Saturday, December 07, 2013

What will happen to the calendar if the Sanhedrin is re-instituted

As of 1910, Pesach was on April 24. Thirty days earlier, March 25, after the equinox, was Purim (14 Adar II). If the Sanhedrin had been re-instituted, that may have been the year that the leap year would have been postponed to the following year. This last occurred in 2005 and will occur again in 2024 (5784). Pesach that year will, according to the fixed calendar that we now use will be on 23 April, making Purim (13 Adar II) on 24 March. Since this is after the equinox, the Sanhedrin could declare that it will not be a leap year. This will pull Pesach back by thirty days (the size of the leap month) to 24 March and move everything else until the following Rosh Hashana.

The "earliest" late Pesach (as seen in the chart) is year 14 of the cycle. In 5790 (2030) Pesach will be 18 April, which is still within the allowable gap before the equinox. That Pesach will not be after April 21 until 15th of Nisan, 6664 = Tue, 22 April 2904. Thus we can theoretically use the current fixed calendar until then.

As of now, Rosh Hshana 5785 will be on 3 October 2020. Thirty days earlier would be on Tuesday, 3 September 2024. Both Cheshvan and Kislev will be 29 day months in 5784. Had we needed to (such as Rosh Hashanah occurring on Wednesday) we could have added the extra day to move it. I will assume for this post that we do not need to move it from Tuesday to Thursday (though it could have been done). Moving the date by thirty days, will change the day of the week by two days.

The following Chanukah, which is scheduled for 26 December, would then occur on Thursday December 26, would then be Tuesday November 26, while Thanksgiving would be on 21 November. With the calendar shift, the first day of Chanukah would be on November 25.

The next Pesach occurs on April 13, 2025 according to the current fixed calendar. The shift would move it to March 13, which is before the equinox. As a result, the Sanhedrin would probably declare 5785 as a leap year moving Pesach back to April 15. The following year would then be back according to the fixed schedule, except that the cycle would have moved. Similar results can be expected with the new calendar, except that the Sanhedrin would keep track.

Now lets examine the entire cycle for that year  (cycle 304). The first year of that cycle is 5776 which is 2015/2016.

Note that moving Rosh Hashannah by thirty days will cause the day of the week to change by two days. If Rosh Hashanah is on Tuesday, moving it back would cause it to occur on Sunday, which is not allowed. This would mean that it needs to be moved ahead to Monday or back to Shabbos by modifying the "extra" day that could be added in Chesvan or Kislev. Similarly, moving it ahead by thirty days would cause the problem to occur Rosh Hashanah is on Monday and would move ahead to Wednesday. For the following table, I will assume that the equinox on 21 March is the critical date. Thus, if Pesach occurs after 21 April, there would need to be a modification. Those years would shown as yes in the modification column. The following year would be left blank as it would probably need to become a leap year to account for the fact that Pesach would be "too early".


YearSecularCycleleap yearPesachModifyNext Rosh Hashanah
577620160yesSat, 23 April 2016yesMon, 3 October 2016
577720171noTue, 11 April 2017
Thu, 21 September 2017
577820182noSat, 31 March 2018noMon, 10 September 2018
577920193yesSat, 20 April 2019noMon, 30 September 2019
578020204noThu, 9 April 2020noSat, 19 September 2020
578120215noSun, 28 March 2021no7 September 2021
578220226yesSat, 16 April 2022no26 September 2022
578320237noThu, 6 April 2023noSat, 16 September 2023
578420248yesTue, 23 April 2024yesThu, 3 October 2024
578520259noSun, 13 April 2025
Tue, 23 September 2025
5786202610noThu, 2 April 2026noSat, 12 September 2026
5787202711yesThu, 22 April 2027yesSat, 2 October 2027
5788202812noTue, 11 April 2028
Thu, 21 September 2028
5789202913noSat, 31 March 2029noMon, 10 September 2029
5790203014yesThu, 18 April 2030noSat, 28 September 2030
5791203115noTue, 8 April 2031noThu, 18 September 2031
5792203216noSat, 27 March 2032noMon, 6 September 2032
5793203317yesThu, 14 April 2033noSat, 24 September 2033
5794203418noTue, 4 April 2034noThu, 14 September 2034
579520350yesTue, 24 April 2035noThu, 4 October 2035


The following tableshows what would happen if the Sanhedrin "skipped" a leap year that has Purim fall after the vernal equinox so that Pesach is after April 21. The following year would then be made a leap year so that Pesach would not fall "too early". In order to estimate the date of Pesach, it will be assumed that adding the thirty days just compensates for the thirty day subtraction, so that Pesach will occur on the same day as that shown on the fixed calenday, allowing for any Cheshvan Kislev modification. This will also account for Rosh Hashanah. Instead of calculating the following Rosh Hashannah according to the full set of rules, I will subtract the 30 days and adjust if necessary by modifying the preceding Cheshvan Kislev calculation so that it does not occur on Sunday, Wednesday, or Friday. I will otherwise assume that Cheshvan and Kislev appear as they would in the current fixed calendar. The modification value will be -1 either Chesvan or Kislev must be changed from 30 to 29 days, +1 if either is changed from 29 to 30 days, and 0 if it is left as set in the current fixed calendar. It will be left blank if Rosh Hashanah did not change from that in the current fixed calendar.

YearSecularCycleleap yearPesachModifyNext Rosh Hashanah
577620160noWed, 23 March 20160Sat, 3 September 2016
577720171yesTue, 11 April 2017
Thu, 21 September 2017
577820182noSat, 31 March 2018
Mon, 10 September 2018
577920193yesSat, 20 April 2019
Mon, 30 September 2019
578020204noThu, 9 April 2020
Sat, 19 September 2020
578120215noSun, 28 March 2021
7 September 2021
578220226yesSat, 16 April 2022
26 September 2022
578320237noThu, 6 April 2023
Sat, 16 September 2023
578420248noSat, 23 March 20240Tue, 3 September 2024
578520259yesSun, 13 April 2025
Tue, 23 September 2025
5786202610noThu, 2 April 2026
Sat, 12 September 2026
5787202711noMon, 22 March 20270Thu, 2 September 2027
5788202812yesTue, 11 April 2028
Thu, 21 September 2028
5789202913noSat, 31 March 2029
Mon, 10 September 2029
5790203014yesThu, 18 April 2030
Sat, 28 September 2030
5791203115noTue, 8 April 2031
Thu, 18 September 2031
5792203216noSat, 27 March 2032
Mon, 6 September 2032
5793203317yesThu, 14 April 2033
Sat, 24 September 2033
5794203418noTue, 4 April 2034
Thu, 14 September 2034

This becomes the new cycle. As can be seen several of the leap years would not require modification. It is possible that the Sanhedrin would not want to change the cycle until the date of Pesach moves farther from the equinox so that all the leap years of the cycle occur after the equinox. As I said earlier, we could use the current calendar until 6664 (2904). The "latest" leap year Pesach on 23 April is only 5 days away from the "earliest" leap year Pesach so that the determination based on "season" and the physical aspects of the year would be more significant than the astronomical calculation.


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