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# CHAT: Telling time

From: Mark J. Reed Tuesday, September 16, 2003, 18:25
```On Tue, Sep 16, 2003 at 10:00:17AM -0700, JS Bangs wrote:
> How do orthodox Jews go about calculating the date of Passover these days?
> For maximum authenticity, shouldn't we[1] just do as they do, only
> celebrate on Sunday?
The early Christians were reluctant to keep the Jewish calendar, so
came up with their own means of approximating it, as I said in my last
message.

Jews in general have no need to calculate the date of Passover,
for it is fixed: it is the fifteenth day of the month of Nisan.
It's only when you try to correlate that to some other calendar,
like the Gregorian, that you have to do calculations - and then only
if you're the one printing up the calendars that everyone else will
just look at. :)

As to the details, it's fairly complex.   You begin with
the assumption that the moment of the first New Moon ("molad") of
Creation was on Sunday, October 6, 3761 BC (Julian) at 20 hours and
906 halakim, that is, 20:50:20, Universal Time.  (Note that
since this was after sunset it was actually almost 3 hours
into Monday by Jewish reckoning.) You then assume
that each subsequent molad is exactly 29 days, 12 hours, and 793
halakim later.

Years are grouped into 19-year cycles of 235 months each.  The year
beginning this fall will be 3761 + 2003 = 5764 Anno Mundi (year of
the world): its Tishri molad will therefore be 5,763 years after
the first one.  5763 / 19 = 303 (with 6 years left over), so for
starters we know that there have been more than 303 * 235 + 6 * 12
= 71,277 months.  The 3rd and 6th years of each 19-year cycle are
leap years which have thirteen months instead of twelve, so that's
two more months.  Therefore, the molad of Tishri, 5764 AM is 71,279
months after the first molad.  71279 months * 765433 halakim/month
is 54,559,298,807 halakim, which his 50,517,869+287/1080 hours,
which is 2,104,911+(5+287/1080)/24 days.  Going forward that many
days from October 7, 3761 BC at 20:50:20 UT takes you to Saturday,
September 27, 2003 at 02:06:16+2/3 UTC, which is therefore the
molad of Tishri for 5764.  Assuming nothing else gets in the way,
therefore, Tishri 1, 5764 will be Saturday, September 27, 2003
(although it will actually begin at sunset on Friday evening,
September 26, 2003, since the Jewish calendar day and weekday
begin at sunset).  However, several things, called the "Dehiyyot" or
"Postponements", can get in the way:

1. If the molad falls between noon and sunset (Jerusalem time),
move the start of the year forward to the following day -
this is just rounding to keep the first visibility of the
crescent moon within the first day of the year.  Not a problem
here.

2. If the first day of the year is now on a Sunday, Wednesday, or Friday,
move it forward one day, to keep various holidays from conflicting with
the observance of the Sabbath.   We're on Saturday, so this is again
not a problem.

3. If the start of the year is now 382 days from the start of the last
year, move it forward one day; 382 days is not a legal year length.

Whups, now we need to find out the molad of Tishri for LAST year,
by repeating the process so far.  I'll skip repeating the details.
That molad fell on Saturday, September 7, 2002 at 6 hours 982 halakim
= 06:54:33 UTC.  That's a Saturday and before noon, so it doesn't move.
2002-09-07 to 2003-09-27 is 385 days, which is legal, so we're good.

4. If the start of the year is now 356 days from the start of the
*following* year, move it forward two days - 356 days is also not
a legal year length.

Sigh.  Now we need the molad of Tishri for the NEXT year, too.
That turns out to be Tuesday, September 14, 2004 at 13:15:56 2/3 UTC.
Since that's between noon and sunset, we move it to Wednesday the
15th, but since that's a Wednesday, it moves again to Thursday the
16th.  From 2003-09-27 to 2004-09-16 is 355 days, so we're still OK.

So we know that Tishri 1, 5764 AM is September 27, 2003.  Now we
just need to find out when Nisan 15 is.

Well, Tishri always has 30 days, so that's easy.  The two months
following, however - Kheshvan and Kislev - have a variable
number of days, which is how the legal year lengths are reached.
In a 353- or 383-day year, both months have 29 days; in a 354-
or 384-day year, Kheshvan has 30 days and Kislev has 29; in a 355-
or 385-day year, both have 30 days.  Since this is a 355-day year,
both months have 30 days.  Other than that, the months regularly
alternate 30/29/30/29.  So after Kislev comes Tevet with 29 days;
then Sh'vat with 30; then Adar, with 29; and then Nisan, which is
the month of Pesach (Passover).  So we have to go forward 30 + 30 + 30 + 29
+ 30 + 29 = 178 days from September 27 to get to Nisan 1, and then
14 more days to get to Nisan 15, which is Pesach.  192 days from
Sept 27, 2003 is April 6, 2004, so that's Passover next year.

There.  Wasn't that simple?  I can't imagine why the Christian
church doesn't do that for Easter.

:)

-Mark
```