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On Shabbat preceding Rosh Chodesh, the time of the molad is generally announced in units of hours, minutes, and chalakim. However, minutes are not a traditional Jewish unit of time; rather hours are divided directly into 1080 chalakim. So why don't we just announce the molad in units of hours and chalakim? On the other hand, if the goal is to make it easier for people familiar with our modern time system to understand the time of the molad, why don't we go all the way and convert the chalakim into seconds. (One chelek easily converts to 3 1/3 seconds.)

So for example, the molad for Kislev 5771 could be announced as "4 minutes and 10 seconds after 9pm on Motzei Shabbat" rather than "4 minutes and 3 chalakim after 9pm".

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Regarding converting Chalakim into seconds, that would only work on the months that it is 3,6,9,12, or 15 Chalakim. However imagine next month Teves when the Molad is Monday 9:48AM + 4 Chalakim, would you announce Monday 48 minutes and 13 1/3 seconds after 9AM?

If you would announce just the hours and Chalakim most people would have no idea what you are talking about and even if they knew would be confused. Imagine next month Teves announcing the Molad is Monday at 9AM + 868 Chalakim. Most people in the Shul would have no idea or would need their calculators to start figuring.

So I guess it is done this way for practical reasons.

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Good point. Accuracy to within one second (or, indeed, to within one minute) should be sufficient to determine the earliest and latest times for Kiddush Levana, but not to determine the next molad, since the time between molads is defined as a number of chalakim. – Isaac Moses Nov 8 '10 at 14:57
I don't really see either of these issues as problems. I don't see a problem per se with using thirds of seconds to announce the molad. And also I don't see why it matters whether people in shul are able convert in their heads from 868 chalakim to... well what would they be converting to anyway? why should people in shul be needing to make any calculations? And if they are calculating the next molad, leaving out the minutes would arguably make it easier, since there is one less unit to deal with. – Sam Nov 22 '10 at 1:50
In fact, the Rambam makes of a unit of time smaller than a chelek, that is, a 'rega', רגע, which is 1/76 of a 'chelek', חלק, .0438 seconds. See Hilchot Kiddush HaChodesh 10:1-2. kodesh.mikranet.org.il/i/3810.htm – chaimkut Dec 21 '10 at 14:51
@Sam, people would convert to familiar units so as to know the earliest and latest times for kidush l'vana. – msh210 Nov 8 '11 at 8:14

Perhaps announcing the molad in thirds of seconds would give the misleading impression that the molad is precise to thirds of seconds. For example, announcing that the molad is 3 1/3 seconds after 9:00pm might give the impression that the molad was calculated to be closer to 3 1/3 seconds after 9 than it is to 3 seconds after 9. By giving the molad in chalakim, it is that the precision of the molad is only to the nearest chelek.

However, I don't fully accept this explanation, because for the halakhic purpose of laying out the calendar, the molad is exact, so precision seems irrelevant. And from the scientific perspective, while the interval between moladot accurately approximates the average time between lunar conjunctions to within a fraction of a chelek, over time the error has certainly accumulated to more than a chelek.

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I have heard it announced in minutes and chalakim and then also in minutes and seconds, but, no, that's not the norm (in my experience).

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