יום חמישי, מרץ 08, 2007

 

Lunar eclipses and Jewish holidays

This Jewish year--my first of living a Jewish life--has been full of convergences of astronomical events with the holidays. The cosmic convergences began at the very head of the year, when the autumnal equinox and Rosh Hashanah coincided (which also happened to be the beginning of Ramadan). Then Chanukah coincided with the winter solstice. Of course, we also have had a year in which many of the holidays (starting with Rosh Hashanah and continuing with Sukkot, Chanukah, and Tu Bishvat) coincided with Shabbat. And now we have just had a lunar eclipse on Purim!

This sent me researching the frequency with which holidays and lunar eclipses might converge. As BZ noted at Mah Rabu (and at Jewschool), Purim may be the perfect holiday for a lunar eclipse:


The moon gets covered up (bad for the Jews [according to the Talmud, Sukkah 29a]), and then becomes visible again (good for the Jews!). This exactly parallels the structure of the book of Esther: during the first half, it appears as though the Jews are going to be annihilated. In the end, this ominous darkness is chased away, and everything works out ok: LaYehudim hayetah orah vesimchah! The Jews had light and joy!

So, how rare was this convergence? It was not easy to piece together the answer, but two terrific websites made it possible. The first was Hebcal, which allows you to generate a calendar of the overlay of Jewish holidays with any year on the Gregorian calendar. This allowed me to compare with dates of lunar eclipses throughout many centuries.

(It would have been so much easier if someone had a list of lunar eclipses according to the Hebrew calendar, but several pages deep into a Google search, nothing of the sort had turned up on-line.)

In any event, if I compared the two pages accurately, it looks to me like we last had a lunar eclipse on Purim in 1978. It looked like the list of lunar eclipses showed one in March, 1978, to be the night after Purim, but that presumably is not possible. So, I think this convergence happened in 1978. I am more confident that it happened in 1960 (which happens to be the year I was born, though not till fall). The next time appears to be in 2025. Mark your calendar!

So, it is a pretty rare occurrence. What about other holidays? Granted, for the reasons BZ gives, Purim may be the perfect holiday on which to have a lunar eclipse, but obviously we can't enjoy such coincidences very often.

Again, with the caveat that I am not claiming my results are definitive, relatively recent and upcoming convergences of lunar eclipses with holidays other than Purim appear to be as follows:

1967 Sukkot and Pesach
1968 Sukkot and Pesach
1986 Sukkot and Pesach
1990 Tu Bi-Shvat
1996 Sukkot and Pesach
2014 Sukkot and Pesach
2018 Tu Bi-Shvat

Well, I never would have guessed that Pesach or Sukkot would have this occur so often (if you call five times over about a sixty-year period "often"). And in consecutive years. Can that be right?

I also do not have sufficient understanding of the cosmic cycles to understand why every time there is a lunar eclipse on Sukkot, there is also one on Pesach of the same Jewish year. Obviously, these holidays occur at the full moons six months apart; from the table of eclipses, I could see that there are many years (by which I mean any 12-month period) when two eclipses occur six (lunar) months apart (including 2007). However, there are others with none (such as 2005 and 2006). My astronomy is not good enough to understand these patterns, or even to see a clear pattern. (I'm just a social scientist; in fact, despite a nearly lifelong fascination with the topic, an astronomy course was the source of my only C in college!)

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