Tomorrow is Hanukkah, and we’ll be doing all kinds of fun Hanukkah coverage this week, but I wanted to let people know about another important thing happening for Jews tomorrow:

There’s one place in the siddur where we go by the Gregorian calendar rather than the Jewish calendar, and it has to do with when we start praying for rain. If you open a siddur to the middle of the Amidah, you’ll notice that there’s one place with two options. Either v’ten tal umatar livracha (give us rain and dew for a blessing), or just vten bracha (give us a blessing). The longer option is a specific request for rain, and we say it starting on the evening of December 4th. But why December 4th and not shmini atzeret, which is when we go through a whole rigamarole at shul asking for rain?
The Autumnal Equinox: ruins everything
Basically, the Rabbis in the Talmud decided that rain should not be requested prior to the sixtieth day after the Autumnal Equinox. It’s not clear why they chose this day, but some scholars have suggested that for the Babylonian farmers rainfall was considered a nuisance before the conclusion of the date-harvest (there’s a joke here about my dating life, my name, and the phrase “date-harvest” but I can’t quite figure out what it would be). Whatever the reason, it’s clear that the equinox, as a phase in the cycle of the sun, is most conveniently calculated by the civil calendar, which is a solar one.

In the course of the Middle Ages the Babylonian practice came to be accepted--though not without a struggle--by all Jewish communities outside of Israel. Israel itself follows a different, earlier date, defined according to the Jewish calendar (the 7th of Heshvan).

Surprise! Once again, normative practice has rejected the more reasonable precedents of praying for rain either when it is beneficial for our own climate, or when it is required in the Israel--in favor of the unlikely option of linking it to the climate of Iraq (the current inhabitant of the land that was formerly called Babylonia, where the Talmud was written).

But there’s still a bit of a math problem to deal with: The Autumnal Equinox actually occurs on the 22nd of September, so that the sixtieth day following should come out on November 20, not December 4!

The discrepancy originates in the methods that we employ for calculating the solar year. The Talmud assumes that a year consists of precisely 365 1/4 days and halakhic practice bases its calculations on that premise.

The calculation is very close, but it's not fully accurate, since an astronomical year falls eleven minutes and fourteen seconds behind that estimate. The margin is admittedly a tiny one, but when stretched across the centuries of Jewish history the minutes begin to add up. Every 128 years the Jewish reckoning pulls a full day ahead of the astronomical equinox.

Anyway, though it seems crazy to pray for rain based primarily on bad math, bad astronomy, and what’s good for Iraq, it’s pretty much the norm, so starting tomorrow night I’ll be praying for rain.

And in case you were wondering, no, I don’t know this stuff just off the top of my head. For more info, check out this source sheet, and this overview.