Tel Aviv study links Qumran calendar abandonment to astronomical drift and Hasmonean politics

September 20, 2026 at 05:29 PM2 min

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A Tel Aviv University study proposes the Qumran sect used its 364-day calendar in its early history before abandoning it due to astronomical drift and shifting relations with Hasmonean ruler Alexander Jannaeus. The findings, published in Tarbiz Quarterly for Jewish Studies, suggest the calendar's one-and-a-quarter-day annual shortfall caused festivals to drift by almost four weeks over twenty years. The research was conducted by Prof. Eshbal Ratzon of the Department of Jewish Philosophy and the Cohn Institute.

Key Facts

The Qumran calendar contained exactly 364 days, divisible by seven, producing 52 complete weeks each year.
A 364-day year is about one day and a quarter shorter than the astronomical year, causing festivals to shift by almost four weeks over twenty years.
Prof. Eshbal Ratzon of Tel Aviv University conducted the study, published in Tarbiz Quarterly for Jewish Studies.
The Qumran sect rejected the Jerusalem Temple's authority to set religious dates, believing God fixed them at Creation.

Calendar Structure

The Qumran calendar differed sharply from the lunisolar calendar used in Jewish religious life during the Second Temple period. Its year contained exactly 364 days, divisible by seven, yielding 52 complete weeks and ensuring festivals always fell on the same day of the week. Members of the Qumran sect regarded this orderly structure as reflecting a perfect divine system. The calendar also carried political significance, as the sect rejected the Jerusalem Temple's authority to set dates, believing God had fixed them at Creation.

Astronomical Drift

The 364-day year is about one day and a quarter shorter than the roughly 365-day astronomical year. If followed for twenty years without correction, religious festivals would shift by almost four weeks relative to the seasons. After several decades, a festival associated with spring could fall in winter or even autumn. The problem is comparable to a clock that loses one minute every day, eventually no longer reflecting actual time.

Historical Reconstruction

Prof. Ratzon's study argues the sect likely used the calendar during its early history, when disagreements over religious dates contributed to its separation from the Jerusalem establishment. Later, the system may have been abandoned because of the astronomical problem and changing political relations with the Hasmonean leadership under Alexander Jannaeus. The findings were published in the Tarbiz Quarterly for Jewish Studies.