The process of determining Rosh Chodesh in times of the Sanhedrin?
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:
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.] 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? Although we explained above that Rosh Chodesh [on the 30th day] is determined by the moon’s 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: The ability to sanctify the new month was only given to the Sanhedrin, the Beis Din Hagadol in Eretz Yisrael, or to a lower Beis Din in Eretz Yisrael who received permission from the Sanhedrin, 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.
Calculating the new moon? 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. 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. 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? 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. 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. 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. 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.
Sanctifying the month versus setting the new month: 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. 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.
 Rambam Kiddush Hachodesh 1:3-8; 5:1-3
 Rambam Kiddush Hachodesh 1:3
If Beis Din makes a mistake: See Likkutei Imrim Maggid 96
 Rambam ibid 1:4
 Rambam ibid 1:5
 Rambam ibid 1:8 and 5:1; Sefer Hamitzvos 153; Chinuch 4
 Rambam ibid 5:1
 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.
 Rambam ibid 5:1
 Sefer Hamitzvos ibid
 Rambam ibid 1:6-7
 Rambam ibid 1:7
 Rambam 18:1
 Rambam 18
 Rambam ibid 18:8; Tur 427
 Rambam ibid 18:5
 Rambam ibid 18:8
 Rambam ibid 18:10
 Rambam ibid 2:8
 Rambam ibid 18:8
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