SIMBAD references

We report measurements of the sky brightness at the Calar Alto Observatory during 18 moonless nights in the years 1989, 1990, 1991, and 1993. Most of the measurements were performed in eight intermediate band filters between 350nm and 820nm. A set of measurements was obtained in the broad band UBVRI filters during three nights in 1990. Typically 10 to 20 readings were obtained each night for one or two fixed positions (fixed in α, δ), selected to be free of stars down to a limiting magnitude of about B=20. The sky brightnesses were calibrated using standard stars. We discuss the sky brightness variations both during individual nights and from year to year and compare the observed sky brightnesses to measurements from other observatory sites. We have found that the long-term variations are well correlated with the solar activity, for which the 10.7cm radio flux is used as a measure.