Tuesday, November 16, 2010

Vayishlach: Quick Zman Krias Shma Calculation


I feel bad about this lazy streak, so here's a little-known zman krias shma shortcut without having to figure out sha'os zmanios.
Time of sunrise, plus twelve, divide by two.  That's zman krias shma.  (Be careful to adjust for Daylight Savings.)
(Rav Baruch Epstein ob'm)

 Eli points out that due to the Equation of Time, this is off a few minutes.  Sorry, Reb Boruch.
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4 comments:

  1. EliNovember 25, 2010 at 2:43 PM

    The zman KS trick works only if 12 is indeed midday. However, 12 is not midday for two reasons - (i) we use today the timezone clock, so our time is offset depending on how our longtitude differs from the center of zone (ii) even at the center of zone, e.g. NYC, the "equation of time" (google it!) tells us that midday is not at 12, and fluctuates around the years.

    [I know about the Tshuva in IGM, but I don't understand it]

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  2. bNovember 25, 2010 at 5:32 PM

    Hi, Eli. Sorry I didn't get back to you about the Maharil Diskin. I suppose I saw it in one of the recent Brisker sefarim on Chumash.

    As for the equation of time, I'm given to understand that the magnitude of the variation is usually in the range of minutes over the course of a year, and may be ignored by the halacha.

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  3. EliNovember 26, 2010 at 12:20 AM

    minutes, yes, but about 30 of them (15 each direction). Here in EY, the earliest Hatzot is about 11:25 and the latest about 11:55. For someone like myself who often gets to Shma a few minutes before deadline, the extra ten minutes are important.

    I agree this could have been a pshat in R Moshe's father, but I can't accept RM didn't know that for someone out of NYC the formula does not work (e.g. in EY there is around 20min average shift of midday due to 5degrees distance from Cairo, the zone center)

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  4. eeNovember 26, 2010 at 12:24 PM

    I just found a table for solar noon in Jerusalem, and found that the earliest is 11:23 on November 3 and the latest is 11:54 on February 14.

    http://www.solar-noon.com/sn_calc.php?Location=Jerusalem&LatDec=31.47&LatNS=N&LongDec=35.12&LongEW=E&TimeZone=-30&Type=SolarNoon

    So I don't know.

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