In the weekday Amidah, in the prayer for a good year, we add the phrase ten tal umatar, “give dew and rain.” This is recited from the evening of December 4 until Passover.
This date is, in a word, weird, since it is the only date in the Jewish calendar explicitly linked to a set date on the solar calendar.
What does make sense is that this prayer is added in the winter months. In Israel, there is a six month dry season and a six month rainy season. Jerusalem gets more rain than London, but it all comes in the six months of fall and winter.
So why don’t we add this prayer at the beginning of fall, that is, the autumnal equinox on September 22? Theoretically we should, but it is commonly explained that the kickoff date for ten tal umatar is set as 2 months after the autumnal equinox.
Why? Because the autumnal equinox is always close to Sukkot (the biggest pilgrimage holiday). If it started raining immediately, the pilgrims on the road home would get soaked. So the rabbis wanted to allow the pilgrims who came to Jerusalem a full two months to return to their homes before the rains began. Although it can rain in Israel as early as the end of September, the real heavy rains don’t generally start until December.
And if you do the math, 60 days is exactly what you would need to travel on foot from Jerusalem to Baghdad. (And I do mean exactly—from Jerusalem to Palmyra (Syria) to Baghdad (Iraq) is 623 miles. At 12 miles per day, the average rate, that’s 52 days. Plus 7 or 8 for Shabbat—depending on what day the holiday is over and the return home started—gives you 59-60 days!)
But there’s something wrong with this picture. 60 days before December 4 is October 5. The autumnal equinox is September 22! Looked at the other way, 60 days after the autumnal equinox is November 21.
So how do we explain a roughly 13 day difference between November 21 and December 4?
For this, we need to know some history and some astronomy. The determination of when the equinoxes and solstices fall was made by Rabbi Shmuel Yarchinai in the 2nd century CE. According to his theory, the length of the solar year was 365 days and 6 hours. [A more exact calculation was made by Rabbi Adda, of 365 days, 5 hours, 55 minutes and 25.438 seconds.
Unfortunately, this was too complicated to be used easily. In addition, up until the 16th century the figure of 365 and ¼ days was commonly used in non-Hebrew solar calendar systems].