  Under stable atmospheric conditions, the zenithal brightness of the urban sky
varies throughout the night following the time course of the anthropogenic
emissions of light. Different types of artificial light sources (e.g.
streetlights, residential, and vehicle lights) present specific time
signatures, and this feature makes it possible to estimate the amount of sky
brightness contributed by each one of them. Our approach is based on
transforming the time representation of the zenithal sky brightness into a
modal coefficients one, in terms of the time course signatures of the sources.
The modal coefficients, and hence the absolute and relative contributions of
each type of source, can be estimated from the measured brightness by means of
linear least squares fits. A method for determining the time signatures is
described, based on wide-field time-lapse photometry of the urban nightscape.
Our preliminary results suggest that artificial light leaking out of the
windows of residential buildings may account for a significant share of the
time-varying part of the zenithal sky brightness, whilst the contribution of
the vehicle lights seems to be significantly smaller.
