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The Jewish calendar today is set based on calculations made by Hillel II in the time of the Gemara. Each month has either 29 or 30 days, so that over time, the months will stay in sync with the moon. As we have learned, during Talmudic times, the new month was based on testimony received from witnesses who saw the new moon, although the Sages who declared the new month had a fair amount of latitude to choose to postpone the announcement if they felt it necessary for one reason or another.
Rabbeinu Chananel and the Geonim point out that a decision on establishing the new month was dependent on a number of issues, some of which were well known, but others were known only to a small group of Sages who participated in sod ha-ibur – the closed assembly that actually made the final decision on this matter.
We explore the history of the lunar calendar from inner biblical analysis with Prof Sacha Stern to an astronomical historical comparison with other intercalating traditions by Prof Ari Belenkiy.