Monday, January 14, 2013

Hanukkah and Thanksgiving: A once in eternity overlap

Next year features an anomaly for American Jews – The first day of Hanukkah coincides with Thanksgiving, on 11/28/2013 (meaning the first night of Hanukkah is actually the night before Thanksgiving). I was curious how often this happens. It turns out that it has never happened before...and it will never happen again.
(Correction: it happened once before, in 1888: see addendum below.)

Thanksgiving is set as the fourth Thursday in November, meaning the latest it can be is 11/28. 11/28 is also the earliest Hanukkah can be. The Jewish calendar repeats on a 19 year cycle, and Thanksgiving repeats on a 7 year cycle. You would therefore expect them to coincide roughly every 19x7 = 133 years. Looking back, this is approximately correct – the last time it would have happened is 1861. However, Thanksgiving was only formally established by President Lincoln in 1863. So, it has never happened before. Why won't it ever happen again?

The reason is because the Jewish calendar is very slowly getting out of sync with the solar calendar, at a rate of 4 days per 1000 years (not bad for a many centuries old calendar!) This means that while presently Hanukkah can be as early as 11/28, over the years the calendar will drift forward, such that the earliest Hanukkah can be is 11/29. The last time Hanukkah falls on 11/28 is 2146 (which happens to be a Monday). Therefore, 2013 is the only time Hanukkah will ever overlap with Thanksgiving. You can see the start date of Hanukkah as a function of time in the attached plots. In the long timescale plot, the drift forward is clear.

Of course, if the Jewish calendar is never modified in any way, then it will slowly move forward through the Gregorian calendar, until it loops all the way back to where it is now. So, Hanukkah will again fall on Thursday, 11/28...in the year 79811.

Update 1/16/2013: In response to numerous questions and comments, I've posted an addendum below.




Addendum:

I have gotten quite a few comments since I first posted this, so I thought I would respond to a few of them:

1) A "day" in the Jewish calendar starts at night.  This leads to an inherent ambiguity when talking about Jewish calendar days overlapping Gregorian calendar days, because each Jewish calendar day actually starts on the previous Gregorian calendar day.  This means that although this coming year the first day of Hanukkah falls on Thanksgiving, candles will be lit for the first NIGHT of Hanukkah the night BEFORE Thanksgiving.

2) Related to (1), when the first day of Hanukkah falls the day after Thanksgiving, the first night's candles are lit the night OF Thanksgiving.  This will happen two more times, in 2070 and 2165.

3) The 79811 date is accurate, but was meant to be tongue in cheek.  But, as a few people pointed out, Jewish law requires Passover to be in the spring.  Therefore, the Jewish calendar will have to be adjusted long before it loops all the way around.

4) For the real nitpickers, it is also true that it is not necessary for Hanukkah to to get all the way back to Thursday, November 28 to hit Thanksgiving again.  The actual next time the two will overlap is when the LAST day of Hanukkah falls on the earliest Thanksgiving date, which is in 76695.  In all honesty, though, all of these dates are unfathomably far in the future, which was really the point.

5) I was considering here Thanksgiving to be the fourth Thursday in November, which is what it currently is.  However, originally it was the LAST Thursday in November.  This changed in 1942. If you use the last Thursday rather than the fourth Thursday, then the overlap has happened once before, in 1888.  There were also various inconsistent dates for Thanksgiving going back centuries, which I did not consider.

6) These calculations were all done in Mathematica.

7) I am not the only person to note this coincidence. At least two other people have also written about it:


18 comments:

  1. Hi Jonathan, I would love to speak to you about this for a story I'm writing. I'm Leanne Italie of The Associated Press and can be reached at litalie@ap.org. Best, Leanne

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  2. Hi, Jonathan,
    Any chance you can rework the math with an adjustment for the requirement that Passover fall in the spring? I.E., can you find the date when Passover occurs before the vernal equinox, and at that point add a periodic adjustment that ensures the earliest possible Passover falls before the vernal equinox from that point forward, and re-plot? Then you can accurately determine the true next time this coincidence will happen. It might also spur our community to plan for that situation if the date is within a reasonable timeframe. (For us Jews, anything under 6,000 years would be considered a reasonable timeframe--we're used to delayed gratification!)

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  3. Nice graphical analysis and discussion!
    Your statement "Jewish law requires Passover to be in the spring" is incorrect, although widely held. Astronomically, from the point of view of solar insolation, the spring equinox marks the MIDDLE of the spring season. Jewish law actually requires that the first OMER offering, on the 16th of Nisan shortly before noon, had to be after the spring equinox (without specifying how the equinox is reckoned). For full explanation and sources, see my web page at:
    http://www.sym454.org/hebrew/drift.htm
    For obvious practical reasons, that should be at the earliest possible opportunity, so it should not be a leap year if the 16th of Nisan at 11am Jerusalem Local Time is already after the spring equinox. Nevertheless, with the current Hebrew calendar, the 16th of Nisan at 11am is more than 30 days after the astronomical spring equinox in 4 out of 19 years. This causes the Hebrew calendar to run "one month late" for an entire year, until the following spring. If this drift were corrected, as explained in the last section of my cited web page, then American Thanksgiving would more often coincide with a day within Chanukah -- but anyway, nobody ought to care about this coincidence. -- Dr. Irv Bromberg, University of Toronto, Canada

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  4. Your term "drift forward" is ambiguous, I suggest "drift later".
    You claim that the Hebrew calendar drifts (later) by 4 days per 1000 years relative to the solar calendar. Unclear what you mean by that. The Gregorian calendar is an approximation to the solar cycle, almost 12 seconds too long per year. Relative to the Gregorian calendar, the Hebrew calendar drifts one day later in exactly 2462400/10643 or about 231+1/3 Gregorian years. That is exactly 4+3967/12312 or just under 4+1/3 days per 1000 years. Relative to the astronomical solar cycle numerical integration is accurate, but using fixed arithmetic one can only approximate the drift. Taking the current mean northward equinoctial year as the reference = 365d 5h 49m 0s (mean solar days) the current drift rate is 6 minutes and 25+25/57 seconds per year, or about 224+1/6 solar years per day of drift, or just under 4+1/2 days per 1000 years.

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  5. This comment has been removed by the author.

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  6. Should be clear that it is the Latest date the LAST Day of Hannukah comes out on the Earliest Thanksgiving

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  7. Are you still at Sandia in Albuquerque? The math and science of the calendar would make the coolest lecture the JCC has put on in since 1510!

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    1. I laughed out loud at that one. But it raises the critical question of what the JCC lecture in 1510 (which calendar) was actually about.

      The Earth going around the Sun perhaps? If so, that's a tough act to follow. :-)

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  8. This Thanksgivukah, ‘Bubbie and Zadie’ are at the White House

    November 9, 2013




    By Dan Bloom

    ThanksgivukahAccording to Paul McFedries ”Wordspy” website, there’s a new holiday on tap this year, this month in fact, and it’s been dubbed “Thanksgivukah.” Jewish humor. Yes, this year for the first time in almost 5,000 years, Thanksgiving and Hannukah occur on the same calendar day.

    Obviously, God was trying out a new tack. According to Wordspy, Thanksgivukkah is “a combined celebration of the U.S. Thanksgiving holiday and the start of the Jewish festival of Hanukkah.”

    The Houston Chronicle mentioned the new holiday in passing, as did The Boston Globe and a host of other newspapers and websites nationwide.

    McFedries, always the alert and tech-savvy editor, noted: “It should be noted that the Thanksgivukkah Facebook page was created by Dana Gitell on November 15, 2012. For the record, she also received a U.S. trademark for the term on July 26, 2013.”

    So as ”Thanksgivukah” approaches on November 28, here’s a short anecdote about how I plan to celebrate the holiday this year, with the hope and a prayer that some savvy e-publisher can find a way to turn a little book I wrote in 1985 into an illustrated children’s ebook in time for the holiday season in 2014. Do they make illustrated ebooks yet?

    Long story short: Bubbie and Zadie — Yiddish terms for ”Grandma” and ”Grandpa” — had been many places in their lives, but never the White House. So when a kind rabbi in Virginia was invited to the annual Chanukah Party at the Obama White House a few years ago, he thoughtfully brought my Jewish children’s picture book with him as a gift and left it with the Obama family.

    The title? ”Bubbie and Zadie Come to My House: A Story for Hanukkah.”

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  9. Jonathan Mizrahi, a quantum physicist at Sandia National Laboratories in Albuquerque, N.M., puzzled on the convergence last January, in a blog post with buzzed-about line graphs picked up by others online. More than 100,000 people have visited the blog since then, he said, including some who questioned his calculations and prompted him to post a couple of clarifications.

    He hadn't made it clear that he was referring to the "second candle" night of Hanukkah, and he hadn't realized Thanksgiving had shifted from the last to the fourth Thursday of November.

    The interest, Mizrahi said, "has truly blown me away. I've just been totally flabbergasted at the number of responses.

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  10. Thanks for pointing all this out. It was, in part, the inspiration for my creation and Trademarking of Hanugivingkkah. Greeting cards for the celebration of Hanukkah & Thanksgiving together: A once in a lifetime Holiday event, happening in OUR lifetime. Thanks again!

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  11. According to both the Hebrew and Gregorian calendars of 1888 the first DAY of both were on Nov 29th!

    http://www.timeanddate.com/calendar/?year=1888&country=1
    http://www.timeanddate.com/calendar/?year=1888&country=34

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  12. Are you sure? Given that Thanksgiving follows the midnight to midnight schedule of the secular calendar, while Chanukah follows the sunset-to-sunset schedule of the Jewish calendar, why couldn't it happen again when the first NIGHT of Chanukah is 11/28? This year, the first night is 11/27, and granted, it won't happen that early some other year that Thanksgiving falls on the 28th. Still, sometime in the next 216 years (approximate slippage of Jewish vs. Gregorian calendars), couldn't it happen that 11/28 is Thanksgiving in some year when it's also the first NIGHT of Chanukkah?

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  13. Check out

    Thanksgivukah: Giving Thanks for Miracles


    http://www.jewcology.com/content/view/Thanksgivukah-Giving-Thanks-for-Miracles

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  14. Your calculations are wrong. The next time Hanukkah and Thanksgiving coincide will be in 2070. It will be the first night of Hanukkah.

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  15. The year 79811 is actually (somewhat) correct. But in fact, 79811 is not the very first time that it will happen. The first time will be in 79043.

    To learn more you can read

    http://www.math.rutgers.edu/~zeilberg/mamarim/mamarimhtml/hanukkah.html

    A precise estimate by Doron Zeilberger.

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  16. The Jewish calendar only runs until the Hebrew year 6000, and the current date is 5774, meaning it will have to be redone in only 227 years. I suspect the hardest part of this will be the question of who makes this adjustment and how they will get the whole Jewish community on board with it.

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  17. The date of Thanksgiving is on a 28 year cycle not 7 years. Each date from November 22 to November 28 will occur on Thursday 4 times in that 28 years, but the sequence of years between years that the same date falls on Thursday is not a consistent 7, but rather 11, 6, 5, 6. (It's ciircular so you can start at different points in the cycle.)

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