The Rudolf Wolf Society (RWG), founded in Zurich in 1992, aims at the continuation of the series of sunspot relative numbers founded by Rudolf Wolf in the 19th century as well as the preservation and scientific utilization of Wolf's legacy. It is an association ruled by Swiss law.
Since 1986, about 15 active amateur solar observers from Switzerland and abroad have been monitoring solar activity according to a standardized observation program. The number, grouping and developmental status of all visible sunspots are recorded. The daily logs are stored and evaluated monthly in a central database. Since January 1986 more than 50'000 individual observations are available.
The real value of the Wolf series lies in its homogeneity, i.e. in the scale factor which has been more or less constant for centuries. This homogeneity was achieved by carefully calibrating each individual observer to the Wolfs normal refractor and by passing on the Wolfs counting method from one generation of standard observers to the next. Since Rudolf Wolf's historical Fraunhofer refractor came into the care of the Rudolf Wolf Society in 1996 after the final closure of the observing station at the former Federal Observatory, the daily calibration observations could be continued seamlessly (standard observer: Dr. Thomas K. Friedli, Belp) and the scale factor could be maintained homogeneously.