(Alternate version) (R. Nachman): We have a supporting Mishnah that the Adar closest to Nisan is always abridged:

(Mishnah): The witnesses of the moon for the months of Nisan and Tishrei can transgress Shabbos.

That makes sense if Adar closest to Nisan was always abridged, as it is a mitzvah to sanctify the month on the basis of eyewitness testimony.

But if the months were sometimes full and sometimes abridged, why should they be allowed to transgress Shabbos - let Beis Din make it full, and sanctify it on the following day!

The Gemara explains why this is not a good proof:

That would only hold true if the 30th fell on Shabbos.

But the Mishnah is referring to where the 31st fell on Shabbos.

In this case, Beis Din have to sanctify the new month today, so the witnesses may transgress Shabbos (and the Mishnah therefore makes sense even if it is sometimes a full month).

Question (R. Kahana): We see from a Mishnah that it couldn't be that the Adar closest to Nisan is always abridged:

(Mishnah): When the Beis ha'Mikdash was standing, the witnesses could transgress Shabbos for all months.

The reason was so that the Korban of Rosh Chodesh would be brought on the correct day.

We see that we are not concerned about the months being sanctified on the basis of eyewitness testimony.

If the Adar closest to Nisan is sometimes abridged and sometimes full, it makes sense that the witnesses can transgress Shabbos, so that we know when Pesach should be.

But if the Adar closest to Nisan is always abridged, why would the witnesses be allowed to transgress Shabbos?

This refutes the view that the Adar closest to Nisan is always abridged.

Ulla came to Bavel and told them that the Beis Din of Eretz Yisrael had made Elul a full month, which would be good for them.

Question: Why was it good for them?

Answer (Ulla): The resultant separation of Shabbos and Yom Tov by a day would prevent picked vegetables from withering (new ones can be picked for Yom Tov).

Alternate answer (R. Acha bar Chanina): It would prevent bodies awaiting burial from decomposing.

Question: What is the difference?

Answer: Where Yom Kipur followed Shabbos:

There would still be a benefit in the case of bodies awaiting burial.

But there would be no benefit for vegetables; if needed after Yom Kipur, new ones can be picked.

Question: But even according to the opinion that the benefit is for vegetables, there would also be the benefit for bodies, so Elul would still be extended!?

New answer: The difference is in the case of Yom Tov immediately preceding or following Shabbos:

There would still be a benefit in separating Yom Tov for the vegetables.

But there is no benefit for bodies; they can be buried by non-Jews.

Question: Even according to the opinion that the benefit is for bodies, there would still be benefit for the vegetables?

Answer: They can be freshened by being soaked in hot water.

Question: Isn't the benefit as much for Eretz Yisrael as for Bavel?

Answer: Bavel is lower and therefore hotter.

Question: We see that is it not permitted to extend a month for an ulterior motive:

(Rabah bar Shmuel): One might think that just as years may be extended for an ulterior motive, months can also be extended;

The Pasuk therefore teaches us that months must be set according to when the moon appears.

Answer (Rava): One must distinguish between extending the month, and sanctifying it (before the correct time), reading it as follows:

One might think that just as years and months may be extended for an ulterior motive, months can also be sanctified (prematurely);

The Pasuk therefore teaches us that months can only be sanctified according to when the moon appears.

This follows R. Yehoshua ben Levi:

We intimidate witnesses into keeping silent so that the month can be extended.

But we do not intimidate them into giving false testimony so that it can be abridged.

Question: Surely R. Yehudah was taught by R. Yochanan that we intimidate people into testifying that they saw the moon so that the month can be abridged?

Answer (Abaye): That is only true for Nisan and Tishrei (when we want to set the date for the festivals).

Alternate answer (Rava): Rabah bar Shmuel's law (that one may not alter the calendar for ulterior motives) is the view of the Acheirim:

(Acheirim): One festival differs from the previous occurrence of that festival in that the following year it falls on the weekday four days later (i.e. the months always alternate between being abridged and full). If it is a leap year, it falls on the weekday five days later.

(R. Dimi of Naharda'a): The opposite of R. Yehoshua ben Levi's rule is true:

We intimidate people into giving false testimony so that the month can be abridged.

But we do not intimidate them into keeping silent so that it can be extended.

The reason is that the latter is noticeably fraudulent (as the new moon is seen by everyone), whereas the former isn't.

(Shmuel): "I am expert enough to set the calendar for all the Diaspora without witnesses."

(Aba, father of R. Samla'i, to Shmuel): "Do you know the difference between whether the Nolad is before or after midday?"

(Shmuel): "No."

(Aba, father of R. Samla'i): "Then there are bound to be other things that you don't know."

(R. Zeira): The first day of the month must be a night followed by a day.

If the Nolad is before midday, the moon will be visible before sunset; if it is after, it won't.

Question: What is the relevance of this?

Answer (R. Ashi): For catching out witnesses.

(R. Zeira citing R. Nachman): The moon is invisible for 24 hours between months:

In Bavel, 6 hours of the old moon and 18 hours of the new one;

In Eretz Yisrael, 6 hours of the new moon and 18 hours of the old one.

Question: What is the relevance of this?

Answer (R. Ashi): For catching out witnesses.

Question: How do we know that the first day of the month must be a night followed by a day?

Answer (R. Yochanan): The Pasuk says that Yom Kipur lasts "from evening to evening."

Alternate answer (Resh Lakish): The Pasuk states that Chametz may not be eaten until the 21st of Nisan in the evening.

Question: What is the difference between these answers?

Answer (Abaye): They are different Scriptural exegeses.

Alternate answer (Rava): According to R. Yochanan, it begins with the onset of night; according to Resh Lakish, it begins at midnight.

(R. Zeira citing R. Nachman): Doubts about when the month began result in an additional day of Yom Tov being observed a day later, not a day earlier.

We see that Yom Tov is kept on the 15th and 16th, not on the 14th.

Question: It should also be observed on the 14th, in case Av/ Shevat were abridged?

Answer: The months preceding them are always abridged, and if two consecutive months were to be abridged, people would find out about it in time.