1. Question / Issue :

If a New Moon cannot be seen, then it seems that the first visible confirmation of a New Moon is actually the Waxing Crescent Moon, which can only be observed after the New Moon - minimally - after 12.1 hours with a telescope, and 15.5 hours without. (See Astronomy.SE/How Soon Could a Waxing Crescent Moon Be Seen?).

According to Judaism 101, the Jewish Calendar:

Used to be calculated by observation

Calculated mathematically since 4th century

In view of this, is there any Rabbinical commentary that helps explain how the 1st of the Month / New Moon was determined during the Temple Period?

  1. Was this pre-calculated - to "safeguard" Rosh Chodesh?
  2. Or, was it determined "After-the-fact" and the calendar day was set to the first, retroactively?

I am hoping for references that can be cited, thank you.

  • 1
    ר'ה דף כ .yes of course they knew what to expect, and if nobody saw on day 30, than day 31 was automatically מקודש
    – Stam
    Apr 5, 2017 at 2:00
  • @stam - A.) Yes, this is exactly what I am asking. I am just hoping for citable references in support of it. B.) However, I am hoping for very early source references, preferably Tannaitic, or at later Mishnaic. Apr 6, 2017 at 2:52
  • Why the downvote? This is a great question.
    – Shimon bM
    Apr 7, 2017 at 1:26
  • Under the right conditions (just after sunset, just before sunrise) you can see a thin crescent the day before and day after a new moon. I have a picture but can't put in a comment.
    – Perry Webb
    Sep 2, 2021 at 9:06
  • 1
    Picture of crescent day after new moon. scontent-dfw5-1.xx.fbcdn.net/v/t1.18169-9/…
    – Perry Webb
    Sep 22, 2021 at 0:50

3 Answers 3



The date of Rosh Chodesh was determined by testimony. However, it was also calculated. Since the calculations weren't binding in and of themselves, the testimony was made to align with the calculations by various methods.


The new month was pre-calculated in that it couldn't be more than 30 days, and on the 31st day the previous month automatically became a 30-day month and the next month began. However, they wouldn't start a new month on the 30th day (thus making the previous month 29 days) without witnesses; if only the court saw it, they would bring people to testify for them.

According to the Mishna (Rosh Hashana 3:1, translating the most relevant parts):

ראוהו בית דין וכל ישראל--נחקרו העדים, לא הספיקו לומר מקודש, עד שחשיכה--הרי זה מעובר. ראוהו בית דין בלבד--יעמדו שניים ויעידו בפניהם, ויאמרו מקודש מקודש. ראוהו שלושה, והם בית דין--יעמדו שניים, ויושיבו מחבריהם אצל היחיד ויעידו בפניהם, ויאמרו מקודש מקודש: שאין היחיד נאמן על ידי עצמו

If the court and all of Israel saw it (the new moon), the witnesses were checked, but they didn't manage to say "sanctified" (making the previous month 29 days), it is a leap month.

If only the court saw it, two should stand and testify before them, and they they say "sanctified, sanctified."

Whether they actually verbally sanctified the new month if it was 29 days long or 30 days long is a matter of dispute (Mishna Rosh Hashana 2:8 and BT 24a). The anonymous Mishna says they would sanctify it whether or not they saw the moon on the 30th or 31st day; Rabbi Eliezer son of Rabbi Tsadok says that it was automatically sanctified if it appeared on the 31st day, and there was no need to sanctify it; Plimo says they would only sanctify it if it appeared on the 31st day; and Rabbi Eliezer the son of Rabbi Shim'on says they wouldn't sanctify it at all.

Rabban Gamliel had a tradition regarding the length of the month (BT 25a) and seems to have accepted witnesses whose testimony conformed to his calculations, even when their testimony was somewhat unreliable (Mishna 2:8). Similar to the case mentioned above where the court brings two people to testify for them. According to Rabbi Yehoshua son of Levi (20a), they would intimidate (מאיימין) the witnesses into extending the month (saying they didn't see the moon), but not into shortening it (saying they did see the month); Rabbi Yochanan used to teach also that they would intimidate them into shortening it.

Second Temple Period

As for the question of how much of this process was observed during the Second Temple period, I haven't really found a detailed source. The Mishna (2:5) mentions with regard to the place (בית יעזק) to which the witnesses came to testify:

בראשונה, לא היו זזים משם כל היום; התקין רבן גמליאל הזקן, שיהו מהלכין אלפיים אמה לכל רוח

Since this source quotes the elder Rabban Gamliel who lived in that period, I think it's clear that they accepted witnesses, but the source doesn't speak about calculations. However, the statement of Rabban Gamliel (the younger) in BT 25a that he had a family tradition regarding calculations (כך מקובלני מבית אבי אבא) may date to his ancestors from the Temple period.

The Tosefta (2:7) says:

אמר ר' יהושע בן קרחה דברים אלו התקינן רבן יוחנן בן זכאי משחרב בית המקדש לכשיבנה הבית במהרה יחזרו דברים אלו ליושנם

Rabbi Yehoshua the son of Korcha says: Rabban Yochanan ben Zakkai enacted these things after the Temple was destroyed. When it will be built soon, things will return to the way they were.

Given that this is said in the context of Rosh Chodesh (cf. Mishna 4:4), it might be that at least some of the practices relating to the New Moon started after the destruction of the Temple. It's not clear to me from this source exactly which.


At the time where observing the moon was necessary to know the months of the year, poor weather conditions would simply delay calendrical events. However, eventually systems in which sightings could be predicted came into use.

This is described in detail throughout the book Calendar and Community by Stern. See especially chapters 3.1.2, 3.4.3 and 5.1.3.


As the author of the question - I feel that this answer does not adequately explain whether a visible Waxing Crescent Moon was accepted "As and For" a "New Moon".

chabad.org, 1 Shemuel 20:18 - And Jonathan said to him, "Tomorrow is the new moon, and you will be remembered, for your seat will be vacant.

Scriptural texts indicate that from at least as early as King Saul's reign, Israel had been "pre-calculating" the New Moon.

So, even if perhaps the New Moon was confirmed through eye-witness testimony, it remains true that Israel would plan in view of a pre-calculated expectation.

  • 1
    Not necessarily: tomorrow can be the new moon if today is the 29th day of the month and the moon has not yet been sighted.
    – Shimon bM
    Apr 7, 2017 at 1:27
  • No no, I understand: I think it's a very good question. My only issue with this answer is that knowing that tomorrow is the new moon need have nothing whatsoever to do with the new moon's being sighted. Months can only be 29 or 30 days. If the moon does not get sighted by the 29th day, it is a 30 day month. If Yonatan were speaking on the 29th day, he would know that the following day is the new moon regardless of whether or not the moon was going to be sighted by witnesses.
    – Shimon bM
    Apr 7, 2017 at 2:32
  • In the period of the monarchy, they may have reckoned the months in the Egyptian fashion, from the first disappearance of the waning crescent. The new month began the next day. That is how Saul could know that the new moon was "tomorrow". May 5, 2021 at 1:37
  • @Mockingbird - Do you have a reference for that? Could you post it as an answer? May 27, 2021 at 8:06
  • It wouldn't really be an answer to the question, since your question asks for Rabbinical commentary. May 29, 2021 at 15:50

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