In Parshas Bo G-d says to Moses to sanctify a new Moon, but Moses "had trouble understanding" and G-d showed him exactly the picture of a new Moon, saying "כזה ראה וקדש".

Obviously, Moses knew the idea and the calculation of new moon either by the tradition his tribe kept, or simply by being educated in Egypt, he probably knew astronomy very well.

The Mishna (Bavli R"H 23b) says:

"How do they examine the witnesses who come to testify about the new moon? ... Say how you saw the moon: Was it in front of the sun or behind the sun? To its north or to its south? How high was the moon over the horizon, and in which direction did it tilt? And how wide was it?*"

Rashi explains it is about its "measure" to be suitable for Kidush, but I don't recall a discussion about Moon's width, except for Yerushalmi (12) says it should be a barley wide (Bavli omits it) with no discussion in place. We also hold that the minimum time the Moon can be seen from the time of Molad is 6 hours but not its width.

So what exactly was G-d's Chidush to Moses about New Moon?

  • You made it sound simple. I dont know the answer but the rambam follows the Greeks which I believe today has been discounted. So it isnt that simple.
    – patient
    Jan 20, 2018 at 23:35
  • (1) Why is it obvious Moses already knew the idea of new moon? As an aside, according to the Wikipedia article on the Egyptian calendar, the new month, of their original lunar calendar, began "when the waning crescent moon could no longer be seen". (2) According to Rashi on Shemot 12:2, Moses was unsure how much of the moon must be visible before it would signify the beginning of a new Jewish calendar month. How would knowledge of astronomy help him?
    – Tamir Evan
    Jan 21, 2018 at 15:35
  • (3) What does the Mishnah quote (and the emphasis made in it) add to the question?
    – Tamir Evan
    Jan 21, 2018 at 15:36
  • @TamirEvan That (2) sounds mighty similar to an answer there. Jan 21, 2018 at 19:57
  • @Yez After seeing the Mekhilta there, I'm inclined to agree with you, except (a) I would have thought (and actually assumed, when writing that comment) that the OP was aware of the Rashi, but overlooked parts of it, and more generally, (b) I'm not entirely sure what the OP is asking, or what the basis of his question is.
    – Tamir Evan
    Jan 21, 2018 at 20:28

2 Answers 2


The Netziv in his pirush on the Mechilta addresses what was difficult.

He quotes Tosafos in Pesachim 6b that this event happened on Rosh Chodesh and the difficulty was simply seeing the sliver of the moon as per Rosh Hashana 24a.

He also quoted the words of the Rambam in the beginning of Hilchos Kidush Hachodesh who wrote in the name of the Rabbis that Hashem showed Moshe a vision of the moon in a prophetic vision. According to this the Netziv suggests, the vision did not happen on Rosh Chodesh which is why it required a special vision.

(In fact, Rabbi Yaakov Kaminetsky quoted this Rambam and since he was assuming based on the gemara in Pesachim that the vision was on Rosh Chodesh, he questioned what forced Rambam to say Hashem showed Moshe the moon in a prophetic vision. He left it unresolved.)

So the difficulty was either seeing a tiny unnoticeable sliver of a moon, or seeing something which would be physically impossible at that point in time.

  • OK, but what כזה stands for? Wasn't "sanctify a new moon" enough?
    – Al Berko
    Jan 22, 2018 at 0:06
  • Either He was actually pointing out what was difficult to see, or He pointed out in the prophecy what exactly a 'new moon' looks like. Think about it. Moshe in the prophecy could've seen something not visible to a regular person looking at the moon from the ground.
    – user6591
    Jan 22, 2018 at 0:16
  • Moon phases are astronomical facts, hard to miss. How about new Moon width? How important is it to halacha?
    – Al Berko
    Jan 22, 2018 at 0:27
  • Again, he quotes Rosh Hashana where we find it is difficult. If you disagree I suggest asking a different question. I don't understand what you are asking about what is important to Halacha.
    – user6591
    Jan 22, 2018 at 0:31

Rav Hirsch explains Bo 12:2

הַחֹ֧דֶשׁ הַזֶּ֛ה לָכֶ֖ם רֹ֣אשׁ חֳדָשִׁ֑ים רִאשׁ֥וֹן הוּא֙ לָכֶ֔ם לְחָדְשֵׁ֖י הַשָּׁנָֽה

That this was a method of giving the command, not that Moshe Rabbeinu did not understand the cycle of the moon.

Just as Hashem told Noah to turn his looks away from the earth which had just been regiven to Man, and direct them to the heavens, and, showing him the rainbow, said זאת אות הברית, this is tobe the sign, the guarantee, of My covenant with the new future which I promise to humanity, in the same way, in Egypt, at the threshold of the new Jewish future, He called Moses and Aaron into the open, showed them the silver crescent of the new moon, and said: "This renewal is to be to you a beginning of New-moons" (literally a beginning of renewals, of revivals), "and it, the month that is beginning with it, is to be for you the first of the months of the year!". There are two מצות contained in this verse, (a) קידוש החודש על פי ראית הלבנה בחידושה, fixing the beginning of the month with the new moon becoming visible, and (b) to make the order of the monts of the year start with Nissan, the month of the redmption.

Indeed, the knowledge of the full cycle was already known and the calculation was used as the basis of determining when the witnesses were to be expected (as explained in Rosh Hashana, 23, 24, 25). That is why (for example) we see in the haftarah that we read for מחר חודש that Dovid and Yonasan knew that it was going to be Rosh Chodesh and that two days of Rosh Chodesh were kept at that time.

Thus we see that Hashem shows Moshe the first crescent of the new moon and tells him that this well known astronomical event is to be declared by witnesses and consecrated by the bais din. Otherwise, Hashem could have instructed Moshe to start from the actual conjuction of Nisan and alternate 29 and 30 day months or to use the calculated molad (as we do now) rather than waiting until witnesses can see the moon. . As Rav Hirsch says.

This is striking evidence that it is not the actual condition up above, but the consecrating declaration of the representatives of Israel that is the decisive factor on which the beginning of the month depends.


I think the OP is asking about where the Mekhilta says (on 12:2) "החדש הזה לכם וגו'... זה אחד מג' דברים שנתקשה משה והראהו המקום את כלן באצבע", and the OP understands it to mean that Moshe didn't know till then what a 'new moon' was. – @TamirEvan

If this is the case then I would read it as meaning that Moshe Rabbeinu knew exactly what a new moon was (the actual conjunction - when no moon is visible at all). Hashem was taking the opportunity to show him that the month would be determined by Bais Din on the basis of witnesses. I would also read it more like Rav Hirsch that Hashem was showing the deeper meaning of the new moon to Moshe Rabbeinu.

  • I think the OP is asking about where the Mekhilta says (on 12:2) "החדש הזה לכם וגו'... זה אחד מג' דברים שנתקשה משה והראהו המקום את כלן באצבע", and the OP understands it to mean that Moshe didn't know till then what a 'new moon' was.
    – Tamir Evan
    Jan 21, 2018 at 19:16
  • 1
    (Regarding the update:) If Moshe Rabbeinu knew exactly what a new moon was, a question remains: What did he have difficulty with ("שנתקשה משה")?
    – Tamir Evan
    Jan 21, 2018 at 20:20
  • The Gemorah is very extensive on learning every word of this verse, also about witnesses, however the word כזה is learned for graphical representation. Of what?
    – Al Berko
    Jan 21, 2018 at 23:39
  • @alberko of how Moshe Rabbeinu was shown what the witnesses should see. Jan 21, 2018 at 23:42
  • @tamirevan I think it is to teach that I it was not the new moon of astronomy but that witnesses should see it which is a little time later. Jan 21, 2018 at 23:45

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