Astronomical observatory publications: information exchange before the Internet era

For decades, perhaps even centuries, the exchange of publications between observatories was the most important source of information on new astronomical results, either in the form of observational data or new scientific theories. In particular, small observatories or institutions used this method. The exchange of physical material between observatories has now been replaced by the exchange of information via the Internet. Yet much of the ancient material has never been digitized and can only be found in the few existing collections of observatory publications. A recent donation of such a collection from the University of Copenhagen to our own library at the University of Southern Denmark has led us to investigate the uniqueness of such collections: Which observatories and publications are represented in the collections that still exist today? We also examine the availability of the material in the collections.