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(6a) The Jewish Calendar

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(Based on a talk given in Greenbelt, Maryland, 12 February 2000)


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6. The Calendar

6a. Jewish Calendar


8. The Round Earth

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8c. Moon dist. (1)

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9a. Earth orbits Sun?

9c. Copernicus
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[IMAGE: Mosaic floor in Beit Alfa, Israel]
    Mosaic pavement of a 6th century synagogue at Beit Alpha, Israel. Signs of the zodiac surround the central chariot of the Sun (a Greek motif), while the corners depict the 4 "turning points" ("tekufot") of the year, solstices and equinoxes, each named for the month in which it occurs--tekufah of Tishrey, (tekufah of Tevet), tekufah of Ni(san), tekufah of Tamuz
Larger version (172 Kb) here.

The Months

In the Jewish calendar, today is the 6th day of the month of Adar--and in this year, that is the "first Adar" or "Adar Aleph", because this is a leap year. The month is followed by "Adar Beth" or "Ve-Adar" ("and-Adar"), the second Adar, an extra month added now and then.

This happens to be year number 5760, and in a while you will also know how one tells by that number if it is a leap year or not. Of course, you realize that the Jewish calendar follows the Moon: "Rosh Chodesh," the beginning of the month, is always supposed to fall on the new Moon--the time when the Moon's position in the sky passes that of the Sun. Soon after that we may see a thin crescent, right after sunset.

It takes the Moon a little over 27 days to go around us, but meanwhile the Sun also shifts its position in the sky--each year, it circles the entire sky. So the Moon needs about 2 extra days to catch up with the Sun, and it takes 29 days, 12 hours, 44 minutes and a fraction, to go from one new moon to the next.

Thus most Jewish months alternate between 29 and 30 days--1st Adar, 30 days, second Adar, 29, Nissan, 30, Iyar 29, Sivan 30 and so on, except that Cheshvan and Kislev, in the fall, are adjustable--to take care of those 44 odd minutes, and for other adjustments.

One important adjustment of this type applies to the new year's day known as Rosh Hashanah ('head of the year')--it may never fall on Sunday, Wednesday or Friday. This is done to prevent the fast of Yom Kippur ("day of atonement"), 10 days later, to be next to a sabbath, because two consecutive days of rest would hinder proper observance of either; and also, to avoid having sabbath hit another holiday later that month. So, strange as it may seem, because of such adjustments, Rosh Hashanah often does NOT fall on the new moon. That is the price one pays for holidays!

There is a strange music in the names of Jewish months--Adar, Nissan, Iyar, Sivan, Tamuz, Av, Elul, Tishrey, Cheshvan, Kislev, Tevet, Shvat, the words have an ancient sound, distinctly non-Hebrew. And in fact, they are not Hebrew but Babylonian, from the homeland of Abraham. Likewise, what we call "Hebrew letters" are Babylonian, too, picked up in the Babylonian exile; Jews in the days of the first Temple used a completely different alphabet.

    (Note: The information on the Babylonian calendar, given below, relied on the "Encyclopaedia Britannica," 1967 edition).

If you take into account that the Babylonians had a special liking for the sound "U", you find that the Babylonian names are very similar:

You will also note some differences: "Shabatu" comes before "Tebetu", whereas in today's Jewish calendar, Tevet comes first, Shvat later. In the three names containing the letter "m" it was replaced in the Jewish calendar by "v" (or "w"), "Du'uzu" is now Tamuz, the name of the god of spring (later known as Adonis), and "Arach-samna" has become "Marcheshvan" or "Cheshvan" for short.

The Start of the Year

When does the Jewish year begin? Tradition has been somewhat ambiguous here. Rosh Hashanah, the official Jewish New Year's Day, is celebrated on the 1st day of Tishrey--but the Bible never calls it "Rosh Hashanah," it is always "Yom Hazikaron," the day of remembrance. And in the biblical chronology, this is not (as you might think) the first day of the first month--no, it is the first day of the seventh month, counting Nissan as the first.

In fact, none of these ancient names is in the Bible--all of them were handed down strictly by oral tradition. The Bible does mention though that the exodus from Egypt was in "the month of Aviv," and while in modern Hebrew that would mean "the month of spring," many believe that "Aviv" was the ancient name of that month.

According to the scriptures, this was to be the first month of the year, in memory of the exodus from Egypt; in all other dates, months are only referred to by their number. At the beginning of chapter 12 in the book of Exodus you will find:

    "This month shall be unto you the beginning of months; it shall be the first month of the year to you."

Still, it is interesting that the Babylonian calendar had the same ambiguity. There too, two months served as pivots--Nissan and Tishrey. Maybe one was the start of the religious year, one of the tax year--since in Nissan the harvest just starts and in Tishrey the crops are all in--maybe they date to different periods or localities, maybe the year was divided into two 6-month sections. One can only speculate.

Keeping up with both Sun and Moon

If the Jewish calendar only followed the Moon, it would get quickly out of step with the seasons, which follow the cycle of the Sun--the holidays would migrate and you might get Passover in the fall, Rosh Hashanah in the spring, Chanukkah in mid-summer. That is what happens in the Moslem calendar, which only follows the moon.

Consider the month of Ramadan, when observant Moslems fast from sunrise to sundown. Ramadan this year was in early winter--the best time, because days are short, nights are long, your fasts are short too and you do not get too thirsty from heat. But wait 15 years! Then Ramadan will have migrated to mid-summer, when days are at their longest, the heat makes you quite thirsty (especially in countries like Arabia and Egypt), and fasting all day long is a much greater ordeal.

But the ancient Babylonians found a way to keep up with both the moon and the sun. Their priests were excellent astronomers--helped, no doubt, by the clear skies in a country perched at the edge of the desert.

    (By the way, the Jewish Talmudic sage Mar Shmuel, who lived in Babylonia in the 3rd century, was also experienced in astronomy. He used to say "I am familiar with the pathways of the heavens as I am familiar with the pathways of [my home town] Nehard'a--except for the comet-star, I don't know what it is.")

By the 9th century BCE, after centuries of observations, Babylonian astronomers concluded that in a cycle of 19 years of 12 lunar months each, if you added 7 more months, you returned almost exactly to the same season.

Today this system is known as the Metonic cycle, because the Greek astronomer Meton introduced it in Athens in the year 432 BCE. However, the Babylonians already knew this at least 400 years earlier. The Chinese also have used this system--and the Jewish calendar does the same.

So how do you know which year has a seond month of Adar? Simple: in every 19-year cycle, years 3, 6, 8, 11, 14, 17 and 19 are leap years. This 19-year sequence is known as the "machzor," meaning "cycle" in Hebrew. In the general calendar, we now have Y2K, a special "millenium" year--but I still remember in 1940 or 1941, when I was 9 years old, the teacher told us that the current year 5700 was special, because it marked exactly 300 cycles since the creation of the world, according to the Jewish calendar.

So--3 times 19 is 57, 300 times 19 is 5700, and the year 5757 would complete 303 cycles. We are now in the year 5760, 3 years into cycle number 304, so by the rule assigning extra months to years 3, 6, 8, 11, 14, 17 and 19, it is a leap year. And the additional month is Adar, because it is the 12th and last month. The ancient Babylonians also added a 2nd Adaru--though sometimes they would add instead a second "Ululu" just before the day which became Rosh Hashanah.

One could go on and on. The Babylonians did not have a sabbath--but they preferred not to work on the 7th, 14th, 21st and 28th day of the month, because they thought those were unlucky days. Is there a connection? Who knows!


I have not yet touched at all our civil calendar, by which this February of the year 2000 is a very special leap month, instituted to fine-tune the calendar. Or that yesterday's date, February 11, was the day George Washington was born--in 1732, on February 11, old style. Then in 1751 the calendar was advanced 11 days, to make up for 17 centuries when this fine-tuning was not yet known, so now America celebrates Washington's birthday on February 22nd.

The Jewish calendar is much more accurate than the old-style calendar used when Washington was born. Still, in the 3500 years or so since the exodus from Egypt, it has slipped by about two weeks. According to the Bible, the exodus took place at the spring equinox, around March 21, and now Passover is about 2 weeks later. However, as long as one uses a calendar that tries to keep up with both the Sun and the Moon, there is nothing one can do about it--except maybe, wait a few thousand years more and then omit an entire leap month.

For Further Exploration: http://www.friesian.com/calendar.htm

Questions from Users:  
What were the origins of the Jewish Calendar?
                 ***       Leap years on the Jewish calendar

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol)phy6.org .
Last updated 25 November 2001

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