Sanctified For Now And Thereafter
‘In Front of the Sun or Behind It?’
(Rosh Hashana 23b)
The Mishnah on this daf is central to any discussion of the role of beis din and its power to set and sanctify a new month. It describes at length how beis din would inaugurate the new month based on the testimony of witnesses who sighted the moon. Sadly, today, we no longer have the Beis Hamikdash and we no longer have a beis din authorized to receive testimony to declare a new month. Instead, we make use of a fixed calendar based on a variety of halachic principles (see Rambam, Hilchos Kiddush HaChodesh chapter 6-8 and Tur O.C. 427-428). Based on the principles known by our Sages, we can predict when the new moons will appear.
A Fixed Calendar
When did this calendar first come into use? The Gemara does not explicitly tell us. The Rashba (Teshuvos 4:254) writes: “We have a tradition that this system was instituted by Hillel, the son of Rabbeinu HaKodesh (Rebbe Yehuda HaNassi). However, I know of no source for this tradition.”
The earliest source we can find in which Hillel is attributed with developing the calendar, is the writings of R’ Avraham ben R’ Chiya HaNassi, one of the early Rishonim, who is known mostly for his sefarim on astronomy. In one of his works, Sefer Ha’Ibbur (3:7), he cites Rav Hai Gaon, who writes that the Jewish calendar was developed by Hillel in the year 4119 (1,655 years ago). The Ramban (Sefer HaZechus on Gittin, 36a) writes the same. He explains that when the Sanhedrin in Eretz Yisrael was disbanded and there were no longer any judges authorized to inaugurate new months based on the testimony of witnesses, Hillel devised a calendar which could be used by all future generations until the Sanhedrin would be restored.
The Rambam (Hilchos Kiddush HaChodesh 5:3) writes that this coincided with the end of the era of Abaye and Rava (who passed away in the years 4098 and 4113, according to the Iggeres Rav Sherira Gaon). Indeed, we find in the Gemara that the new month was still inaugurated based on the testimony of witnesses during the time of Abaye and Rava. For example, Rosh Hashana 21a reports that one year Rava was uncertain on which day to keep Yom Kippur. Also, Taanis 29b states that Tisha B’Av occurred on erev Shabbos during the time of Abaye (which is impossible under Hillel’s calendar – see Or Samei’ach on the Rambam, ibid).
On Whose Authority
From where does Hillel’s calendar derive its authority? A calendar is not a beis din. On what authority does it declare that such and such a day should be the first of the month?
The Rambam (Hilchos Kiddush HaChodesh 5:2) maintains that Hashem entrusted Moshe Rabbeinu with two different ways of inaugurating the new months: 1) via testimony when the Sanhedrin was active and 2) via a practical system after the Sanhedrin was disbanded. Hillel’s calendar is of the second method.
The Ramban, however, argues that Moshe was told only to inaugurate the new month via testimony before beis din. According to the Ramban, beis din in the time of Hillel ruled that Hillel’s calendar is binding on all future generations until the Sanhedrin would be reinstated. It had this authority since beis din, when necessary, is allowed to determine the day of Rosh Chodesh for a certain month long before that month arrives.
The Principles
Hillel’s calendar was based on a number of halachic principles including “molad zakein – do not consider a previous appearance of the moon.” According to this principle, if the moon appears in the afternoon, that day is not Rosh Chodesh, but rather the day after.