When is Rosh Chodesh?

When is Rosh Chodesh?[1]

Rosh Chodesh is determined based on when the new orbit of the moon begins and when the new moon can be seen. This itself can be determined in one of two ways: a) Calculating the Molad and calculating as to when the moon can be seen, and b) accepting testimony as to whether the moon was seen. In the times of the Beis Din Hagadol, both methods were used to determine the day of Rosh Chodesh; at times Rosh Chodesh was determined solely by the method of calculation, and at times it was determined also based on testimony. In today’s times, since the abolishment of the Beis Din Hagadol and the cease of acceptance of testimony, only the first option remains, and Rosh Chodesh is thus sanctified based on calculation alone. The following will feature the detailed laws on this subject: 

A. In times of the Sanhedrin-Sanctification based on testimony:[2]
During the times of the Sanhedrin, in which the moon can be sanctified based on testimony, Rosh Chodesh is determined by when the moon becomes visible after it has completed its orbit and begun a new orbit. From the day in which the new moon is seen one counts 29 days, and waits for the 30th day to see if the moon will be seen. If the moon is seen on the 30th day, the 30th day is Rosh Chodesh and is the first day of the next month. If the moon is not seen on the 30th day, Rosh Chodesh is automatically set for the 31st day, which serves as the first day of the next month. This applies irrelevant to whether one sees the moon on the 31st day, as there is never more than 30 days between orbits of the moon. [A month in which Rosh Chodesh is on the 30th day is called a Chaseir/lacking month, being that the month only had 29 days. A month in which Rosh Chodesh was on the 31st day is called a Malei/full month, being that the month had 30 days.[3]] Now, what remains to be explained is how to determine when the moon is seen. This is determined by calculation and testimony of visibility, and at times only by calculation, as will be explained.
Who determines Rosh Chodesh based on visibility of the moon?[4] Although we explained above that Rosh Chodesh [on the 30th day] is determined by the moons visibility, nonetheless, this determination was not given into the hands of the individual viewer but rather to the Beis Din. Beis Din would first have to calculate as to whether it is possible for the new moon to be seen on the 30th day, and then collect testimony from witnesses who viewed the moon. After corroboration of the testimony, the Beis Din would have to declare on the 30th day that it is Rosh Chodesh. If for whatever reason Beis Din did not make this declaration, the month remains a 30 day month [Malei] and Rosh Chodesh only begins the next day on the 31st day of the month. [The exact Nussach said by the Beis Din to sanctify the new month is found in Miseches Sofrim, and is documented in the end of this chapter.]
Which Beis Din determines Rosh Chodesh-The Beis Din of Eretz Yisrael:[5] The ability to sanctify the new month was only given to the Sanhedrin[6], the Beis Din Hagadol[7] in Eretz Yisrael, or to a lower Beis Din in Eretz Yisrael who received permission from the Sanhedrin[8], or to an individual who received Semicha in Eretz Yisrael and is considered the Gadol Hador. The new month can never be sanctified by a lower level Beis Din who did not receive permission from the Sanhedrin, or by a Beis Din that is outside Eretz Yisrael, or by an individual that is not the Gadol Hador or did not receive Semicha in Eretz Yisrael. Once the Beis Din Hagadol became dispersed and there no longer remained any person who received Semicha from Eretz Yisrael, the ability to sanctify the new month in Beis Din became abolished, similar to the abolishment of the Karbanos once the Temple was destroyed.[9]
Calculating the new moon?[10] Even in the times of testimony, part of the Biblical command to sanctify the new month included that the Beis Din was required to calculate as to when the new moon would become potentially visible.[11] If the moon would not be visible on the 30th day, based on their calculations, then the Beis Din would not accept testimony and any potential witnesses would be automatically disqualified as false or in error of what that which they saw. Rosh Chodesh would automatically be set for the 31st day. If Beis Din calculated that the new moon could be seen on the 30th day, then they would await witnesses to testify and be properly interrogated, corroborating their testimony. At times, despite the calculations, the moon is not visible due to clouds and position, and therefore the Beis Din must wait and see if in truth the moon was seen or not.[12] We will now explore a case in which the new month on the 30th was determined based only on calculation and not on witnesses even in the times of the Sanhedrin.
Relying only on calculation-What was done in the event that witnesses did not come forward month after month?[13] In the event that the moon was not seen for one month after the other, such as due to clouds or positioning, and hence no witnesses came forward, the future months are set not in accordance to testimony of the moon’s visibility but rather in accordance to calculation, having one month Malei and one month Chaseir, or two months Malei or two months Chaseir, in accordance to the calculation.[14] The reason why this is done is because if the Beis Din would make every month Malei due to lack of testimony, then it would end up that after 10 months the new moon begins to appear 5-6 days before Rosh Chodesh, and there is no greater matter of ridicule than this, and it destroys the whole concept of basing the new month on the new moon.[15] Thus, in conclusion we see that at times, even during the era of the Sanhedrin when the new month was sanctified based on testimony, they would have to set the new month based solely on calculation and negate testimony in order to prevent a catastrophic misalignment between the new moon and the new month. This matter was a tradition of Moshe from Sinai, that at times the Beis Din must forgo the testimony and rely solely on calculation.[16] All the areas in the Talmud that mention the new month being set based on calculation alone refers to the above situation, in which the tradition applies.[17]
Sanctifying the month versus setting the new month:[18] When the new month is set for the 31st day, whether this is due to a lack of testimony or due to the calculation showing that the moon cannot appear prior, the new month was set [Kavua] and not sanctified [Mekudash]. Furthermore, if the new month was set on the 30th day based on calculation alone and not on testimony, then the new month was set and not sanctified. The new month can only be sanctified on the 30th day and only based on testimony.[19] The difference between setting the new month and sanctifying the new month is that when the month is sanctified the Rosh Beis Din has to declare the month as Mekudash/holy in front of three people, and have them respond “Mekudash Mekudash”. All this had to be done during the day. If for whatever reason the Beis Din did not sanctify the new month through performing the above procedure then the new month is set for the 31st and not the 30th, despite the testimony. Hence, the establishing of the new month on the 30th based on testimony is only finalized by the sanctification of the Beis Din within the above procedure. When however the month is set and not sanctified then there is no necessity for the above procedure to be performed.

In times of the Sanhedrin or Beis Din Hagadol the new month was sanctified by the Sanhedrin or Beis Din based on calculation and/or testimony. If the calculation showed that the moon could not be visible on the 30th, then Rosh Chodesh would be set based on this calculation for the 31st day. There was no need for witnesses. If the calculation showed that the moon will be potentially visible on the 30th [depending on weather and other conditions] then if witnesses came forward and were corroborated, the month was set for the 30th day. If no witnesses came forward then the month was set for the 31st, despite the above calculation. In the event that this occurred two months in a row, that the moon was potentially visible on the 30th but witnesses did not come forward, then the Beis Din sets the remaining months based solely on calculation and not witnesses.

B. In today’s times:[20]
In today’s times that we no longer have a Sanhedrin, [or Beis Din, or even an individual who received original Semicha from Eretz Yisrael[21]] we can no longer sanctify the new month based on testimony of the moon’s visibility. This however does not mean that the ability to sanctify the day of Rosh Chodesh has become lost, but rather that we now have to resort to setting the new month solely based on our calculations, and not based on the testimony that follows after the calculations.[22]
Does sanctification based on calculation have Biblical status?[23] Even though today we do not sanctify the new month based on testimony but rather solely on calculation, nevertheless, this sanctification has the same Biblical status as did the sanctification in the times of Sanhedrin. The Rambam explains that we received a tradition, a Halacha Lemoshe Misinai, that the Mitzvah of sanctifying the new month is performed through [calculation and] testimony during the times of the Sanhedrin and through [only] calculation during times that the Sanhedrin is abolished. Thus, even from a Biblical perspective, since we do not have a Sanhedrin, we do not need testimony in order to sanctify the new month, and relying solely on calculation suffices.
Who can calculate and set the new month today? Some Rishonim[24] learn that in times that the Sanhedrin is nullified, the Beis Din or Sages have ability to sanctify the new month based on calculation even though they do not have original Semicha. Nonetheless, this calculation and sanctification can only be done by the Sages and Beis Din of Eretz Yisrael, and the Sages of the Diaspora have absolutely no jurisdiction over the calendar.[25] Other Rishonim[26] however learn that the new month can only be calculated and sanctified by one who has original Semicha, and hence today since Semicha has been abolished there is no one that is able to sanctify the new month. Now, if you ask how according to this approach we are able to have set months, this is because we have a tradition that Hillel Hanassi set and sanctified all the future months based on calculation.[27] The following are the details of the tradition of Hillel’s calendar.
The tradition of Hillel’s calendar: Rav Haiy Gaon[28], and many Rishonim[29] record a tradition that upon seeing that the sanctification of the new moon through witnesses would be nullified, Hillel Nesiah set and sanctified all of the new months until the coming of Eliyahu Hanavi.[30] This Hillel is not to be confused with the famous Hillel Hanassi, mentioned in the Mishneh, and otherwise known as Hillel Hazakein, the head of the academy of Beis Hillel. His father’s name was Rebbe Yehuda Hanassi/Nessiah.[31] Hillel was the Gadol Hador of his time and had received Semicha in Eretz Yisrael.[32] There is much confusion as to exactly which Hillel he was, and which Rebbe Yehuda Hanassi was his father, as throughout the generations of the Tanaaim and Amoraim there were many people named Hillel with fathers or grandfathers called Rebbe Yehuda Hanassi. Some Rishonim[33] imply that this Hillel is the Hillel mentioned in the Talmud [first generation of Amoraim], who was the son or grandson[34] of Rebbe Yehuda Hanassi. Some[35] however explain that this Hillel, although being a descendent of the first Hillel Hanassi and Rebbe Yehuda Hanassi, lived hundreds of years later in the times of the 5th generation of Amoraim. As far as we know, this Hillel is not mention at all in the Talmud, neither the Bavli or Yerushalmi. 

C. When did we stop sanctifying the new moon based on testimony?[36]
The sanctification of the new moon based on testimony is dependent on the establishment of a set Beis Din [i.e. Sanhedrin] in Eretz Yisrael. It is not dependent at all on the establishment of the Temple, and hence for many years after the destruction, the moon was still sanctified through testimony. This sanctification continued throughout the times of the Mishneh, and Talmud up until the end of the Talmudic period, during [and including[37]] the generation of Abaya and Rava. In their generation, the Jewish settlement of Eretz Yisrael became dispersed and there did not remain a set Beis Din to sanctify the moon. From that period and on the moon has been sanctified through the set calendar. Historically, the last Sanhedrin/Beis Din to have sanctified the new moon based on testimony was the Sanhedrin headed by Hillel Nesiah[38] [the same Hillel mentioned above who created the calendar] in the year 359.[39] Since that time, the new month is determined based on calculation. From the above it is understood that the original Semicha continued to exist well past the times of the Tanaaim and until at least the year 359 when Hillel sanctified all the future months. Some Rishonim[40] learn that the Semicha was indeed nullified in the generation of Hillel. Other Rishonim[41] learn that it was nullified a generation after Hillel.

D. Will we sanctify the moon based on testimony after Moshiach comes?[42]
After Moshiach comes and the Sanhedrin is reestablished, we will once again sanctify the moon based on testimony.
After Techiyas Hameisim:[43] In the period of the resurrection, the moon will have its own light and hence the concept of sanctifying the new moon will no longer be relevant.


[1] Rambam Kiddush Hachodesh 1/3-8; 5/1-3

[2] Rambam Kiddush Hachodesh 1/3

[3] Rambam ibid 1/4

[4] Rambam ibid 1/5

[5] Rambam ibid 1/8 and 5/1; Sefer Hamitzvos 153; Chinuch 4

[6] Rambam ibid 5/1

[7] Sefer Hachinuch 4; Sefer Hamitzvos Rambam 153; omitted in Mishneh Torah chapter 1/8 and 5/1, however in 5/1 he mentions the Sanhedrin, which is equivalent to the Beis Din Hagadol.

[8] Rambam ibid 5/1

[9] Sefer Hamitzvos ibid

[10] Rambam ibid 1/6-7

[11] Rambam ibid 1/7

[12] Rambam 18/1

[13] Rambam 18

[14] Rambam ibid 18/8; Tur 427

[15] Rambam ibid 18/5

[16] Rambam ibid 18/8

[17] Rambam ibid 18/10

[18] Rambam ibid 2/8

[19] Rambam ibid 18/8

[20] Rambam ibid 5/1-2; Chinuch 4

[21] Chinuch ibid

[22] As explained above, even in the times of the Sanhedrin the sanctification process included two steps: 1) calculating when the new moon can potentially become visible, and b) if it can become visible on the 30th, waiting for testimony from witnesses. In today’s times we can only perform the first step.

[23] Rambam ibid 5/2

[24] See Rambam 5/1-2 that implies there is no need to have original Semicha and that any Beis Din in Eretz Yisrael can calculate and establish the new month, and so seems also from the fact that the Rambam omits entirely the episode involving Hillel and his sanctification of the new months for all future times; Rashba 4/254 defends the ruling of the Rambam and says he knows no source for the tradition that Hillel established the months.

[25] Rambam ibid 5/13

[26] Sefer Hachinuch 4 “Without the above conditions [of Semicha] there is no permission for anyone to sanctify the new month”; Ritva 25a; See Sefer Yesod Olam 4/9; Torah Sheleima p. 26

[27] Sefer Hachinuch ibid

[28] The stating of this tradition by Rav Haiy Gaon is brought in a letter that he wrote which is printed in the Sefer Haibur of Rav Avraham Ben Reb Chiya Hanassi

[29] Sefer Hachinuch 4; Ramban on Sefer Hamitzvos of Rambam 153; Ran Sanhedrin 11a; Ritva 25a; Rashba 4/254; Riei Beitza 5a; Baal Hamaor Rosh Hashanah 20b; Yesod Olam [of Rav Yitzchak Yisraeli, Berlin 1777]; Machatzis Hashekel 417; Derech Mitzvosecha p. 69

Those who do not record this tradition: This tradition from Rebbe Hillel is not mentioned in the Talmud, nor in the writings of the Geonim such as Rabbeinu Chananel or the Rasag, nor in the Rambam in Mishneh Torah. The Rashba 4/254 writes that he knows no source for the tradition that Hillel established the months.

[30] Sefer Hachinuch 4; Ramban on Sefer Hamitzvos of Rambam 153; Derech Mitzvosecha p. 69; However see Machatzis Hashekel ibid who writes “until the end of time”.

[31] Ramban in Sefer Hazechus; Sefer Hachinuch ibid

[32] Sefer Hachinuch ibid

[33] Sefer Hachinuch ibid; Baal Hamaor Rosh Hashanah 20b

[34] Some Rishonim write son [Rashba ibid;] while some write grandson [Chinuch ibid; Baal Hamaor Rosh Hashanah 20b]

[35] See Torah Shleima p. 32 who writes he was the 10th generation from Hillel Hazakein and the 6th generation from Rabbeinu Hakadosh.

[36] Rambam Hilchos Kiddush Hachodesh 5/3; See Rosh Hashanah 21a; Sanhedrin 12a; Beitza 6a

[37] Machazik Bracha 527; Beis Efraim O.C. 51

[38] Sefer Hachinuch 4; Machatzis Hashekel 417; Derech Mitzvosecha p. 69

[39] Igeres Rav Haiy Gaon

[40] Ramban; Reah; Yesod Olam, brought in Torah Sheleima 13/6-119 [p. 97]

[41] Ramban in debate of with Friar Paul [Pablo Christiano] p. 87, brought in Torah Shleima ibid, that the Semicha continued for 400 years past Yoshke; see Torah Shleima there for a Likkut of other Rishonim

[42] Levush 421/1; Sefer Hachinuch 4 “Hillel sanctified all the months until the coming of Eliyahu”; Sefer Hasichos Mishpatim 1992; See Beitza 5b and Toras Menachem 1984 3/8251

Other sources: See Machatzis Hashekel 417 that “Hillel sanctified all new moons until the end of time”

[43] Toras Menachem 1984 3/1528

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