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The HaMaor Mikraos Gedolos chumash has a chart of molad (forecasted first visibility of the new moon in Jerusalem, based on averages observable to the naked eye, assuming Jerusalem local time; am I getting that right?) times in the back, which is great, but it seems like it's off by a day every so often.

For instance, they announced in shul that the Molad for Av, 5770, is Sunday, 6:07 (+ 17/18) PM, and rosh chodesh (the official New Moon day) is Monday. But my chart had it as Monday night, 6:07 +17/18.
(My chart also has a note that they call 6PM-6AM "night.") Is my chart just wrong, or am I reading it incorrectly? This isn't the first time this has happened.

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2 Answers 2

I wonder if maybe they misinterpreted the source from which they got the times. Suppose that the original said "ליל שני" or the like; that would properly mean "the night leading into Monday" (i.e., Sunday night), but the printers of your chumash may have misunderstood that to mean Monday night.

Out of curiosity, then, can you post several months' worth of molados as given in your chumash? Then we can see whether that might be the case.

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Since the "day" is defined as "night followed by day", then Sunday night after sunset is actually and technically Monday. Thus, the original Hebrew declaration shows that the Moled is "Monday" after the time specified for sunset. The time for "day" is defined as sunrise to sunset and divided into 12 "hours". Similarly for the time from sunset to sunrise. The number of minutes each hour will therefore vary. 6 PM to 6 AM is the time for the period around the equinox when the day and night are equal in length (12 sixty minute hours each). Some calculations will define 6 PM as sunset and 6 AM as sunrise, with midnight and noon half the time in between. This will the use the "variable hours" as the position of the other times. This will not be the same as the time on the clock (fixed hours of 60 minutes each).

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