4 Measurements

Measurements of night sky brightness in Venezuela were carried out from October 1997 to March 1998 and from October 1998 to February 1999. In Italy, a six-month, uninterrupted observing campaign started on April 1998. Results are summarized in Table 2. Values refer to zenith brightness on moonless, clear nights. The latter is very important, since effects from the scattering layer depend strongly on transparency. However, on exceptionally clear nights, especially in winter, measurements show that sky brightness can be 0.3 magnitude fainter than typical values. Also, special care was used to select a patch of sky far from the Milky Way and the Ecliptic. At the same time, before a reading could be taken, at least 20 minutes in subdued light were spent to reach a good level of dark adaptation. This is sometimes troublesome when the observer is near bright sources, as is the case in inhabited centers. In this situation, it is very useful to wear special, deep-red goggles in order to preserve the eyes dark adapted. A measurement comprises three to five consecutive readings of the sky brightness. The mean is reported in a special form that must be also filled with relevant data such as the site name, date, local time and a brief description of sky conditions. Measurements at Caracas, Mérida, Florence and Siena were carried out from downtown areas.