Date: Sun, 21 Dec 1997 11:25:15
From: Simon Cassidy <scassidy@EARTHLINK.NET>
Subject: Re: Lunar systems (Jewish)
>.............. The Jewish calendar has been rule-based since Biblical
> times; see, e.g., I-Samuel 20:5 : "And David said unto Jonathan, Behold,
> to morrow is the new moon, and I should not fail to sit with the king at
> meat" -- that is, the rules were accurate enough for king Saul to have a
> monthly new-moon feast which was prepared well in advance.
Simon notes: The ability to predict the evening of the first crescent moon (the new moon) just one day ahead of time (the maximum predictive ability implied by this Biblical passage) does not demonstrate that the Jewish calendar was completely (arithmetically) rule-based when this passage was written.
> The rules are indeed complex, though not horrendously so; the main
> complication is due to the way lunar dates are matched with days of the
> week (usually by specifying which days of the week certain holidays can
> *not* fall on). Thus quite often the first day of a month is delayed one
> or even two days after the observed new moon.
Simon notes: This weekday sensitivity in the current Jewish calendar rules is not the only reason that the first day of the Jewish months often fall later than the day of the new moon. The major (most problemmatic) reason is as Peter asserted, that the lunar motion is too complicated to be predicted (to the day) by rules (such as the Jewish calendar rules) which utilize only low order approximations of this complex motion. This problem is not as evident with Solar calendars because the higher order perturbations of the annual solar cycle are of much smaller magnitude than the perturbations of the lunar phase cycle.
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