[Originally posted in November 2005 on the now-defunct Maven Yavin blog. The original comment thread has disappeared.]
First, a bit of a history lesson:
The calendar that the western world uses, called the Gregorian calendar after the Pope who made it the official calendar, isn’t that old. You can read up on it here, but suffice it to say that the current system, which contains 97 leap years every 400 years, is just over 400 years old. Before then, the Julian calendar, which contains a leap year every 4 years, was used. When the change was made, first in 1582 and whenever it was adopted afterward by other countries, the calendar had to be adjusted so that certain features of the astronomical year match consistently with the calendar year. For example, that the solstices and equinoces would not vary, and so that certain holidays (especially Easter) would remain in the appropriate season.
In order to retroactively adjust the calendar, 10 dates were dropped that year; those who went to sleep the night of October 4, woke up on the morning of October 15.
As is known, the Jewish calendar adjusts itself to the solar year as well by introducing an extra month in 7 of every 19 years, also to insure that certain holidays (especially Pesach) occur in the appropriate seasons. Additionally, the 4 seasons of the solar year are called in Rabbinic parlance the Tekufot of Tishrei, Tevet, Nissan, and Tammuz corresponding to Autumn, Winter, Spring, and Summer, respectively. The beginning of these Tekufot correspond to the beginning of the calendrical seasons as well: the autumnal equinox, the winter solstice, the vernal equinox, and the summer solstice. The days upon which those occur are called ‘yemei tekufah’.
Now, a bit of an astronomy lesson:
Solstices and equinoces are not days; they’re events. The equinox is the moment in time where the part of the earth closest to the sun lies at the equator, where somewhere on earth along the equator it is noon and the sun lies directly, absolutely, at the zenith of the sky, or when the line which goes from the center of the earth to the center of the sun is perfectly perpendicular to the Earth’s axis (all of these mean the same thing). We notice its effects in that the length of days and nights are as close to even as they get during the course of the year.
The solstices are the moments when the point on the Earth closest to the sun is as far north or south as they will get during the course of the year, when at High noon, somewhere along the tropic of Cancer or Capricorn, the sun will be directly overhead, when the angle formed by the intersection of the Earth’s axis and the line from the center of the Earth to the center of the Sun is at its most acute (or obtuse, depending on how you’re measuring).
The Gregorian calendar is arranged so that these events nearly always occur on the same four dates, which correspond to the beginnings of the four seasons.
Now, a lesson in Halakha:
According to the Gemara, (Ta’anit 10a), in Babylonia, which didn’t require all that much rain, they began to pray for rain (what Ashkenazim say as ‘Ve-ten tal u-matar’) on day 60 of the Tekufah of Tishrei, i.e., the 60th day after the autumnal equinox.
The autumnal equinox was originally designated to fall out on September 24 or so in the Julian calendar, but over the course of the centuries had slid back about 11 days. Nevertheless, September 24 was still treated as the equinox. Thus, Halakhic authorities such as Tashbetz (3:123) and Avudraham (Shmoneh Esrei, s.v. “Ha’revi’it Be-birkat) can rule that we begin praying for rain on the 22nd or November, and the 23rd on leap years.
However, astronomically speaking, when they wrote those words, the 60th day after the solstice was already 9 or 10 days earlier. Thus, just as the Catholic and Protestant calendars shifted, the Jewish calendar should have shifted as well, and the proper time to begin praying for rain, for us Babylonians, should be back at November 20 or 21 (when the equinox is on September 23, as it normally is).
Sadly, but not surprisingly, the Halakha never adjusted. We will continue begin praying for rain on December 5th or 6th until the year 2100, when the Halakha will observe a leap year but the rest of the world won’t, and we’ll begin praying for rain on December 6th or 7th. Eventually (i.e., in about 16,000 years), we’ll never pray for rain because Pesach will be before the 60th day from the Julian equinox. By then, though, hopefully we’ll all be living in Israel so we won’t calculate by the methods of the accursed Babylonians. For the time being, it’s another example of what happens when Halacha goes into exile.