[Avodah] Where is the Molad announced for?

[David Cohen]

Some of the following is copied from Facebook comments where R' Micha and I
had more or less this same discussion 6 months ago, but I suppose we're
repeating it here for the benefit of a different audience.  :-)

The length of the mean synodic month (expressed in mean solar days***) is
decreasing *very* slowly, such that it takes about 10,000 years to decrease
by an entire chelek. If your degree of precision is that you're rounding to
the nearest chelek, then the value of 29 days + 12 hours + 793 chalakim was
accurate in the time of the Neo-Babylonian astronomers, it was accurate in
the time when our calculated calendar was set up, and it's still accurate
today. (The accumulated error of ~2 hours that we have now is due to the
cumulative effect of the "rounding error.") It was, indeed, most *precise*
-- in the sense of the actual value being exactly 793.000 chalakim -- in
the 4th century CE, but if your level  of precision is whole chalakim, then
I wouldn't say that it's been *inaccurate* at any point.

*** In objective (i.e. atomic) time, the length of the mean synodic month
is actually slowly increasing, but it's increasing more slowly than the
length of the mean solar day is, which means that it's decreasing when we
measure time, as we customarily do, in mean solar days and divisions
thereof.

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As far as my main point, I share your objection to the assumption that the
calendar moladot were intended to be referring to Jerusalem mean time.  I
just don't think they were intended to be referring to the exact mean time
of any deliberately selected meridian, but rather that the determining
factor was having the very first molad come out exactly on the hour.
There's a wide range, spanning 15 degrees of longitude, over which rounding
that first molad to the nearest hour would get you the same result (14
hours into Friday), and I doubt that it particularly interested anybody at
the time the calendar molad was established to figure out exactly what
meridian it would be precisely accurate for.  For the purpose of the
calendar, it doesn't matter.  We only need to know if we want to translate
the molad into an actual time that we can point to on our watches and say
"the molad is.... now."  But the only reason we'd have any interest in
doing that is if we're using the calendar molad to determine our window of
time for kiddush levana, and I think that the practice of doing that came
long after it was established for the purpose of the calendar.

-- D.C.
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