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?

  • 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

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
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    @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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