  In 2018, Solar Cycle 24 entered into a solar minimum phase. During this
period, 11 million zenithal night sky brightness (NSB) data were collected at
different dark sites around the planet, including astronomical observatories
and natural protected areas, with identical broadband Telescope Encoder and Sky
Sensor photometers (based on the Unihedron Sky Quality Meter TSL237 sensor). A
detailed observational review of the multiple effects that contribute to the
NSB measurement has been conducted with optimal filters designed to avoid
brightening effects by the Sun, the Moon, clouds, and other astronomical
sources (the Galaxy and zodiacal light). The natural NSB has been calculated
from the percentiles for 44 different photometers by applying these new
filters. The pristine night sky was measured to change with an amplitude of 0.1
mag/arcsec$^2$ in all the photometers, which is suggested to be due to NSB
variations on scales of up to months and to be compatible with semiannual
oscillations. We report the systematic observation of short-time variations in
NSB on the vast majority of the nights and find these to be related to airglow
events forming above the mesosphere.
