[Avodah] new moon

[Micha Berger]

On Wed, Oct 22, 2025 at 06:02:03AM +0300, Joel Rich via Avodah wrote:
> Do we believe that moshe rabbeinu and/or shlomo hamelech did not know how
> to calculate when the new moon would appear (as Hillel II established)?

Rabbi Hillel didn't establish when the new moon would appear. He
presided over the last Sanhedrin who established when Rosh Chodesh
will be from then until the next Sanhedrin (bimheirah beyameinu).

It could be that Moshe Rabbeinu and Shelomo haMelekha knew the molad and
still didn't use Rabbi Hillel's calendar. Further down I will explain
why I suspect the molad was given to Moshe miSinai. (But not "halakhah
leMoshe miSinai", since the average time between New Moons is closer to
a fact than a law.)

(According to the She'iltos. The Rambam agrees that our calendar was
made at the end of the last Sanhedrin, but that it was in the days of R
Ashi. He needs this to explain the authority of the Bavli -- that it was
notes from the last time there was a Sanhedrin to legislate nationally.)

Rabbeinu Bachayei on "HaChodesh hazeh lakhem"
<https://www.sefaria.org/Rabbeinu_Bahya%2C_Shemot.12.2.3>
cites Rabbainu Chananel, and they both hold that the calendar was always
calculated. R Eliahu Monk's translation:

   Rabbeinu Chananel also states clearly [Torah Shlemah by Rabbi M. Kasher
   pages 46-48] that we base our calendar on astronomical calculations
   rather than the sighting of the moon. For proof of this, consider the
   fact that during the Israelites wanderings in the desert for 40 years
   when their encampment was totally enveloped by the [ananei hakavod]
   G-ds clouds of distinction, neither the people nor their leaders
   were able to sight the moon. They did not even see the sun by day
   either... How would the Israelites have been able to fix the date of
   the new moon unless they had based themselves on their astronomical
   calculations? Clearly, the principal method of determining the new
   moon is based on calculations.

Leshitasam, receiving eidus was pro-forma part of Qiddush haChodesh
ritual, but not used to set the date.

But as I said above, this machloqes *could* be limited to whether there
were algorithms for Rosh Chodes before R Hillel, not whether the molad
was known.

It's pretty clear to me that R Chananel must had held that the Beis Din
haGadol / Sanhedrin didn't always use the same calculations, just that
they used *a* calculation, and from there decided whether or not to open
to accept eidim. Otherwise, what kind of sefeiqa deyoma would they have
had in Abayei's day to cause YT sheini shel Goliyos?

For that matter, Abayei's day was just decades before our current
calendar. And he is the one who wrote to Jews in Bavel that there is
now a derabbanan to continue acting as though the day is still in doubt.
So apparently they tried to set up a calculated calendar before the end
of the Sanhedrin, and the last Sanhedrin updated the calculation.

Or the comment that Anshei Keneses haGedolah made sure Hoshanah Rabba
would never fall out on Shabbos -- for a reason that didn't exist until
the concept of Hoashanos. Moshe Rabbeinu or Shelomo haMelekh wouldn't
have had that rule.



In years past, I posted here a theory that the molad was miSinai. We
definitely knew the current length of a molad in the days of Galus Bavel,
since they knew them. Historians assume we got it from them. But if you
think about Galus Yechaniah, they took our intellegsia. Knowledge flowed
the other way.

Now it can get a bit mathematically complicated. In short, the length of
a day and a month change with time. (Tides are sapping energy from the
system.) The month got longer, but the change in the length of a day will
also change how long "29 days, 12 hours and 793 chalaqim" is. So, glossing
over that part, there was a time when the molad was its most accurate.

In human history, the average time between astronomical new moons hasn't
been a full cheileq off from the molad. The difference wouldn't have
mattered much until the drift started adding up.

But it when it was most accurate "just happened to be" the same period
as Rabbi Hillel???

So I figure, this shows we didn't figure out the molad. Nor did Bavel.
Hashem must have given Moshe Rabbeinu the molad, but gave the power
to use it as part of deciding when to make Rosh Chodes to BY, and our
representative, the Sanhedrin. (RH 25a)


And after all, just knowing the molad would solve R Chananel's problem
of the how the Dor haMidbar could be meqadeish hachodesh when couldn't
see the moon. Even if the decision of when to start the month didn't
plug that molad into an algorithm.

Chodesh Tov!
Tir'u baTov!
-Micha

-- 
Micha Berger                 A cheerful disposition is an inestimable treasure.
http://www.aishdas.org/asp   It preserves health, promotes convalescence,
Author: Widen Your Tent      and helps us cope with adversity.
- https://amzn.to/2JRxnDF       - R' SR Hirsch, "From the Wisdom of Mishlei"