Daily Talk
The Rambam has just finished handing us a calculation exact to a 1080th of an hour, and the first thing he does with it is overrule it. Rosh Chodesh Tishrei may never be established on a Sunday, a Wednesday, or a Friday, whatever the arithmetic says. Why build an instrument that precise and then legislate when not to trust it?
The Days the Calendar Refuses
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About This Talk
Chapter 7 of Kiddush HaChodesh is the chapter of the postponements. Rosh Chodesh Tishrei is never fixed on Sunday, Wednesday, or Friday; a molad at midday or later moves the day forward; a molad that lands on the night of the third day at nine hours and 204 parts pushes an ordinary year all the way to Thursday; and in a year following a leap year a Monday molad past three hours and 589 parts moves to Tuesday. If the postponed day is itself one of the forbidden three, the day moves again. Then the Rambam gives his reason, and it is not the one most people expect: the whole calculation tracks only the mean motion of the sun and the moon, so the court builds a day of fixing and a day of deferring in order to catch the true conjunction.