The auxiliary science of historical chronology and its role for climatologists

For a long time, auxiliary sciences in history such as palaeography and diplomatics have constituted an essential part in the education of history students to be prepared for source criticism. For historical climatologists, knowledge of historical chronology is probably the most important auxiliary science of historians. However, non-historians are in many cases not aware of the problems caused by different dating systems in the sources. This concerns on the one hand different starting dates of the New Year causing sometimes “doubled events” in two subsequent years. On the other hand, the Gregorian calendar reform of 1582 correcting the gap between the date in the Julian calendar system used so far and the actual date of the solar year has to be considered for long-time climate reconstruction based on proxy data such as grain and vine harvests or snow/ice cover. The problem even becomes more complicated as the majority of catholic territories adopted Pope Gregory XIII’s reform immediately, whereas the territories with Lutheran or Calvinist churches dominating refused this reform until around 1700 or even longer (parts of the Swiss canton of Grisons remained with the old calendar until the early 19th century!). In particular, sources from countries such as Switzerland and Germany with a mix of confessions have to be evaluated even more accurately before they can be used for climate reconstruction. The same is true for documentary evidence from orthodox countries such as pre-revolution Russia, i.e. before 1917, where the Gregorian calendar system has been introduced even two centuries later. Contact zones of western European and orthodox culture such as the Baltic States or territories on the Balkans therefore require specific attention when dealing with climatologically relevant data. This paper will give an overview of the different systems (based on the handbook by German historian Hermann Grotefend) and will provide examples how climatologists can avoid fake dating of their sources.