time. The paper addresses also the determination of the correlation
timescales with solar activity and the possible connection with the flux of
charged particles emitted by the Sun.
