Jewish Calendar Cycles

Jewish Calendar Cycles in Medieval Manuscripts

The Jewish calendar and its calculation, as still used today, became standard and fixed by the 9th-10th centuries. However, an alternative calendar system, traditionally attributed to Rav Naḥshon Gaon (9th-century), existed alongside it until the end of the medieval period and even beyond. This system assumes that the Jewish calendar fully repeats itself every 247 (19×13) years, so that a calendar calculated for 247 years can be re-used indefinitely. The calendar produced by the cycle of Rav Naḥshon Gaon is incompatible with the normative rabbinic calendar, which by definition is not cyclical. The use of the 247-year cycle could have led to holidays being celebrated at different times.

The cycle of Rav Naḥshon, although controversial, seems to have enjoyed popularity because of its convenience. It was used in Yemen in the 12th-14th centuries, and appears in many European medieval Hebrew manuscripts, including the very influential law-code Arba’ah Turim of Jacob b. Asher. Because of the way they were constructed, 247-year cycles were never identical to one another. This raises questions regarding the extent to which the Jewish calendar, in the Middle Ages, was standardized and fixed.

Despite its wide dissemination, the 247-year cycle did not receive the scholarly attention that it deserves. Early 20th-century historians of the Jewish calendar, such as H. Y. Bornstein and Z. H. Jaffe, were only interested in the attribution of the cycle to Rav Naḥshon Gaon and in the part it may have played in the formation of the normative rabbinic calendar. More recently, Yosef Tobi analyzed the medieval sources from Yemen. But the bulk of the evidence in European manuscripts and printed works is yet to be properly studied.

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Our researcher in this area is Dr Nadia Vidro.

Our aim is to systematically survey the evidence of the cycle of Rav Naḥshon in medieval manuscripts and in early prints, Hebrew and Latin, to analyze how it was constructed and used, and to investigate its possible historical implications.

Our research questions are:

1. What are the origins of the 247-year cycle?

2. How was the cycle of Rav Naḥshon produced: was it copied by scribes from master copies, or each time freshly calculated and designed?

3. Were the cycles of Rav Naḥshon used in practice, and did the use of these cycles lead to diversity of practice between different Jews or Jewish communities?

4. Were attempts made to reconcile the 247-year cycle and the normative computation?

5. What were the attitudes of rabbinic leaders towards the use of the 247-year cycle and its potential for calendar diversity?