פירוש על ראש השנה 2:8

צורות לבנות – the measurement of the moon and to which side its horns incline.

Rabban Gamaliel had diagrams of the moon on a tablet [hung] on the wall of his upper chamber, and he used to show them to the unlearned and say, “Did it look like this or this?”
It happened that two witnesses came and said, “We saw it in the morning in the east and in the evening in the west.” Rabbi Yohanan ben Nuri said: they are lying witnesses. When they came to Yavneh Rabban Gamaliel accepted them.
On another occasion two witnesses came and said, “We saw it at its proper time, but on the night which should have been the new moon it was not seen,” and Rabban Gamaliel accepted their evidence. Rabbi Dosa ben Harkinas said: they are lying witnesses. How can they testify that a woman has given birth when on the next day her belly is between her teeth (? Rabbi Joshua to him: I see your argument.
This mishnah and the next one contain one of the most famous stories in rabbinic literature. In it Rabban Gamaliel and Rabbi Joshua clash over accepting what seems to surely be false testimony as to the new moon.
Section one: This section sets up the character of Rabban Gamaliel, who will play the lead role in the story. Rabban Gamaliel was the head of the court in Yavneh and he was the rabbi who would ultimately decide whether the witnesses’ testimony would be accepted and the new month sanctified. In order to facilitate this, he would have pictures of the moon in its different phases hung up in his upper chamber, where he would interrogate them.
Section two: The story now begins. Two witnesses come in front of some rabbi (it’s not clear whom they come in front of) and tell him that they saw the new moon in the morning in the east and in the evening in the west. This is impossible and hence Rabbi Dosa ben Harkinas calls them lying witnesses. The witnesses proceed on to Yavneh and come in front of Rabban Gamaliel, who sanctifies the month based on their testimony. We as readers of the mishnah are properly shocked wasn’t Rabban Gamaliel the very rabbi who was especially fervent in checking the witnesses?
The same thing seems to happen a second time. Again witnesses offer impossible testimony, which Rabbi Dosa ben Harkinas promptly rejects, this time using a graphic analogy. In Hebrew there is a pun which is lost in translation. The word for pregnant and the word for a thirty day month are the same.
In the final line of the mishnah, a new character is added, Rabbi Joshua. Rabbi Joshua agrees with Rabbi Dosa ben Harkinas, setting the stage for his eventual clash with Rabban Gamaliel in tomorrow’s mishnah.

קיבלן ר"ג – not because he thought that it was possible that the moon could be seen in the morning in the east at the start of sunrise, and on that day itself, it would be seen in the west with the setting of the sun, for that is impossible, but rather, he accepted their testimony in what they said that they saw the moon in the west in the evening, because he knew the manner of the calculations that it was possible that it would appear that same night. And concerning that which they said that it (i.e., the moon) appeared in the east in the morning, he said, that they are in error and that clouds in the form of the moon appeared to them in the firmament, and that is what is taught in the Baraitha (Tractate Rosh Hashanah 25a): One time, the sky became covered with clouds and the image of the money appeared on the twenty-[ninth] of the month, etc. Rabbi Yohanan ben Nuri said that they were false witnesses, for it appeared to him, according to his calculations, that it was not possible that moon should appear on that night that the witnesses said that they saw it, and he said that they are false witnesses because there was no time between the appearance and the beginning of the first quarter of the moon (i.e., New Moon) according to his opinion, so much time that it would be possible that it should appear on it. And on this, Rabban Gamaliel responded in the Baraitha: So I have received from the house of my father’s father, that sometimes it comes at lengthy [period of time] and sometimes it comes in a short [time], that is to say, that the time that there is between the beginning of the first quarter of the moon until the moon appears is not always equal. But there are times when the movement of the moon is quick and times when it is late. And according to the speed of the movement of the moon at that time, Rabban Gamaliel judged that it was appropriate that the moon should appear on that night, and therefore, he accepted their testimony.

ראינוהו בזמנו – on the night of the thirtieth

ובליל עבורו – on the light of the thirty-first.

וקבלן ר"ג – not because it is stated that the moon removed itself from the sun on the night of the thirtieth [of the month] until it would appear, and on the night of the thirty-first, it returned to being behind it and drew closer to the sun until it was covered, for the moon does not return to being behind it, but makes its path as it goes around in its circuit, but that Rabban Gamaliel knew in the calculations for that particular night so that when the witnesses said that they saw it, the moon had already distanced itself from the sun until it was it was possible that it would be seen. Therefore, he accepted their testimony, and regarding what they said on the night of its intercalation (i.e., proclamation of a full month) do not appear [to be valid]. It is possible that it was because the cloud covered it or another reason that caused that they (i.e., the witnesses) did not see it.

רואה אני את דבריך – to intercalate the month