The Jewish Calendar

Part of science education web site "From Stargazers to Starships"
      at http://www.phy6.org/stagaze/Sjewcale.htm
      home page http://www.phy6.org/stagaze/Sintro.htm
Optional extension of http://www.phy6.org/stagaze/Scalend.htm

The Months

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.

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 names of Jewish months ring like strange music--Adar, Nissan, Iyar, Sivan, Tamuz, Av, Elul, Tishrey, Cheshvan, Kislev, Tevet, Shvat, words with 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.)

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

The Start of the Year

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:

Keeping up with both Sun and 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.

Ancient Babylonians, however, 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, Babylonians had already known this for at least 400 years. 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, considering them unlucky. Is there a connection? Who knows!

Accuracy

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.