who watches the watchers?

תלמוד בבלי מסכת ראש השנה דף כ עמוד א

I
[what precedes this is a discussion of what reasons are good ones to manipulate the length of months (eg, to keep yom and shabbat from falling out consecutively to allow burial before decomposition begins). the whole discusion presupposes that the rabbis can manipulate the lengths of te months, at least to some degree, hence the following discussion:]

II
איני? והתני רבה בר שמואל: יכול כשם שמעברין את השנה לצורך - כך מעברין את החדש לצורך?תלמוד לומר +שמות יב+ החדש הזה לכם ראש חדשים - כזה ראה וקדש.

Is it really so (that the rabbis can lengthen the months as they need)? but Rabbah Bar Shmuel taught a baraita: One might think that just as they lengthen the year (by inserting a 13th month to create a leap year) out of necessity, so they can lengthen the year (by declaring Rosh Hodesh on the 30th instead of th 29th day from the last Rosh Hodesh). thus it teaches us: "this month is for you" (read: this new moon is the one you should use. see rashi Ex 12:2) - see this and sanctify the month.*

III
אמר רבא: לא קשיא; כאן - לעברו, כאן - לקדשו. והכי קאמר: יכול כשם שמעברין את השנה ואת החדש לצורך - כך מקדשין את החדש לצורך? תלמוד לומר החדש הזה לכם, כזה ראה וקדש. וכי הא דאמר רבי יהושע בן לוי: מאיימין על העדים על החדש שנראה בזמנו לעברו, ואין מאיימין על העדים על החדש שלא נראה בזמנו לקדשו.

Rava said: it is no difficulty. Here - to extend it. Here- to sanctify it (earlier than the 30th day), and thus goes the statement: One might think that, just as they extend the year and the month ot of necessity, so too they may sanctify the month out of necessity. thus it teaches us "this is the new moon for you" - see this and sanctify. (ie, "see this" is necessary before declaring Rosh Hodesh early, but merely "seeing this" does not require the rabbis to declare rosh hodesh).
And (if so, this opinion is) similar to that which Rabbi Yehoshua ben Levi said: They may threaten the witnesses regarding the new moon that was seen in its time (preventing their testimony) in order to lengthen the month, but they may not threaten the witnesses regarding the new moon that was not seen in its time (to testify that they saw it anyway, in order) to sanctify it (early).

IV
איני? והא שלח ליה רבי יהודה נשיאה לרבי אמי: הוו יודעין שכל ימיו של רבי יוחנן היה מלמדנו: מאיימין על העדים על החדש שלא נראה בזמנו לקדשו, אף על פי שלא ראוהו - יאמרו ראינו! -
Is this really so? But behold, Rabi Yehudah Nesiah sent to Rabbi Ami: Be aware that all of Rabbi Yohanan's life he would teach us: they may threaten te witnesses regarding the new moon that was not seen at its time to sanctify it. Even though they did no see it, they should say "we saw it."!

V
אמר אביי: לא קשיא; הא - בניסן ותשרי, הא - בשאר ירחי.

Abaye said: it is no difficulty. Here (they may threaten the witnesses even to shorten the months) regarding Nissan and Tishrei, Here regarding the rest of the months.
(at this stage, the fate of the positionin part III is unclear: it was initially introduced to explain how the rabbis may lengthen the months to prevent issues from coming u re: timing of holidays. Now the the gemara, per Abaye, says that in holiday-containing months the rabbis may even shorten the month, and in the rest of the months they may even lenthen it. However, I do not understand the reason they would want to lengthen a non-holiday month... help?)

VI
...
. רב דימי מנהרדעא מתני איפכא: מאיימין על העדים על החדש שלא נראה בזמנו לקדשו, ואין מאיימין על העדים על החדש שנראה בזמנו לעברו. מאי טעמא? האי - מיחזי כשקרא, האי - לא מיחזי כשקרא.

Rava Dimi of Nehardaa teaches the opposite (of the position of RYBL in part III):
They may threaten the witnesses regarding the new moon that is not seen at its time to sanctify it, and they may not threaten the witnesses regarding the new moon that was seen at its time to lengthen it.
What is the reason? This (lenthening the month) appears to be a lie, this (starting the month early) does not appear to be a lie.
(if we were to relate the drashah of "see this..." to this opinion, we would say that it indicates that if one sees the moon, one must sanctify, but not that one may not sanctify the month eaqrly without a moon)

*Refresher (correct me if i'm wrong...): Rosh Hodesh is when 2 witnesss come and say they saw the new moon, except that even without witnesses a month can't be more than 30 days long. So, the point of saying "see this and snactify" is that you must sanctify the new month upon testimony of a new moon - there is no rabbinic discretion to extend the month.

Discussion:

I think this sugya-segment is intersting because it brings up two different reasons the Rabbis would check their own authority. (here, leading to opposite results!)
the first is implicit, I htink, in the position of part III: the rabbis may ignore testimony in order to lengthen the month, but they cannot cause testimony to be fabrcated. This position seems to be concerned with the internal legitimacy of the bet din: what has it been given the authority (from God) to do? It may be that the rabbis can ignore the new moon that God has put up there in the sky (akin to "shev ve-al taaseh?), but they cannot fabricate a new moon where none exists.
the background here is the many midrashim that tie the power of the rabbis over the calendar (eg, "asher tikrau atem be-moadam...) to the same verse (hachodesh hazeh LAKHEM), thus citing this verse to limit rabbinic power seems to me to indicate a discomfort with sweeping rabbinic discretion and an attempt to make sure the rabbis can't make arbitrary/counterfactual decisions with impunity. At this stage, the gemara is concerned with the fundamental legitimacy of rabbinic authority: how much authority did God actually give the rabbis?

The second check is external: legitimacy in the eyes of the people. thus, in part VI, the rabbis apparetnyl have the power to coerce false testimony: to create a moon where none exists, but they must be afraid of counterfactual prounouncements that can easily be discovered. Thus, if everyone has seen the new moon, the rabbis cannot legitimately turn away witnesses and say there was no moon, since this will appear as a lie (essentially, to coopt a metaphor from elsewhere, putting the baby back into the womb - it's there one day, allegedly absent the next...) However, they can "create" the moon via coerced testimony since this does not blatanlty contradict the order of nature (perhaps only these two people were in the right place at the right time, given visibilty conditins, etc... and in any case everyone will see the moon the next day, presumably, giving some sort of "retroactive" feeling of truth to the poronouncement that it was present even the night before).

This second check is in some ways more cynical, though I can think of at least one other source that is explicitly concerned with the same issues. (precise location evades me. the story about the two rabbis discussins non-Jewish cheese, etc) that said, I find it interesting (though not sure how to articulate why) that the two approaches appear in one sugya, and produce opposite results.

PS - if you made it this far, thanks. I am told I have a problem with length.