Hebrew calendar software creators: can you notify this list when updating the calendar?

Nadav Har'El nyh at math.technion.ac.il
Tue Apr 13 16:41:12 IDT 2010


On Tue, Apr 13, 2010, Ron Varburg wrote about "Hebrew calendar software creators: can you notify this list when updating the calendar?":
> 
> The file /usr/share/calendar.judaic is out of date.

Where did this file come from? It doesn't exist on Fedora, for example.

> The creators of Hebrew calendars software probably track 
> updates of the calendar.  Perhaps they might update 
> this list regulary about updates to the calendar, so that
>  others can send an updated file to their favorite distro.
> 
> In fact, how do these people track updates to the calendar?

I don't know what you mean by "updates to the calendar".

There are two separate issues:

The first issue is that the Jewish lunar calendar (obviously) doesn't work by
the laws of the Christian solar ("Gregorian") calendar.

The good news is that for hundreds of years, the Jewish calendar is defined
not by guessing or by humans observing the moon - but rather by well-
known and predictable rules, so it is possible to write software which will
calculate the correct Jewish of calendar for hundreds of years in history
and in the future, and it won't need "updating" in our lifetime. See for
example http://www.math.technion.ac.il/~rl/docs/gauss.pdf for one formula
for calculating when Pesach occurs - and from the day of Pesach you can
calculate the rest of the calendar (the details are quite complex, and not
necessary in this post).

Nevertheless, even after you know the Jewish dates, the question is where
to put holidays. Again, established Jewish holidays have been fixed hundreds
of years ago and will not change in our lifetime. The bigger problem is
the new, secular, holidays. The most annoying problem is the upcoming 
Yom Haatsmaut, the rules for which have changed several times, the most
recent change was in 2004: see

	http://www.knesset.gov.il/laws/special/heb/chok_yom_hazikaron.htm

When is Yom Haatsmaut? If you ask any child he'll tell you it's on he be'iyyar.
The sad truth is that is rarely true! Only once in 3 years does it fall on
that date! Why is that? Here's the detailed explanation for those who are
curious (I already explained this on this list back in 2004):

He be-Iyyar can't happen on every day of the week, because of intrinsic
rules in the Hebrew calendar that determine the day of pesach (15 in Nisan).
He be-Iyyar can only fall on a Monday, Wednesday, Friday or Saturday.
Since 1980 (I believe), Friday and Saturday were deamed undersirable for
Yom Haazmaut, and caused it to move earlier (to 4 or 3 of Iyyar).
But in 2004, our ever-so-wise politicians decided that Monday is also
undesirable, moving the holiday to 6th of Iyyar. What's wrong with Yom
Haazmaut being on Monday? Nothing really... Except that it makes Yom
Hazikkaron on Sunday, which in turn makes the "Erev Yom Hazikkaron" on
Motsei Shabat. This in turn, supposedly, means that the people preparing
the cerimonies need to work on Shabat, and also that the cerimonies will
have to start very late, to allow families to start travelling after the
Shabat.

All this means that the only chance we have for ever seeing the Holiday on He
Be-Iyyar is when it happens on a Wednesday, which happens about once every
3 years.

So in 2004, authors of Jewish (or rather, Israeli) calendar software needed
to change a little if() in the algorithm. There wasn't any change (as far
as I know) since, and hopefully there won't be further changes in the
forseable future.

I hope this was helpful, or at least interesting ;-)

Nadav.


-- 
Nadav Har'El                        |      Tuesday, Apr 13 2010, 29 Nisan 5770
nyh at math.technion.ac.il             |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |New! Divorcee Barbie! Comes with all the
http://nadav.harel.org.il           |usual accessories, plus all of Ken's stuff



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