Last week we touched on the concept of lo te’hey shemiya gedolah m’reiya (Rosh HaShana 25b). If beis din sees the new moon, they can declare rosh chodesh without witnesses coming forward – hearing about an event from a witness cannot be greater proof than personal observation. Yesh lachkor: does this mean that observation substitutes for testimony, i.e. it is as if those dayanim who saw the new moon have testified, or does this mean that when the moon is directly observed by dayanim no testimony is needed? Rashi seems to take the latter view, as he writes that lo te’hey shemiya gedolah m’reiya works because witnesses are not required for beis din to declare rosh chodesh -- Hashem told Moshe to declare Rosh Chodesh based on his personal observation, ka'zeh re'eh v'kadesh, without witnesses. The implication of Rashi is that if we were dealing with an area of halacha that required proof in the form of witnesses, lo te’hey shemiya gedolah m’reiya would not apply. Tosfos disagrees, as we find the concept of lo te’hey shemiya gedolah m’reiya applied even where beis din sees someone commit murder. How can Rashi cite the special din of ka’zeh re’eh v’kadesh told to Moshe in the context of Kiddush hachodesh as the source for lo te’hey shemiya gedolah m’reiya when the concept appears in other places as well?
The process of kiddush hachodesh as described in the Torah involves beis din declaring rosh chodesh based on witnesses’ testimony or beis din’s own observation of the new moon’s appearance. Our present calendar, however, is based on calculations of when the moon will appear -- we have no beis din which declares rosh chodesh and no witnesses coming forward to testify. What gives our calendar validity? Ramban in his comments to Sefer haMitzvos (153) explains that when Hillel II formalized the cheshbon/calculations that would predict all future roshei chodesh, his beis din in effect did a kiddush hachodesh for all those future events. Rambam disagrees and writes that it is the observance of rosh chodesh by the Jewish community, meaning the community in Eretz Yisrael (an important aside: the Rambam tells us that there will always be a remnant of the Jewish people in Eretz Yisrael), which effectively makes it rosh chodesh. According to the Rambam, if rosh chodesh is a function of observance by the people, what are we to make of all the halachos describing beis din’s role in vetting witnesses and declaring rosh chodesh?
R’ Soloveitchik in an essay in Koveitz Ch. Torah distinguished between two roles of beis din. We are familiar with the role of beis din acting as a judicial body, but beis din also serves a secondary role as the representatives of klal yisrael. Where halachos call for a consensus of the people, this consensus is not measured by popular vote, but is reflected in the action of beis din (sanhedin in particular). The halachos of rosh chodesh that speak of a declaration by beis din in concert with the nasi being present reflects this role. Rosh chodesh is established by the people; beis din is simply their representative.
Based on this analysis of the Rambam, I think we can perhaps suggest an explanation of why Rashi cites a pasuk to justify lo te’hey shemiya gedolah m’reiya in the context of kiddush hachodesh. True, lo te’hey shemiya gedolah m’reiya applies in other contexts, even in capital cases, but in those other areas the substitution of beis din’s own observation for witnesses’ testimony is part and parcel of the judicial process of establishing the facts of the case and deciding on a verdict. Rashi felt compelled to cite a pasuk to justify lo te’hey shemiya gedolah m’reiya, allowing private observation in place of public testimony, even in the context of kiddush hachodesh, where beis din serves not as judges, but as representatives.
(See Moadim u'Zmanim vol 1 for a different explanation of Rashi).