1 Introduction

The diffuse brightness of the moonless night sky is produced by natural and artificial sources. Natural processes include airglow, tropospheric scattering of starlight, light from faint stars and the zodiacal light. Updated levels of natural sky brightness have been reported by Leinert et al. (1998) and Schaefer (1998).

On the other hand, artificial night sky brightness is due to man-made lighting systems that, unfortunately, also illuminate the sky, both directly and by single or multiple reflections from the ground and buildings. Both natural and artificial sources are variable. It is well known, for instance, that airglow emissions vary according to changes in the upper atmosphere and the solar cycle.

Unlike the natural change of night sky brightness, which at some point eventually returns to a minimum, light pollution has increased steadily over time. Even the casual observer is able to realize that the night sky is much brighter now than in the past. This issue has concerned not only the astronomical community but also common citizens, at the point that several regulations are currently in effect to protect sky conditions at major observatories and at special sites, such as natural parks and reservations.

Nevertheless, the efficacy of these and future regulations can only be assessed by the direct monitoring of the night sky quality over time. This is usually carried out at most observatories, but measurements of night sky brightness at other common places, such as cities, towns and villages, which represent the main polluting sources, are scarce. In Venezuela, measurements of night sky brightness have been carried out at the Venezuelan Astronomical Observatory (VAO) only recently. Located near the equator at an altitude of 3600 m in the Andes, the VAO is protected by a recent regulation (Gaceta Oficial 1991), intended to minimize light pollution in its vicinity. In Italy, the first light pollution map of the territory was compiled by Bertiau et al. (1973). At present, several regional laws regulate local lighting systems and a national law is being discussed (Cinzano 1997).

Since these legal tools will hopefully reduce, or at least stop, light pollution and improve artificial lighting standards, this paper reports recent zenith night sky brightness at selected places in Venezuela and Italy, including large cities, town and villages, with the aim to provide reference values that can be used for future comparisons and also to test the feasibility of night sky brightness predictions using satellite images of the Earth at night.

At the same time, a small spectrograph was used to take representative spectra of the night sky from about 4100 to 6400 Å. This is because forty years ago public lighting relied basically on incandescent lamps; these have a continuum spectrum corresponding, essentially, to a blackbody source at 2600 K. Nowadays, the majority of the lamps used in public lighting are of the gas discharge type, namely High Pressure Sodium and Mercury, with strong emission lines in their spectra. For this reason, a more comprehensive description of night sky conditions is given not only by the zenith brightness, but also by the spectral distribution of the light that pollutes the sky at a specific site.