At the outset, I want to point out that two separate issues are involved
here. The first is the question of whether Lo Sasur (You shall not deviate
from the ruling of the Sages) applies to the issue of the Jewish calendar
5757, and the second is regarding the correct understanding of the
prohibition of Lo Sasur from the standpoint of the Reshonim (early
commentaries), and also understanding the Torah Temimah's explanation
of the Jerusalem Talmud.
Regarding the first matter, upon further reflection I would agree that
the question of the accuracy of the Jewish calendar would not fall under
the category of "Lo sasur", at least not in the strict sense of the verse.
This is neither a question of halacha nor of a basic precept, as in the
Rambam's 13 Principles of Faith.
However anyone who has studied Talmud seriously can attest to the pervasive
principle found throughout the Oral Torah, that "eylu ve'eylu divrei
Elokim chaim". Which means, any Torah which was learned and taught by our
Sages, is pure Torah and pure truth.
For example, we follow the rule of Bais Hillel that one may say the Shema
while either sitting or standing. Still and all, no one would dare say that
"Bais Shamai were mistaken and wrong". Rather, Bais Hillel are correct, and
we follow their halachic ruling. Bais Shamai are also correct, albeit on a
different plane; we do not follow their ruling.
Some would say that this is only regarding "theoretical halachic matters"
as opposed to a psak halacha (ruling) based upon what would appear to be
primitive pre-scientific observations; this is not the view of the
mainstream community of Torah scholarship. Furthermore, the Talmud and
Shulchan Aruch have numerous examples to the contrary. Perhaps those who
are learning the Daf Yomi could recite examples where the halacha to this
day follows principles of "treifa" (mortally injured animal/person) that
are not in accordance with the understanding of modern medicine.
In non-halachic matters we see the famous episode, Gittin 56a&b. We find
R' Yochanan b' Zakkai insisted on calling the general who was besieging
Jerusalem "Caesar" based upon his tradition as to the interpretation of the
pasuk (verse) in Isaiah, "And the Lebanon will fall to the mighty one".
This in spite of the fact that as far as anyone knew, Caesar was in Rome,
and R' Yochanan would surely be killed if he were wrong. The Talmud relates
that at that point a messenger arrived to inform Vaspasian that the Senate
had declared him the new Caesar.
Thus in matters of halacha, and in matters of mesorah (tradition) in
practice, we do NOT defer to common knowledge if it contradicts the
directive of chazal.
With regard to the second issue, i.e. how to understand the issur of Lo
sasur, I am responding in a separate detailed posting.
Eliezer Gordon