Gemara Clarity

Rosh Hashanah 20b

ראש השנה כ׳ ע״ב

Learn Rosh Hashanah 20b step by step: Aramaic beside English, every question, answer and proof colour-coded.

Open Rosh Hashanah 20b in Gemara Clarity

Topic

Calendar calculation and witnesses: the מולד, the hidden moon, the night-and-day rule for ראש חודש, and where a day of doubt goes

Overview

The amud opens by finishing an earlier answer: pressuring witnesses so that ראש חודש is delayed would look like a lie, but setting it a day early would not. Next, שמואל claims he can fix the calendar for the whole גולה by calculation. אבא אבוה דרבי שמלאי tests him with a line from סוד העיבור about a מולד before or after חצות. שמואל admits he does not know it, so his claim is pushed back. רבי זירא then sends a message from ארץ ישראל explaining that line. Both the night and the day of ראש חודש must belong to the new month. A מולד before midday means the moon will be seen near sunset, and a מולד after midday means it will not. רב אשי explains that this rule, like רב נחמן's rule that the moon is hidden for twenty-four hours, is used to catch false witnesses. The גמרא then gives two sources for the night-and-day rule, from רבי יוחנן and from ריש לקיש. אביי and רבא disagree about whether those two sources lead to different results. Finally, רב נחמן teaches that a day kept out of calendar doubt is pushed forward, and the גמרא challenges why the 14th is not kept too.

Opening lines

הַאי מִיחֲזֵי כְּשִׁקְרָא,

This looks like a lie,

הַאי לָא מִיחֲזֵי כְּשִׁקְרָא.

that does not look like a lie.

אָמַר שְׁמוּאֵל:

Shmuel said:

יָכֵילְנָא לְתַקּוֹנֵי לְכוּלַּהּ גּוֹלָה.

I am able to fix (the calendar) for the entire diaspora.

The steps (17)

  1. ResolutionOne move looks like a lie, the other does not
  2. Statementשמואל claims he can fix the calendar for the whole diaspora
  3. Questionאבא tests שמואל with a puzzling line from a ברייתא
  4. Rejectionשמואל admits he does not know, and his claim is pushed back
  5. Statementרבי זירא: both the night and the day must belong to the new month
  6. ResponseExplaining the line about a מולד before or after midday
  7. ResponseThe practical use: catching false witnesses
  8. StatementThe moon is hidden for twenty-four hours
  9. ResponseAgain, used to catch false witnesses
  10. QuestionWhere do we learn that night and day must both be in the new month?
  11. Responseרבי יוחנן's source: from evening to evening
  12. Responseריש לקיש's source: until the twenty-first day at evening
  13. QuestionWhat difference is there between the two sources?
  14. Responseאביי: they only differ in how they read the פסוק
  15. Responseרבא: they differ about the time before midnight
  16. Statementרב נחמן: a day of doubt is pushed forward
  17. ChallengeWhy not also keep the fourteenth?