The natural night sky brightness is a relevant input for monitoring the light pollution
evolution at observatory sites, by subtracting it from the overall sky brightness determined
by direct measurements. It is also instrumental for assessing the expected darkness of the
pristine night skies. The natural brightness of the night sky is determined by the sum of
the spectral radiances coming from astrophysical sources, including zodiacal light, and the
atmospheric airglow. The resulting radiance is modified by absorption and scattering before it
reaches the observer. Therefore, the natural night sky brightness is a function of the location,
time and atmospheric conditions. We present in this work GAMBONS (GAia Map of the
Brightness Of the Natural Sky), a model to map the natural night brightness of the sky in
cloudless and moonless nights. Unlike previous maps, GAMBONS is based on the extraatmospheric star radiance obtained from the Gaia catalogue. The Gaia-DR2 archive compiles
astrometric and photometric information for more than 1.6 billion stars up to 𝐺 = 21magnitude.
For the brightest stars, not included in Gaia-DR2, we have used the Hipparcos catalogue
instead. After adding up to the star radiance the contributions of the diffuse galactic and
extragalactic light, zodiacal light and airglow, and taking into account the effects of atmospheric
attenuation and scattering, the radiance detected by ground-based observers can be estimated.
This methodology can be applied to any photometric band, if appropriate transformations
from the Gaia bands are available. In particular, we present the expected sky brightness for
𝑉 (Johnson), and visual photopic and scotopic passbands.
