In the 20th century the archival documents were organized in three series (Osservazioni astronomiche, Osservazioni meteorologiche, Buste) by Guido Horn d’Arturo (1879-1967), for a long time Director of the Astronomical Observatory of the Bologna University. At the end of the same century the Archives were enriched by the legacy of the Fondo Guido Horn d’Arturo.
At the beginning of 21st century a reorganization gave to the Archives its present structure in five series:
- Serie Osservazioni astronomiche: they contain the documents collected by Horn under this name and more documents found afterwards;
- Serie Osservazioni meteorologiche: they contain the documents collected by Horn under this name and more documents found afterwards;
- Serie storica Specola XVII-XIX secolo: they contain the documents arranged by Guido Horn d’Arturo in the series Buste, enclosing the main part of the archival materials, among which a relevant number of letters between astronomers working in Bologna and scientists in the world: Gian Domenico Cassini, Paris Maria Salvago, Luigi Ferdinando Marsili, Georg Christoph Eimmart, Johann Jakob Scheuchzer, Antoine-François Laval, Ottaviano Fabrizio Mossotti, Lorenzo Respighi and many others;
- Serie nuova Specola XVII-XXI secolo: they collect documents not arranged by Horn and more documents found afterwards;
- Fondo Guido Horn d’Arturo: it contains the correspondence of the astronomer from 1912 to 1939 donated by his heirs and more documents found afterwards.
The documents of the Archives have been the subject of numerous studies and publications (see “Bibliography”).