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I recently read on Matzav.com an article by Rabbi Dovid Heber discussing the 247 year cycle in the calendar. He mentioned that every 247 years the Hebrew calendar is exactly identical. Now in the article he mentions that every 247 years the Molad is almost identical. It is only a mere 905 chalakim (50 minutes and 16.67 seconds) earlier.

My question is at what point along the 247 year cycles, due to the constant slow change and creeping up in the Molad will the calendar be forced to change?

Gershon Gold
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  • http://judaism.stackexchange.com/questions/7955/shiva-vearbaim-umatayim-mi-yodeya/7963#7963 – Gershon Gold Sep 21 '15 at 17:39
  • Related? http://judaism.stackexchange.com/a/60034/2 – Isaac Moses Sep 21 '15 at 17:40
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    I'm not sure I understand the question... Are you asking after how many shifts of 50 minutes = 0.83 hours, will we drift a whole day? The answer is 24/0.83 hours = 29 cycles. – Double AA Sep 21 '15 at 17:51
  • @DoubleAA: At what point will the 247 year cycles not be the same anymore. – Gershon Gold Sep 21 '15 at 17:58
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    @GershonGold I still don't understand what's unclear to yuou. Every cycle it drifts 50 minutes. Is that the answer you want? What about what do you want to know will be different when? – Double AA Sep 21 '15 at 18:01
  • @DoubleAA: No. I wrote that in the question. At some point due to the 50 minutes per 247 year cycle, Rosh Chodesh will have to be declared earlier. When does this happen? – Gershon Gold Sep 21 '15 at 18:03
  • After 29 cycles it will shift back a day, as I showed above. Different months will cross the day-line at different points. You want to know when the next time a month will be different than it was 247 years prior? The last time that happened? The first time that happened? – Double AA Sep 21 '15 at 18:05
  • @DoubleAA: Going forward in the calendar when will be the next time it happens? – Gershon Gold Sep 21 '15 at 18:07
  • Let me know if http://individual.utoronto.ca/kalendis/hebrew/molad.htm answers your question. If so, I will extract what I can & post as an answer (or you are welcome to post your own answer.) The article gets a bit too complex for me after a while. You may have better luck than I. – DanF Sep 21 '15 at 18:50
  • @DanF: Wow is that confusing! – Gershon Gold Sep 21 '15 at 18:57
  • @GershonGold OK. I thought that I would give it a try. Perhaps by scanning through it, you might have seen something that addresses your question, that I may have missed. Sometimes two brains are better than one. But, I guess two confused brains are worse :-) – DanF Sep 21 '15 at 20:38
  • @GershonGold I think I found a "simpler" article that addresses the "seasonal shift" of the calendar. I don't think it will directly answer your question about the 247 year cycle, but, it may provide a different perspective on when the calendar, in general may need adjusting. Unless you think it may be useful, when I can locate it, I will place as a comment rather than an answer. – DanF Sep 21 '15 at 20:41
  • I think he means that, due to the way the calendar is set up, the rules will be forced to change because the 905 chalakim will add up. The answer is around the year 6000, when the Rambam says Mashiach will come, and we have no need for a calendar. (Something I tried explaining in this question, though my comment seems to have been deleted. It was years ago, anyway.) – DonielF Jul 26 '16 at 23:49

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According to the Pri Chodosh, in his notes on the calendar (in the back of the Shulchan Aruch on Orach Chaim siman 428), the next time there's a difference from 247 years earlier will be in תתמז-תתמח (in machzor שח), which is 2087-2088 on the secular calendar. The last time was in תשז-תשח, which is 1947-1948.

Shamiach
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