Commentary: On the Babylonian Talmud, Tractate Rosh Hashanah

When R. Zera ascended [from Babylonia to the land of Israel], he sent [a letter] to them: It is necessary for the night and the day to be of the new month. And this is what Abba, father of R. Simlai, said: They calculate its birth—if [the new moon] was “born” before midday, it is known that it will be visible near sunset; if it was not born before midday, it is known that it will not be visible near sunset. What difference does this statement make? R. Ashi said: To contradict witnesses. R. Zera said in the name of R. Naḥman: The moon is covered for twenty-four hours. For us, six of the old [month] and eighteen of the new; for them, six of the new and eighteen of the old. What difference does this statement make? Rav Ashi said: to contradict witnesses. [b. Rosh Hashanah 20b]

Rashi

When R. Zera ascended, he sent to them: back to his colleagues in Babylonia, [he sent] an account of two matters he had learned here.

It is necessary for the night and the day to be of the new month: the day must follow the night, so that the day is part of the month of the previous night. We learn from this that if the old moon was seen after dark on the twenty-ninth day, which is the night of the thirtieth, the new month is not sanctified on the thirtieth day. Now, you might ask, is it not obvious that it is not sanctified, since the new moon will not be seen on the thirtieth, for we maintain that the moon is covered for twenty-four hours, as stated below? Nevertheless, this statement is necessary according to the opinion that the court may intimidate witnesses into testifying about a new moon that was not seen at its proper time, in order to sanctify it.¹ This statement teaches us that if the old moon was seen at night, the court does not perform such an intimidation, as everyone has seen that the night is part of the previous month.

And this is what Abba, father of R. Simlai, said before R. Samuel: and he did not know how to explain this difference between a moon born before midday and one born after midday;² I will explain it to you.

They calculate: the birth of the moon [i.e., the conjunction of the moon and the sun].

Born before midday: then it is known that it will be visible on this day before sunset, as the moon is hidden from the inhabitants of the land of Israel, who are in the west, for only six hours after its renewal, due to its small size [i.e., thinness]. For the moon, six hours after its renewal, is always in the southwestern corner and is visible to them.

If it was not born before midday: it is known that it will not be visible on this day, as it is small for the whole next six hours and is invisible to the human eye.

What difference does this statement make: for our calculations, since we sanctify the new month based on witness testimony.

To contradict witnesses: if the birth of the moon was after midday, and they say, “We saw the new moon before sunset,” in order that it should be sanctified on that day, then they are false witnesses.

The moon is covered for twenty-four hours: close to its renewal, before and afterward, it is hidden from the eye, due to its small size.

Thus, for six hours before its renewal and six hours after its renewal it is hidden from all people. For twelve hours before and after this period, when it is in the east it is visible to the inhabitants of the east but not the inhabitants of the west, because of its small size, and when it is in the west it is visible to the inhabitants of the west but not the inhabitants of the east. Before its renewal it is always in the southeastern corner, and after its renewal in the southwestern corner.

This is what I saw in the work of R. Se’adya. Consequently, from the time when the old moon is covered in the east, the new moon is not visible to the inhabitants of the east until twenty-four hours later; and when it is hidden from the inhabitants of the west, the new moon is not visible to them until twenty-four hours later.

For us Babylonians, who are in the east, six of the old: it is hidden, but no more.

What is the reason? Because the old moon, before its renewal, is in the southeastern corner, and it can be seen by the inhabitants of Babylonia who are situated near the east. After that, it is invisible to them due to its smallness. After six hours of the new moon, it is positioned in the southwestern corner, and it can therefore be seen by the inhabitants of the land of Israel who are near the west, but not by the inhabitants of Babylonia. This state of affairs lasts for a full twelve hours after the first six hours. Thus, the new moon is hidden from the inhabitants of Babylonia for eighteen hours, and after eighteen hours it is already large and is visible everywhere.

For them, six of the new: as I explained, at the end of six hours it is visible to the inhabitants of the west.

And eighteen of the old: as I explained, the moon is always in the east at the end of its cycle, and throughout those eighteen hours of the old moon it is not visible in the west, on account of its smallness. Beforehand, however, it was large and could be seen from a distance.

To contradict witnesses: if they say, “We saw the old moon and the new moon within twenty-four hours of each other,” they have testified falsely.