The Jewish Calendar: A Closer Look
Level: Advanced
When I view Your heavens, the work of Your fingers
The moon and the stars that You have established...
What is man, that You would remember him?
And the son of mortal man, that You would visit him?
-Psalms 8:4-5 (3-4 in gentile Bibles)
The basics of the Jewish calendar were explained on the
previous page, and will be mentioned
only in passing here. This page is intended for those who are interested
in a deeper understanding of the workings of Rabbi Hillel II's fixed calendar,
or those who want to be able to build their own Jewish calendar computer
programs.
Although this page will focus primarily on calendar calculations, I encourage
you not to dismiss this as purely a mathematical exercise devoid of spiritual
value. The sages emphasized the value of studying
astronomy as a way of appreciating the greatness of the Creator's work. This
page does focus on some arcane mathematics, but do not be intimidated by
it: the Jewish scholar Rambam wrote that, "the
method of the fixed calendar is one which an average school child can master
in 3 or 4 days." (Hilkhot Qiddush HaHodesh 11:4). A lot of the confusion
people experience stems from variations in the way different sources say
the same thing, and the way some sources use familiar terms to mean unfamiliar
things. I will do my best to keep these variations straight for you.
This page includes JavaScript and VBScript that performs all of the calculations
described, in the order that the concepts are presented here and with detailed
comments. Those who are comfortable with programming languages may find it
faster and easier to understand the math by looking at the code. This code
is not necessarily the best or most efficient script possible, but it's not
intended to be; it's intended to illustrate how the calendar is calculated.
Despite the inefficiency of this code, I have no doubt that it will be appearing
on other websites in short order. Would it kill you to give me credit and
a link back?
The Jewish calendar is based on three astronomical phenomena: the rotation
of the Earth about its axis (a day); the revolution of the moon about the
Earth (a month); and the revolution of the Earth about the sun (a year).
These three phenomena are independent of each other, so there is no direct
correlation between them. On average, the moon revolves around the Earth
in about 29½ days. The Earth revolves around the sun in about 365¼
days, that is, about 12 lunar months and 11 days.
To coordinate these three phenomena, and to accommodate certain ritual
requirements, the Jewish calendar consists of 12 or 13 months of 29 or 30
days, and can be 353, 354, 355, 383, 384 or 385 days long. The linchpin of
the calendar is the new moon, referred to in Hebrew as the molad.
A new month on the Jewish calendar begins with the molad, (pronounced moh-LAHD).
Molad is a Hebrew word meaning "birth," and refers to what we call the "new
moon" in English. The molad for the month of
Tishri (the month that starts with
Rosh Hashanah) is the most important one for
calendar calculations, and is referred to as Molad Tishri.
Note that the calculated molad does not necessarily correspond precisely
to the astronomical new moon. The length of time from one astronomical new
moon to the next varies somewhat because of the eccentric orbits of the Earth
and Moon; however, the moladot of Rabbi Hillel's calendar are set using a
fixed average length of time: 29 days, 12 hours, and 793 parts (or in Hebrew,
chalakim). The amount of time is commonly written in an abbreviated
form: 29d 12h 793p.
A "part" (or in Hebrew, cheilek) is a unit of time used in the Jewish
calendar, equal to 3-1/3 seconds. There are 18 parts in a minute and 1,080
parts in an hour. Most sources express time from calendar calculations in
days, hours and parts, although some sources break the parts down into minutes.
For example, the period between moladot could be written as 29 days, 12 hours,
44 minutes and 1 part (29d 12h 44m 1p), because 793 parts is 44 minutes and
1 part (793 = 44 times 18 parts plus 1 part) . This makes the resulting times
look somewhat more familiar, but it increases the number of calculations,
so we will stick with days, hours and parts.
The same shorthand can be used to express the time when a molad occurs. The
time is normally expressed as a day of the week, along with the hours and
parts (or hours, minutes and parts). For example, the time of a molad might
be expressed as 2d 12h 1005p (or 2d 12h 55m 15p), meaning that it occurs
on Monday (the second day) at the 12th hour and 1005 parts.
The "hours" used to calculate the molad are standard 1/24 of a day hours.
Note that this differs from the "hours" used for ritual scheduling, which
are 1/12 of the time from sunrise to sunset. For example, at Pesach (Passover),
we are required to stop eating chametz at the end of the "fourth hour "of
the morning on Nissan 14, that is, at the end of 1/3 of the time between
sunrise and sunset. These "seasonal hours" vary depending on the time of
the year; molad hours are constant. The time for the molad is Jerusalem Solar
Time, which is not necessarily the same as your local time. It is also not
necessarily the same as the time on the clock, even in Jerusalem. This fact
has no effect on your calculations, but is worth knowing.
The Jewish "day" starts at sunset, rather than at midnight. If you read the
story of creation in Genesis Ch. 1, you will notice that it says, "And there
was evening, and there was morning, one day." From this, we infer that a
day begins with evening, that is, at sunset. Accordingly, most sources discussing
the molad use 6PM of the preceding evening as the "zero hour." In our example,
2d 12h 1005p, the 12h means the 12th hour after 6PM, that is, 6AM. If a molad
occurs at 2d 4h 0p, this means that it occurs at 10PM on Sunday night,
because the second day (Monday) begins at 6PM of the preceding evening (Sunday).
Some sources, however, use the more familiar Western conventions and use
midnight as the zero hour. Be very careful to check which system is being
used when you rely on times given by any source! If the time is referred
to as "Rambam time" or something similar, then
you know it uses 6PM as the zero hour. On this page, I am using Rambam time,
but some well-respected Orthodox sources in America use midnight as their
zero hour. As long as you are consistent, you will get the same result under
either system.
Here is an overview of the steps involved in calculating the date of Rosh
Hashanah on the Jewish calendar:
-
Start with a known molad (and the corresponding Gregorian date, if you wish
to convert your resulting date to Gregorian).
-
Determine the number of months between the known molad and
Tishri of the year of the date you are calculating.
-
Multiply the number of months by the length of the molad: 29d 12h 793p.
-
Add the result to the known starting molad.
-
Apply the dechiyot (rules of postponement) to determine the date of
Rosh Hashanah for the year of your date.
-
To get the Gregorian date, add the number of days elapsed calculated above
to the Gregorian starting date.
If you want to calculate a date other than Rosh Hashanah, you will have to
calculate either that year's Rosh Hashanah, the following year's Rosh Hashanah
or both and use this information to work out the date.
We will now look at these steps in detail, illustrating the techniques by
calculating the dates of Rosh Hashanah and
Pesach (Passover) in the year 5766 (2005-2006)
using 5759 as our starting point. As I said above, if you are comfortable
with JavaScript and VBScript, you may find that it is faster and easier to
understand these concepts by viewing my code
here. This code is designed to illustrate
calendar principles and is not the most efficient code possible. If you choose
to use it in your own work despite this warning, would it kill you to give
me credit and a link back?
Step 1: Start with a Known Molad
To perform any calculations on the Jewish calendar, you need a starting point,
preferably the molad of Tishri for a specific
year, along with the corresponding Gregorian date if you want to be able
to convert the Hebrew date to Gregorian. It is not possible to work out a
molad from first principles, because the first molad of creation (known as
Molad Tohu) did not occur at 0d 0h 0p!
I like to base my calculations on the molad of Tishri 5759, which occurred
at 2d 12h 1005p (using 6PM as the zero hour), and corresponded to the Gregorian
date 9/21/1998. I use this particular year because it is not subject to dechiyot
(postponements), which complicate the Gregorian conversions. If you will
be calculating dates in the past and would like to avoid the complications
of subtracting dates, you may prefer to work with an earlier molad, such
as Tishri 5661 (9/24/1900), 2d 11h 9p, or even Molad Tishri 5558 (9/21/1797),
5d 11h 607p. I'm sure our Christian friends are primarily interested in knowing
Molad Tishri 3762 (the year 1), or some other year in that lifetime.
Unfortunately, the program I use to calculate molads overflows after 3861
(the year 100), so you'll have to work out the rest yourself: Molad Tishri
3869 (9/22/108, the earliest one I can work out that is not subject to
postponements) is 7d 8h 957p. A more interesting base from my perspective
is Molad Tishri 4120 (9/10/359), 5d 8h 29p, which is the first non-postponed
year after Rabbi Hillel II developed this calendar! Keep in mind, however,
that you can't really calculate a "Gregorian" date before the Gregorian calendar
reforms, which took effect at varying times in varying places.
Step 2: Determine the Number of Months to Tishri of Your Year
The next step is to determine how many months are between your starting point
and Tishri of the year of your end point. There are exactly 235 months in
every 19-year cycle of leap years (12 12-month years plus 7 13-month years),
but if your number of years is not evenly divisible by 19, then you will
have to determine whether each remaining year is a regular year (12 months)
or a leap year (13 months).
Fortunately, the leap year cycle is easily calculated. Leap years occur in
years 3, 6, 8, 11, 14, 17 and 19 of a 19-year cycle, and the 19-year cycle
begins in the year 1, so you can simply divide the year number by 19 and
examine the remainder. If the remainder is 3, 6, 8, 11, 14, 17 or 0 (the
19th year of the cycle) then the year is a leap year. Otherwise, it is not.
There is less than one cycle between 5759 and 5766. The remaining months
from 5759 to 5766 are:
| Year |
Divided by 19 |
Leap Year? |
Months |
| 5759 |
303 remainder 2 |
No |
12 |
| 5760 |
303 remainder 3 |
Yes |
13 |
| 5761 |
303 remainder 4 |
No |
12 |
| 5762 |
303 remainder 5 |
No |
12 |
| 5763 |
303 remainder 6 |
Yes |
13 |
| 5764 |
303 remainder 7 |
No |
12 |
| 5765 |
303 remainder 8 |
Yes |
13 |
| Total |
87 |
Step 3: Multiply the Number of Months by the Length of the Molad
Next, we multiply the number of months by the average length of the molad,
which is 29d 12h 793p:
-
793p * 87 = 68,991p
-
12h * 87 = 1,044h
-
29d * 87 = 2,523d
Of course, we will then have to round up the smaller units into the larger
units, just as we would round 75 minutes into 1 hour and 15 minutes. Here
are the stages of this rounding:
-
68,991 parts / 1,080 parts per hour = 63 hours remainder 951 parts
-
(1,044 multiplied hours + 63 rounded hours) / 24 hours per day = 46 days
remainder 3 hours
-
2,523 multiplied days + 46 rounded days = 2,569 days
We now know the amount of time between our starting molad and our ending
molad: 2,569d 3h 951p
Step 4: Add the Result to the Starting Molad
Next, we add the elapsed time calculated above to the starting date to get
the ending date. Our starting molad is 2d 12h 1005p (using 6PM as 0h). We
will not add the days yet, for reasons that will soon become clear.
-
951 elapsed parts + 1005 starting parts = 1,956 parts
-
3 elapsed hours + 12 starting hours = 15 hours
Now we need to do some rounding:
-
1,956 calculated parts / 1,080 parts per hour = 1 hour remainder 876
parts
-
15 calculated hours + 1 rounded hour / 24 hours per day = 0 days remainder
16 hours
-
2,569 calculated days + 0 rounded days = 2,569 days
At this point, we should note the number of days elapsed between our starting
point and our ending point: 2,569 days. We must note this at this point in
the calculation, after the hours are rounded into the days but before the
weekday of the starting molad is added to the number of days. This number
of days will be necessary to determine the Gregorian date. Note that if the
hours are more than 24 at this point, you will need to round those hours
into the days to get the elapsed time. In this calculation, however, the
number of hours is only 16.
Let's finish the calculation of the molad, adding the days and determining
the day of the week:
-
(2569 calculated days + 2 starting days) / 7 days per week = 367 weeks remainder
2 days
The remainder of 2 days gives us the day of the week for our molad, so the
resulting molad is: 2d 16h 876p, that is, Monday in the 16th hour (10AM)
and 876 parts, with 2,569 elapsed days. Note that if the remainder is 0 days,
the molad is 7d (Shabbat), because 7 days / 7 days per week = 1 week remainder
0 days.
Step 5: Apply the Dechiyot
There are four rules of postponement known as dechiyot, pronounced d'-khee-YOHT,
where "kh" is a throat-clearing noise (singular: dechiyah). These rules postpone
the date of Rosh Hashanah, but do not affect the calculated time of the molad.
One of the dechiyot is a general rule of rounding while the rest are designed
to prevent oddities in the length of the year and the date of Rosh Hashanah.
Dechiyah 1: Molad Zakein
The first dechiyah is molad zakein, meaning an "old" molad. If the molad
occurs at or after noon (that is, 18h where 6PM is 0h), the molad is considered
to be "old" and we round to the next day. This rule is quite commonly applied,
affecting a quarter of all years (half if you use midnight as the 0 hour).
If noon seems a bit early to be considering the molad "old," remember that
the Jewish "day" starts at sunset while this hour measurement starts at midnight.
The rule of molad zakein simply means that a molad at or after noon relates
to the "day" that starts at the next sunset (4-10 hours later) rather than
the previous sunset (14-20 hours earlier). This rationale is clear from the
Rambam notation, where 6PM is 0h and a Molad Zakein is one that occurs at
or after 18h in a 24h day.
Interestingly, Molad Zakein is the reason why you will get the same result
regardless of whether you use 6PM or midnight as your zero hour. With midnight
as your zero hour, Molad Zakein applies to molads after 12h, applying in
half of all years. With 6PM as your zero hour, Molad Zakein only applies
in one quarter of all years, but molads between 6PM and midnight are already
considered to be part of the next day, so the result is the same!
Our molad occurs at 16h in Rambam notation, so it is not a molad zakein and
Rosh Hashanah stays on the calculated date for now.
Note that when dechiyot like this apply, a day must be added to the elapsed
time for purposes of calculating the Gregorian equivalent date, but the molad
does not change. The unchanged molad is used for purposes of calculating
subsequent years and for certain religious purposes. For example, in 5760,
the calculated molad was 6d 21h 801p. Molad Zakein pushed Rosh Hashanah to
the next day, but if you were to calculate a subsequent date using 5760 as
your base, you would calculate from 6d (Friday), not from Saturday.
Dechiyah 2: Lo A"DU Rosh
The second dechiyah is known as Lo A"DU or Lo A"DU Rosh. This rule states
that Rosh Hashanah cannot occur on a Sunday (Day
1), a Wednesday (Day 4) or a Friday (Day 6). The word Lo means "Not," and
the word A"DU is a way of pronouncing Alef-Dalet-Vav, letters with the numerical
values 1, 4 and 6 (see Hebrew Alphabet - Numerical
Values). If the calculated molad occurs on one of these days of the week,
Rosh Hashanah is postponed by a day to prevent other problems with the calendar.
If Rosh Hashanah fell on a Wednesday or Friday, then
Yom Kippur would fall on a Friday or Sunday,
which is undesirable. If Rosh Hashanah fell on a Sunday, the
Hoshanah Rabbah would fall on a Saturday,
making it impossible to observe some of the day's customs.
This dechiyah is also commonly applied, as you might imagine. It applies
to three out of seven days, so one would expect it to occur almost half of
the time.
Note that the dechiyot of molad zakein and Lo A"DU Rosh can work in combination:
a molad at 5d 19h 0m 0p (Thursday at 1PM) is rounded to Friday by the rule
of Molad Zakein, then postponed to Saturday by the rule of Lo A"DU Rosh,
even though the original molad was on a valid day of the week. On the other
hand, a molad at 4d 19h 0m 0p (Wednesday at 1PM) is rounded to Thursday by
Molad Zakein, and Lo A"DU Rosh does not apply: even though the molad occurred
on Wednesday, Molad Zakein has already moved it off of that date so Lo A"DU
Rosh is not necessary. This is why the rule of Molad Zakein must be checked
before the rule of Lo A"DU Rosh.
Dechiyah 3: Gatarad
The remaining two dechiyot are much less commonly applied.
Dechiyah Gatarad holds that if Molad Tishri in a simple (12-month, non-leap)
year occurs on a Tuesday at 9h 204p or later, Rosh Hashanah is postponed
to the next day (a Wednesday, which by the effect of Lo A"DU Rosh would then
be postponed to Thursday).
 The
name, Gatarad, is a mnemonic for the rule. In Hebrew, Gatarad it is spelled
Gimel-Teit-Reish-Dalet. Using letters as numerals, Gimel is 3, and represents
Tuesday. Teit is 9 and represents the 9th hour (that is, 9h in Rambam notation,
but 3h in midnight-based notation). Reish is 200 and Dalet is 4, representing
204 parts.
Why does such a complicated rule exist? This rule prevents the possibility
that a year might be 366 days, an invalid length. Consider: a Molad Tishri
at 3d 9h 204p would not be postponed by Molad Zakein or Lo A"DU Rosh. Add
12 lunar cycles (354d 8h 876p) to the next year's Rosh Hashanah and you get
7d 18h 0p with 354 days elapsed. Molad Zakein applies to the following year,
postponing Rosh Hashanah to the next day, a Sunday, with 355 days elapsed.
Lo A"DU Rosh is then triggered, postponing Rosh Hashanah and leaving 356
days elapsed and making the current year an invalid length.
Note that Gatarad invariably triggers Lo A"DU Rosh. Gatarad only applies
when Rosh Hashanah is Tuesday and Gatarad postpones Rosh Hashanah to Wednesday.
Lo A"DU Rosh then postpones Rosh Hashanah to Thursday. Some programmers like
to check Gatarad before checking Lo A"DU Rosh; others check Gatarad after
Lo A"DU Rosh but use this rule to add two days (the Gatarad day plus the
resulting Lo A"DU Rosh day). Either way, if Gatarad applies, Rosh Hashanah
falls on Thursday.
Note also that this rule is not combined with Molad Zakein. If Molad Zakein
applies to the current year, Gatarad is unnecessary; thus Gatarad applies
only to molads between 9h 204p and 17h 1079p.
As you might imagine, this rule is not commonly applied. It applies only
in non-leap years (12 out of 19) when the molad occurs on Tuesday (1 out
of 7) between the 9th hour and the 18th hour (9 out of 24). It occurs about
three times a century. It last occurred in 5745 (1984-85) and will not occur
again until 5796 (2035-36).
Dechiyah 4: Betutkafot
Like Dechiyah Gatarad, this rule is not very commonly applied and is designed
to prevent a year from having an invalid length. Dechiyah Betutkafot prevents
a leap-year from having 382 days (too few days) by postponing Rosh Hashanah
of the non-leap year following the leap year.
 Also like Dechiyah Gatarad, the name of
the rule tells you how it is calculated: if Molad Tishri in a year following
a leap year occurs on Monday (Beit, 2) in the 15th hour (Teit-Vav, 15 in
Rambam notation, but 9h in midnight-based notation) and 589 parts
(Tav-Kaf-Pei-Teit, 589), then it is postponed to the next day. The rule is
applied only if the actual molad occurs on Monday, not if it is postponed
to Monday. Like Gatarad, the rule really only applies to molads before noon
(18h), because Molad Zakein handles the postponements for molads at or after
noon. Unlike Gatarad, Betutkafot does not trigger Lo A"DU Rosh, because
Betutkafot postpones Rosh Hashanah from a Monday to a Tuesday and Tuesday
is an acceptable day for Rosh Hashanah.
The reasoning behind this rule is similar to the reasoning behind Gatarad:
the 13 lunar cycles of the preceding year are 383d 21h 589p. If this year's
Molad Tishri occurs after 2d 15h 589p, then the preceding year's Molad Tishri
must have occurred on or after 3d 18h 0p. This is 384 elapsed days, but the
preceding year's Molad Tishri was a Molad Zakein postponing Rosh Hashanah
to Wednesday, which triggers Lo A"DU, moving Rosh Hashanah to Thursday. The
two postponements shorten the preceding year to 382 days. Dechiyah Betutkafot
postpones the current year's Rosh Hashanah by one day to increase the preceding
year to a permissible 383 days.
This is the rarest of the four dechiyot, applying only in the year after
a leap year (7 out of 19) when the molad occurs on Monday (1 out of 7) between
the 15th hour and the 18th hour (3 out of 24). It applies once or twice a
century, but it applies next year (5766, that is, 2005-06). The last time
it applied was 5688 (1927-28).
Step 6: Add Elapsed Days to Gregorian Starting Date
To determine the Gregorian date for Rosh Hashanah, you must take the elapsed
days calculated in Step 4, add any additional days triggered by the dechiyot
in Step 5, and add this number of days to the date of Rosh Hashanah for your
known molad. For those following the script: JavaScript isn't very effective
for dates, so we will shift to VBScript here.
If you're trying to do this without writing a program, a spreadsheet such
as Microsoft Excel can easily add a number of days to a date.
To calculate the date of Rosh Hashanah for any year after 5759 using the
principles above:
-
Calculate the number of months between 5759 and your year.
-
Calculate the amount of time elapsed in those months.
-
Add the elapsed time to the molad of 5759 to determine the molad of your
year, stopping to note the elapsed days before adding the day of week from
the 5759 molad.
-
Determine whether any dechiyot apply and if so, add them to the elapsed days
determined above.
-
Add the days elapsed to the date of Rosh Hashanah in 5759.
These steps are performed by the function CalcRH in the JavaScript, used
by the form below. Just type the Hebrew year and the Gregorian date will
appear, using only the functions discussed above. Click the button below
to try it!
The principles and JavaScript above are sufficient to allow you to convert
Rosh Hashanah to a Gregorian date for any year. However, if you want to calculate
a date other than Rosh Hashanah, you will have to calculate either that year's
Rosh Hashanah, the following year's Rosh Hashanah or both and use this
information to work out the date. The information you need varies depending
on the month of the date you are calculating
-
Tishri
-
Tishri is the month of Rosh Hashanah, so you simply add the date of the month
to Rosh Hashanah and subtract 1 (because Rosh Hashanah is Day 1).
-
Cheshvan
-
Cheshvan is the second month of the calendar year, and the preceding month
of Tishri is always 30 days, so you simply take the current Rosh Hashanah,
add 29 days (30 - 1 for Rosh Hashanah) and add the date of the month.
-
Kislev
-
Kislev is the hardest month to calculate. You cannot simply work forward
from the current year's Rosh Hashanah, because the preceding month of Cheshvan
can be 29 or 30 days, nor can you work backward from the next year's Rosh
Hashanah, because Kislev itself can also be 29 or 30 days. To calculate the
length of Kislev, you need to know the date of Rosh Hashanah of both the
current year and the next year, then calculate the difference between them
to determine the length of the current year. If the year is 353, 354, 383
or 384 days, then Cheshvan is 29 days and you can determine a date in Kislev
taking the current Rosh Hashanah, adding 58 days, then adding the date of
the month. If the year is 355 or 385 days, then Cheshvan is 30 days and you
can determine a date in Kislev by taking the current Rosh Hashanah, adding
59 days, then adding the date of the month. (For the programming-inclined:
date differences are best calculated by VBScript).
-
Tevet, Shevat
-
The remaining months of the year are of unchanging length, but the number
of months varies depending on whether the year is a leap year! Tevet and
Shevat are best calculated by working backwards from the following year's
Rosh Hashanah and subtracting an additional 30 days in a leap year. Tevet's
offset in a non-leap year is -266; Shevat's is -237.
-
Adar, Adar I and Adar II
-
Adar is always offset -207 from the following Rosh Hashanah; however, in
regular years, Adar is the 12th month of the year (starting from Nissan),
and in leap years, is known as Adar II and is the 13th month of the year.
Adar I, the extra month inserted as the 12th month in leap years, is always
offset -237 days from Rosh Hashanah.
-
Nissan, Iyar, Sivan, Tammuz, Av, Elul
-
The remaining months of the year are all of unchanging length and not affected
by leap years. Simply subtract the appropriate number of days from the following
year's Rosh Hashanah and add the date of the month.
The form below uses the functions above to calculate the dates of major Jewish
holidays for any Hebrew year.
Calendar scholars use a system of encoding to describe each Jewish year.
This encoding consists of three Hebrew letters
that serve as a shorthand for important features of the calendar, and once
you work out the code, you know everything you need to know about the calendar.
You don't need to know the encoding system to be able to calculate the calendar,
but it may help you understand important features of the calendar.
The first letter is either Pei or Mem. Pei stands for the Hebrew word P'shuta
(simple), and refers to a 12-month regular year. Mem stands for Me'uberet,
and refers to a 13-month leap year.
The second letter indicates which day of the week Rosh Hashanah occurs. Letters
of the Hebrew alphabet also serve as numerals (see
Hebrew Alphabet - Numerical Values), and
this letter indicates whether Rosh Hashanah occurs on a Monday (Beit, that
is, 2), a Tuesday (Gimel, 3), a Thursday (Hei, 5) or a Saturday (Zayin, 7).
Why not Alef (1), Dalet (4) or Vav (6)? Because Dechiyah Lo A"DU Rosh, discussed
above, prevents Rosh Hashanah from falling on Sunday, Wednesday or Friday.
The third letter tells you the length of the year, which can be 353, 354
or 355 days (in a leap year, 383, 384 or 385). This variation comes in part
from the length of the molad cycles (which add about 8 or 21 hours to the
time of day each year, which sometimes rolls over to another day) and in
part from the application of the dechiyot. A year's length can be encoded
as Cheit for Chaseir (deficient or lacking, a 353 or 383 day year), Kaf for
K'Seder (in order, a 354 or 384 day year) or Shin for Shaleim (whole or complete,
a 355 or 385 day year). In a Chaseir year, both Cheshvan and Kislev have
29 days. In a Shaleim year, both Cheshvan and Kislev have 30 days. In a K'Seder
year, Cheshvan has 29 days and Kislev has 30 days.
Under this system of encoding, the current year (5765) is coded Mem-Hei-Cheit,
because it is a leap year (Mem), Rosh Hashanah started on a Thursday (Hei),
and the year will have 383 days (Cheit). Next year (5766) would be encoded
as Pei-Gimel-Kaf because it will be a regular (non-leap) year, it will start
on a Tuesday, and it will have 354 days.
Some people code the years differently: the day of Rosh Hashanah as the first
letter (instead of the second), the length of the year as the second letter
(instead of the third), and the day of the week that
Pesach (Passover) starts as the third. This
third letter can be Alef (1, Sunday) through Zayin (7, Saturday). The advantage
of this system is that it tells you the day of the week that both Pesach
and Rosh Hashanah occur, which has some effect on their observances, and
once these are known, we can infer the days of the other major festivals
(Sukkot and
Shavu'ot). The disadvantage is that nothing
in this system tells you whether the year is a leap year, although this can
be inferred if you know the calendar well enough.
Although there are many theoretical permutations of these three-letter codes,
only 14 of them are actually possible given the constraints of calendar
calculations. This means that there are only 14 different possible layouts
for an annual Jewish calendar. Keep in mind, though, that these 14 different
layouts don't necessarily correspond to the same Gregorian days, but they
do correspond to the distribution of weekly Torah
readings. For example, in the year 5765, a Mem-Hei-Cheit year, the Torah
portion Emor was read on 5 Iyar, which was May 14, 2005. The next Mem-Hei-Cheit
year will be 5768, and Parshat Emor will be read on 5 Iyar in that year too,
but 5 Iyar will occur on May 10, 2008. Contrast this with 5766, a Pei-Gimel-Kaf
year, when Emor will be read on 15 Iyar (May 13, 2006).
The following table shows which parshiyot are combined in which year encodings:
| Year Encoding |
Parshiyot Combined |
| Mem-Beit-Cheit |
Chukat-Balak
Matot-Masei
Nitzavim-Vayeilech |
| Mem-Beit-Shin |
Matot-Masei |
| Mem-Gimel-Kaf |
Matot-Masei |
| Mem-Hei-Cheit |
none |
| Mem-Hei-Shin |
Nitzavim-Vayeilech |
| Mem-Zayin-Cheit |
Matot-Masei
Nitzavim-Vayeilech |
| Mem-Zayin-Shin |
Chukat-Balak
Matot-Masei
Nitzavim-Vayeilech |
| Pei-Beit-Cheit |
Vayakhel-Pekudei
Tazria-Metzora
Achrei Mot-Kedoshim
Behar-Bechukotai
Matot-Masei
Nitzavim-Vayeilech |
| Pei-Beit-Shin |
Vayakhel-Pekudei
Tazria-Metzora
Achrei Mot-Kedoshim
Behar-Bechukotai
Chukat-Balak
Matot-Masei
Nitzavim-Vayeilech |
| Pei-Gimel-Kaf |
Vayakhel-Pekudei
Tazria-Metzora
Achrei Mot-Kedoshim
Behar-Bechukotai
Chukat-Balak
Matot-Masei
Nitzavim-Vayeilech |
| Pei-Hei-Kaf |
Vayakhel-Pekudei
Tazria-Metzora
Achrei Mot-Kedoshim
Behar-Bechukotai
Matot-Masei |
| Pei-Hei-Shin |
Tazria-Metzora
Achrei Mot-Kedoshim
Behar-Bechukotai
Matot-Masei |
| Pei-Zayin-Cheit |
Vayakhel-Pekudei
Tazria-Metzora
Achrei Mot-Kedoshim
Behar-Bechukotai
Matot-Masei |
| Pei-Zayin-Shin |
Vayakhel-Pekudei
Tazria-Metzora
Achrei Mot-Kedoshim
Behar-Bechukotai
Matot-Masei
Nitzavim-Vayeilech |
At one time, the accuracy of the Jewish calendar was proverbial. But how
accurate is it really?
The average lunar month on the Jewish calendar is 29d 12h 793p. The average
lunar month as calculated by modern astronomers is 29d 12h 44m 2.8s, that
is, 29d 12h 792.84p. so the variation is less than two tenths of the smallest
unit of measurement recognized by the system, about half of a second. That
is quite remarkably accurate. Of course, those lost half-seconds do add up:
within in a century, you're off by 10 minutes.
How well does the calendar correspond to the solar year? The
rabbis recognized long ago that the calendar
gains 1h 485p in every 19-year cycle, adding up to a day every 300 years
or so. This was important to the rabbis in scheduling certain rituals that
are based on the solar year rather than the lunar year. We can see this effect
when we examine the dates of Rosh Hashanah over time.
Rabbi Hillel II developed the Jewish calendar in the Jewish year 4119. Using
his calendar methods as described above, and artificially assuming that the
Gregorian calendar we use today was in effect at that time, the date of Rosh
Hashanah ranged from August 29 to September 28 between the years 4100 and
4200 (the 42nd century). In the present Jewish century (the 58th), the dates
of Rosh Hashanah range from September 5 to October 5, a gain of 6 or 7 days.
This is considerably more accurate than the Julian calendar used by Christians
in Rabbi Hillel's time (which had to be corrected by 11 days a few centuries
ago), but you can see that it is gaining some time.
The discrepancy in the Jewish calendar, however, is still less than a lunar
month and is therefore as accurate as it is possible to be in a lunisolar
calendar. In fact, it takes about 9300 years for this discrepancy to accumulate
to a full month of time. The rabbis were aware of the problem, but were quite
confident that a new Sanhedrin will be established long before this discrepancy
becomes problematic. We still have more than 3500 years to go.
 The book that most people recommend
for learning about the Jewish calendar is Rabbi Nathan Bushwick's
Understanding
the Jewish Calendar, which you can buy from amazon.com by clicking the
title above. I ordered this book while I was writing this page; it took about
a month to arrive, and I confess I was a bit disappointed by it. About half
of the book was basic astronomy that I learned in fourth grade, and most
of the calendar calculations I had learned before the book arrived. Nevertheless,
the book did have some interesting insights and thorough citation to
Torah, Talmud
and Rambam that you may find useful or interesting.
© Copyright 5765 (2005), Tracey R
Rich
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