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My due diligence:

I see that I’m walking into a “duplicate question” minefield. So if you’ve got an answer, answer quickly.

Suppose that we use the word “summer” to refer to that part of the year when the northern hemisphere has relatively warm weather, long days and short nights; and we use the word “winter” by contrast to mean that part of the year when the northern hemisphere has relatively cold weather, short days and long nights.

If we do all of that math described in all of those previous questions and conclude that in one century we will begin to pray for rain on December 3, and in the next century we will begin on December 4, and in the next century we will begin on December 5, is that because December is moving from winter to summer, or because our prayer for rain is? Or is there some third alternative?

In other words, I'm not asking how to compute anything. I'm asking what these computations are supposed to achieve, and what they will achieve. In what sense will this set of rules (our complicated rule for this prayer and the complicated rule concerning February 29) preserve the seasonality of our prayer and of the months January through December?

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  • Because the prayer for rain is moving - it is calculated using (the somewhat inaccurate) tekufat shmuel
    – Joel K
    Jan 2 '19 at 14:56
  • @Joel K So we're committed to a system that will sooner or later have us praying for rain in every season?
    – Chaim
    Jan 2 '19 at 14:57
  • @Chaim the system also will sooner or later have Pesach be on every season. No system is perfect. Ours is good enough.
    – Double AA
    Jan 2 '19 at 15:02
  • Since the edit, this question becomes relevant (but not a dupe).
    – DonielF
    Jan 2 '19 at 15:12
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The rules try to keep the date for requesting rain in the late fall. They are not perfect. No rule would be except adding the ever-changing long decimal tropical year length.

If we were to take a simple rule and start requesting rain always 365 days after the previous time, we'd end up shifting a day earlier every four years.

If we were to do that, but every four years add back one day to compensate, we'd end up shifting a day later about every 130 years.

If we were to do that, but three times every four hundred years drop out one day each to compensate, we'd still end up off a day every ~3200 years.

Method 2 is what we use for simplicity because it's good enough. Method 3 is what modern calendars use (by way of including February 29 sometimes). That's why 3 times every 400 years (specifically, in whole century years not divisible by 400) our date shifts relative to modern calendars.

But everyone really is just approximating since the tropical year isn't a rational number of days long. Either you use the precise astronomically calculated decimal value, or you approximate to an accuracy that is "good enough" for your purposes.

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  • What's the halachic source for our current way of determining the date for the prayer for rain?
    – Chaim
    Jan 2 '19 at 18:41
  • 1
    @Chaim the Talmud in the first chapter of Taanit says to start 60 days after the equinox. The equinox is calculated using the approximation detailed above and at judaism.stackexchange.com/q/12674/759 . Were it not for approximating we'd start around November 22 but over the centuries we've drifted all of about two weeks. Not such a big deal.
    – Double AA
    Jan 2 '19 at 18:52
  • So this method of calculating the equinox is described in Ta'anis?
    – Chaim
    Jan 2 '19 at 22:30
  • @chaim shmuel's tekufa is described in Eruvin 56a
    – Double AA
    Jan 3 '19 at 0:07
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The rules you mention in your question are supposed to give a date 60 days after the autumnal equinox as the start date for requesting rain, following the practice in Babylon during Talmudic times.

As noted in @DoubleAA’s answer, the current system is based on the ‘tekufah of Shemuel’ which assumes a mean solar year of 365.25 years, causing a slight drift year on year, relative to the ‘true’ solar year.

R. Moshe Feinstein in Iggerot Moshe Orach Chaim 4:17 notes that we are unconcerned by this inaccuracy for a couple of reasons:

  1. There is nothing inherently special about ‘Day 60 following the equinox’. It’s just that in most years, rain is beneficial from that time on. In order to have a practice which stays consistent from year to year, ‘Day 60’ was selected, and because Shmuel’s estimate is more easily computed, that is what is used in practice, with its inaccuracies.

  2. For us, ‘Day 60 following the equinox’ has even less meaning from an agricultural or climatic perspective. We don’t necessarily need the rain to start precisely then. We just do what they did in Talmudic Babylonia. And in Talmudic Babylonia, they followed Shmuel’s opinion (whose inaccuracy at the time was either not known or not relevant on their timescales).

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  • 1
    Does Rav Moshe say they didn't know Shmuel's tekufa was imprecise or is that your suggestion
    – robev
    Oct 1 '20 at 12:20
  • @robev it’s a little unclear to me. Earlier in the piece he seems to vacillate between saying that we know Shmuel is inaccurate but it doesn’t matter, and also saying that Shmuel was aware of the inaccuracy and was not concerned by it
    – Joel K
    Oct 1 '20 at 12:29
  • So the reason that we will begin to pray for rain on December 3 in one century, on December 4 in the next, and on December 5 in the next, is that December 3 is occurring earlier (in earth's orbit around the sun) from one century to the next, and we must correct for the error of the non-Jewish calendar. Right?
    – Chaim
    Oct 2 '20 at 19:06
  • @chaim No. December 3rd stays (roughly) the same from year to year relative to the sun’s orbit (shifts a day approximately every 3200 years according to DoubleAA’s answer). Our prayer for rain is starting later on in the fall season every year (due to our use of Shmuel’s tekufah which is less accurate than the current secular calendar). The shift amounts to approximately one day every 130 years.
    – Joel K
    Oct 3 '20 at 17:24

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