How does the Jewish calendar work?

The Jewish Calendar: History and Inner Workings by Dr. Fred Reiss; (c) 2017; ISBN 9781544-72382.

By Rabbi Michael Leo Samuel


Rabbi Michael Leo Samuel

CHULA VISTA, California — The study of the Jewish calendar is not one of the easiest topics to research.  It is an area that most rabbis have at best a general knowledge of the history. Dr. Fred Reiss chronicles how the Jewish calendar evolved over the centuries and for that alone, his new book, The Jewish Calendar: History and Inner Workings is a good read.

Although the existence of a calendar is implied in the early books of Genesis, Exodus, and Leviticus, it is only in the book of Numbers the first formal imperative to celebrate the New Moon is found (Num. 10:10), despite the fact that Leviticus 23 mentions all the festivals and holy days! Historically and biblically Rosh Hodesh certainly figures prominently in the original Passover holiday. Reiss points out that “based on the Book of Exodus, the Israelites, as a nation, officially adopted a lunar calendar about the middle of the 15th century BCE, but rejected the foreign names for the days of the week and months of the year” (pp. 29-30).

Unlike the Muslims who employ a purely lunar calendar, the Hebrew calendar, as Reiss explains is a lunisolar calendar, “whose months start with the New Moon and seasons of the year regulated by the sun.” Reiss is correct, for the Passover holiday must always occur in the month of aviv (often rendered as “spring”) but means “barley harvest.” As Reiss noted further that in the event the barley harvest was not ready, the ancients intercalated a month. Interestingly, as Philo noted (I am quoting my research on this matter), the Passover always had to occur at the vernal equinox. Reiss also makes reference to this point as well, but the origin traces back to Philo of Alexandria. Rosh Hodesh (“the head of the month”) has historically been determined by the sighting of the new moon’s crescent. In a lunar calendar, it occurs on a cycle of slightly more than 29 days. In ancient times, the Sanhedrin used to determine whether a month had 29 or 30 days and all this was predicated upon the visual observation of witnesses.

In the first chapter of the book, Reiss chronicles the development of the Hebrew calendar and its various permutations over the centuries. When the witnesses appeared, the Rosh Hodesh was celebrated and that day it was counted was marked as the first of the month. If no witnesses appeared, then the next day was designated as the Rosh Hodesh.

It is often hard for moderns to grasp what the world was like without the Internet. How did peoples communicate with one another about the Rosh Hodesh? Well, fires were lit on the Mount of Olives and this signaled to other communities to light their bonfires, and within minutes, the entire country knew when it was the New Moon. The High Priest used to make the final ruling whether or not to declare a new month.

Reiss documents how the Babylonian Jews used the Assyrian/Babylonian calendar month names, which are pretty much the same names Jews use today. When the Greeks conquered the Ancient Near East, Greek astronomy and mathematics enabled “clandestine” rabbinic councils to ascertain the arrival of the New Moon.
When one considers the number of Nobel Prize winners that happen to be Jewish, it is sometimes difficult for me to imagine why our ancestors struggled so hard in developing a Jewish calendar that was based in mathematics that would supplant the older witnessing system used by the previous generations of Jews.  Why didn’t our ancestors figure this out centuries before?

Historically, after the Jewish revolts against Rome, most Jews were dispersed from Judea, the old system certifying and signaling new moons and months in what the Romans had renamed Palestine, was in dissolution. This problem certainly worried the early generations of rabbis who realized that a new calendar for fixing our months became essential based on mathematical calculations.

A small council of rabbis guarded the secret instructions for constructing the calendar, until the mid-fourth century CE when, due to repressive acts and ultimate dissolution of the Jewish Court by Roman emperors, Hillel II, President of the Jewish Court in Babylonia, revealed those rules, so that Jews are able to construct their religious calendar.

As Reiss noted the Jewish calendar, once established by Hillel the Second in the middle of the fourth century, has never been adjusted. Even though the monthly moon cycle varies by as much as +/- 0.7 days per lunar cycle, and this can complicate the actual times when a holiday is supposed to begin. Interestingly, there has never been an instance where a new moon was ever sighted before the Hebrew calendar date. NASA claims to have improved on the amazingly precise lunar cycle of 29.53059 days used in the Jewish calendar. It has been said that even the great 18th century Vilna Gaon believed there were mistakes in the Jewish calendar.

Toward the end of the book, Reiss discusses some of the problems inherent in the Jewish calendar and the Orthodox rabbinic responses have varied over time. He discusses a number of different suggestions to the problems, but this is one topic that does not seem to have any immediate resolution as the author noted.

Personally, I have read a number of articles in Israeli journals and papers written by Orthodox scholars who think with the help of computer technology, we can now correct a number of these internal flaws embedded in the Jewish calendar we now use.

Although we have two days of Yom Tov in the Diaspora, there is still value in keeping the tradition because it reminds us that the majority of the Jews living today do not live in the Land of Israel. Yet, even in the medieval period, three days dedicated to Yom Tov and Shabbat often made it very hard for a struggling Jewish family to make a living.

Whether such changes will occur, this remains to be seen.

Reiss raises many interesting question readers might find interesting in knowing about the ancient Jewish calendar’s history: What is the definition of lunation time? Why have the rabbis condensed seven days into four days? Why is the High Holy Day of Rosh Hashanah often postponed? The civil calendar is either 365 or 366-days long, why does the Jewish calendar have six different year lengths? The Julian calendar repeats every twenty-eight years, the Gregorian calendar every four hundred years, why does it take 689,472 years for the Jewish calendar to repeat? All calendars have errors, what are the Jewish calendar’s errors and what do they affect?

When you read this book, you will find the answers quite illuminating and informative.
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Rabbi Michael Leo Samuel, spiritual leader of Temple Beth Shalom in Chula Vista, is author of numerous books including the Rediscovering Philo of Alexandria Commentary on the Pentateuch.

 

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