Jewish Calendar

By Mary Jane Chaignot

Categories: Early Christianity


I would like to know if Hillel 2nd might have had any part in advocating for the pagan month names in his calendar in addition to the exiles. Thanks.


It's doubtful. Calendar making, it seems, was a long drawn out process. Ancient Israel relied upon the Mesopotamian calendar. The Hebrew Bible mentions four of those months by name: Aviv (1st), Ziv (2nd), Ethanim (7th), and Bul (8th). These are believed to be Canaanite or Phoenician words. Numbers were used in other references. While in exile (587 BCE), the Israelites adopted names similar to the Babylonian months for their calendar: Nisan, Iyyar, Sivan, Tammuz, Ab, Elul, Tishri, Cheshvan, Kislev, Tevet, Shevat, Adar.

The first day of the Jewish month was noted when they were able to observe the first sliver of moon following a new moon. Such markers were available to ancient people wherever they were and provided uniformity for festival celebrations. They knew the new moon happened every 29 or 30 days, so all they had to do was keep track of the days since the first sighting. Still, there were a few flaws. Let's say, for instance, that it was cloudy that first night and no one could see the moon. That made it harder to plan and coordinate exact meeting times. Moreover, these lunar markers still had to coordinate with the solar year. As it turns out, twelve lunar months make 354.4 days. A solar year is eleven days longer. In order to accommodate these differences, they added extra days in the spring if it seemed too early for spring. That also led to variability in festival celebrations.

In the fifth century BCE, Meton of Athens, a famed mathematician, astronomer, and engineer, noticed a pattern in the lunar calendar. He realized that 19 solar years and 235 lunar months added up to the same number – 6940 days. He was the one who determined that 7 extra months should be added over a period of 19 years. This became known as the Metonic cycle and could be used to accurately predict future events.

The Jews, however, continued to mark their calendars by the first sighting after the new moon. By the time of the first century, people officially reported these moon sightings to the Sanhedrin, and then they would declare when the month started. Messengers were sent out to inform the populace. And, it was up to the discretion of the Sanhedrin to add extra days whenever they deemed it necessary.

By the fourth century, however, the Romans were on the verge of outlawing the Sanhedrin, which would impair their ability to determine the beginning of the month based on the moon sightings. This came about, in part, because the Romans had adopted the solar model during the time of Julius Caesar and wanted it to be uniformly used throughout the Empire. (Not that the solar calendar was perfect—it, too, was off by one day every 128 years. Over the centuries, this would become an issue.)

It is believed that Hillel II was president of the Sanhedrin from, roughly, 330-365 CE. In anticipation of the Romans dismantling the Sanhedrin, Hillel II instituted a fixed calendar using the Metonic system. He did this by adding another month every seven years. In that year, they would have months Adar I and Adar II. Not only was he able to set future dates, but he was also able to make sure that holidays were celebrated at the right times. He wrote his calendar through the year 2240 C.E.

This was considered to be a great advancement for which Hillel II has been given the credit. But some scholars dispute how great a hand he had in the development of the Jewish calendar because ancient writings suggest it was still subject to moon sightings through the 10thcentury. Scholars will continue to debate this issue.

Regardless, by the 16th century, the equinox was off by 10 days based on the Julian calendar. Since this was tied to Easter, Pope Gregory instituted the Gregorian calendar, which has an extra day every four years. It is a solar calendar that completely ignores the cycles of the moon and is most widely used throughout the world to this day.

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