Date: October 15, 2009

Title: “When is Passover this Year Anyway?”


Podcaster: Zachary Kessin & Rav Hillel Maizels

Organization: None

Description: This podcast explores the origins and history of the Hebrew calendar, how it works and how it relates to the lunar cycle and seasons.

Zachary Kessin
Zachary Kessin grew up in Cambridge and Hillsdale NJ. He has lived in the Boston area, London and Israel. Zachary holds a BA in Physics from Brandeis University and works at a high tech startup in Petach Tikva Israel. Zachary escapes down to the Negev desert whenever he can to enjoy dark skies. He also lives in Ariel with his wife, and 4 children.

Rav Hillel Maizels
Rav Hillel Maizels studied for 11 years at Yeshivat Har Etzion where he received Rabbinical ordination (from the Chief Rabbinate of Israel) and served as the Overseas Program Co-ordinator and madrich.  Rav Hillel holds a B.Sc in Computers and Psychology from the University of South Africa and a teaching diploma from Michlelet Herzog. He made aliya from South Africa in 1995 and served in the IDF Tank Corps. Rav Hillel is currently the Rav of the Ohel Ephraim community in Ariel, together with his wife and two daughters. 

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Allen Thomas in memory of Lena Thomas, his best friend, whom he married on this day, many years ago. Lena’s favorite cause was to rescue animals, particularly horses, who were rejected by their owners for illness or age, and she gave a home to many creatures in need. In college, she liked to sit on the boat docks to watch the stars at the George Williams campus on Lake Geneva, Wisconsin, near Yerkes Observatory.


ZK: Hello, my name is Zachary Kessin, I am an amateur astronomer here in Ariel, Israel and I am here with my friend and Rabbi, Hillel Maizels.

HM: Hi, also here in Ariel, Israel the Rabbi of “Ohel Ephraim” shul [synagogue] here in Ariel, Israel.

ZK: And we are here today to talk about the Hebrew calendar. So if you’ve ever wondered why Passover seems to float around at random on the calendar, it’s not actually at random. And what day is it this year, Rabbi?

HM: Ok, this year, Passover actually falls out on March 30th (2010.) People always ask “Is Passover early or is Passover late?” Well, let’s explain how it works.

ZK: So, how does it work?

HM: So, back in Exodus, Chapter 12, the Jewish people, the Children of Israel, are still in Egypt and God comes to Moses and says to him “This month shall be for you the first month of the Jewish calendar.” Being the month as we call it in Hebrew, the month of Nissan. From that moment onwards, the Jewish calendar is defined as a lunar calendar (i.e., based on the moon.) The cycle of the moon, from the beginning of the first sighting known as the Molad til the next first sighting is a lunar month. What is a modern  lunar measurement?

ZK: So a modern lunar – measurement of a lunar month is 29 days, 12 hours, 44 minutes, 2 seconds and 8/10th of a second on average. It’s slightly, it varies slightly of course.

HM: Now that’s fascinating because the Rabbis of the Talmud determined that the lunar month is 29 and a half days which is your 12 hours and they called it 793 parts of 1080 of an hour which works out to almost the same number. I believe it’s 44 minutes…

ZK: It’s 44 minutes, 3 seconds and one third, which is 6/10th of a second longer than the mean solar month.

HM: That’s pretty amazing for a calculation (made) about 2000 years ago.

ZK: Yeah, this was 2000 years ago, they had no notion of a “zero,” certainly no telescope and very primitive instruments. This was probably worked out from cycles of lunar eclipses… any rate…

HM: Well, as you pointed out, that’s an accurate description of a month length. The Rabbis at the time determined that, to determine which is the new month, the beginning of the month, you actually had to have witnesses coming along and saying, “We saw the new moon last night.” Now obviously is could only be one of two nights, either the 29th or the 30th and if no witnesses came on the night after the 29th then automatically the month became a 30 day month because 29 1/2 days is either 29 or 30. Ok, so that’s how things worked for a number of years, all according to the witnesses whether the month would be 29 or 30 [days]. The problem is that the Jewish calendar is not only lunar. If it had been only lunar then our calendar would be like the Muslim calendar, where Ramadan could be in the winter one year and a few years later it could be in the fall, a few years later it is in the summer.

ZK: Just to point out, 12 lunar months is roughly 354 days, so you get an 11 day per solar year discrepancy if you measure versus the eclipses… the equinoxes, for example.

HM: Right, so after a number of years, the festivals actually move around to different seasons. However, in the Jewish calendar, we have another verse in the Bible and that is from Deuteronomy Chapter 16. Deuteronomy Chapter 16 the festivals are listed as being seasonal festivals. For example, Passover is called the festival of Spring, Sukkot – or Tabernacles is the festival of the Fall, of the gathering of the harvest. In which case the Jewish calendar is actually a combination of lunar and solar.

ZK: So what happens is every few years, we insert an extra month – the month of Adar I. Adar II being the month that’s there every year. Now how did that used to work?

HM: So the way it used to work is at the end of Adar, which is the end of the winter, the Sages would go out and check the weather, check out the land. If it was still too cold, they’d say, “OK, obviously the calendar has moved enough, moved back far enough to add another month.” If the weather is warm enough, they’d say, “Fine, carry on as normal.” That was all well and good for a number of years…

ZK: At least 1000 [years].

HM: Yes. However with time, the witness procedure became faulty. People would come along and bear false testimony to try and ruin the calendar. Came along Hillel the Elder, Hillel haNasi, and he actually, shall we say, codified…

ZK: Formalized…

HM: …formalized the calendar. And he formalized it as such that every 19 years there would be 7 inter-calendar months…

ZK: Or leap months, is the more common term.

HM: Sure, which then works out that every third, sixth, eighth, eleventh, fourteenth, seventeenth, and nineteenth year is a leap year. So you have 19 times 12 months plus an extra 7 months… and does that work out Zach?

ZK: Well, that does. This is the Metonic Cycle, it was worked out by Meton in about 400 BCE, so, well, a very long time ago. He worked out that 235 solar synodic months is 6939 days, roughly. And that 19 mean solar years is also 6939 days. The specific values are 6939.88 days for the months, and 6939.602 days for the year. So you’re off by a little bit but not very much. It works out you’re off by about 6 minutes, 25 seconds and a half, roughly speaking, per year in the modern calendar. So this is for 2000 years ago, a very accurate measurement or series of measurements.

HM: The interesting thing is according to this therefore, the festival, let’s say Passover as you asked originally, every year will move back by 11 days but every 2 to 3 years will jump forward by 30 days, and therefore Passover, or the Jewish festivals in general, will move around, but they’ll always stay within the same range, keeping them within their seasonal times as the Torah, the Bible prescribes. And this is a good solution for a balance between the lunar side, our monthly lunar months, and the yearly solar calendar as well.

ZK: So that’s the basics. The other thing is the Molad – the period of the months which is a Molad is announced every month: it’s 29 days, 12 hours, 44 minutes, and 3 seconds and 1/3, about 6/10th of a second longer than a mean solar month. This sort of represents sort of the mean time when the moon would originally have been visible in Jerusalem. Now its off by 6/10th of a second, so that meridian where it originally would have been visible 2000 years ago or so when this was worked out actually has moved from Jerusalem to roughly the region of Kandahar, Afghanistan now.

HM: I don’t think the focus of Judaism is going to shift to Kandahar, Afghanistan so quickly.

ZK: [laughing] Yes, I think we’re going to stick with Jerusalem. The drift of the Hebrew calendar versus the cycle of the equinox is about one day every 224 years, which is significantly better than the Julian calendar, although not nearly as good as the modern civil Gregorian calendar.

HM: Which is an interesting point because as a modern Jew who is both obligated to Halacha but at the same time appreciative and cognizant of modern science, this to me doesn’t pose such a problem. Because although our calendar might be moving off slightly as the years, as the centuries progress, you have to remember that the original calendar was supposed to be adjusted every month and every year. The original calendar had your witnesses, it had the Sanhedrin – the high court deciding these things, and therefore the calendar we have according to Hillel haZaken, Hillel the Elder, is really just an interim measure until the Sanhedrin gets reestablished. And therefore I would venture to say that the fact that it’s going off course is actually a good thing. It’s a sign that really isn’t the ultimate way we should be doing things.

ZK: And it’s also a sign that, you know, Hillel and his colleagues had a very firm understanding of what was going on.
Thank you for listening. This is Zach Kessin and Hillel Maizels of Ariel, Israel and enjoy the rest of the 365 days of Astronomy

End of podcast:

365 Days of Astronomy
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