The Calendar – moon sightings versus set calendar

Some facts are so obvious that mentioning them seems a waste of time. For example, everyone knows that Hillel II established our permanent calendar some 1,600 years ago, while beforehand, Beis Din determined Rosh Chodesh through visual sightings.


Surprisingly, this obvious “fact” is not self-evident after all. No lesser authority than Rav Saadia Gaon (the Rasag) wrote the exact opposite. According to him, the rules of our permanent calendar were the primary rules of Rosh Chodesh ever since Sinai. If so, you may ask, why does the

Gemara in Rosh Hashanah devote so much discussion to the visual-sighting system? The answer to this question is totally counter-intuitive.

In  Parshas  Bo  (Shemos  12:2)Rabeinu Bechaye (citing Rabeinu Chananel) offers the following summation of the Rasag’s approach.

“The determination of months is through calculation and not through visual sightings of the moon,” he begins. “Proof to this principle is that during all the forty years the Jews spent in the desert, the cloud covered them by day and a pillar of fire by night, and during all of them they saw neither the sun by day nor the moon by night… If so, how could they establish months through seeing the moon? Rather, the chief mitzvah of the Torah is [to determine months] according to calculation.”

Rabeinu Bechaye then explains that this calculation was the same calculation our permanent calendar uses. Assuming an average month length of 29.5 days and 693 chalakim, each Rosh Chodesh was determined with a handful of basic rules.

If not so, he argues, why do Chazal consider the determination of Rosh Chodesh as such a complicated procedure? All you have to do is go outside at dusk and look out for the new moon. If it’s there, you announce Rosh Chodesh; if it isn’t, you don’t! Furthermore, in Shmuel I (ch. 20) we find Shaul celebrating two days of Rosh Chodesh as it says, “Shaul said to Yehonasan his son, Why did the son of Yishai not come yesterday or today to the meal? And Yehonasan rose in anger and did not eat bread on the day of the second [Rosh] Chodesh.”

Rabeinu Bechaye then tackles the question we mentioned earlier. If Chazal always relied on the calendar we use today, why do they devote so much discussion to the laws of visual sightings? Why were visual sightings necessary if Chazal always relied primarily on the rules of our present calendar? Rabeinu Bechaye’s answer is an amazing historical chiddush.

“Yisroel used to determine the months according to [our present rules of] calculation for 1,500 years, from the days of Moshe Rabeinu until Antignos [Ish Socho] who was head of the exile and head of the Sanhedrin,” he writes. “Among his disciples were two men, Tzaddok and Baisus… They became heretics and began raising doubts about the determination of Rosh Chodesh, arguing that the way to determine Rosh Chodesh is not through calculations but through visual sightings, and that this was the correct method. The sages of the generation had to prove them wrong and demonstrate [their error] to them with clear proofs.”

How did these visual sightings crush the heretics’ opposition to Chazal? Rabeinu Bechaye explains:

“Concerning   the   statement   of Chazal (Rosh Hashanah 24a) that Raban Gamliel had various shapes of the moon on a chart in his attic on the wall, which he showed simple people, this means as follows: Raban Gamliel made these shapes to demonstrate his knowledge of the moon’s monthly movements to the talmidim of Tzadok and Baisus… And when they saw this month after month, year after year, it became clear to them that the main thing is the calculation and the opinion of the dissenters was nullified. Even though they used to accept witnesses as a remembrance of this, the Beis Din actually relied on the calculation.”

Rabeinu Bechaye sums up with the amazing conclusion that when Hashem commanded Moshe, This month shall be for you the first of months, this was a Halacha leMoshe miSinai that Beis Din has the power to determine Rosh Chodesh and leap years according to the same rules that govern our permanent calendar!

In conclusion, according to the Rasag, Sod Ha’Ibur (the secret of calculating the Jewish calendar) refers not to the complicated calculations used to calculate the position of the moon, but to the calculations of our permanent calendar, and these rules were utilized to determine Rosh Chodesh from the time we left Egypt. Of course, all this is the absolute total opposite of everything most contemporary Jews know of the Jewish calendar.

So novel is the Rasag’s approach that Rav Hai Gaon (Otzar Hageonim) and the Rambam (Peirush HamishnayosRosh Hashanah chapter 2) contend that the Rasag never believed it himself!

“I am surprised at a certain person who denies the obvious and says that the Jewish faith is built not on visual sighting of the moon, but only on calculations, [saying this] even though he believes all these writings [of

Chazal],” the Rambam writes. “I think that this person did not believe it [what he said], but his intent in this statement was to defeat his opponent however possible, either true or false… And what you, the reader should believe, is that our principle faith is built on visual sightings.”

Who is this “opponent” mentioned by the Rambam? Probably the Karaim he mentions in Sefer Hamitzvos (lo taaseh 153):

“This mitzvah [of determining Rosh Chodesh through visual sightings] is performed only by the Beis Din Hagadol and only in Eretz Yisroel. Therefore, visual sightings has ceased nowadays due the absence of a Beis Din Hagadol, just as offering sacrifices has ceased due to the absence of the Bais Hamikdash. And this was the intent of the heretics known as Karaites [to fulfill the mitzvah of determining the month through visual sightings even in our time] and they erred [because we have no Beis Din Hagadol].”

Regarding how we will determine the calendar after Moshiach arrives will depend on the above conflict between the Rasag and the Rambam. According to the Rasag we will continue with a more flexible version of our present system, while according to the Rambam we will revert to the old system of visual sightings.


As mentioned earlier, the Rambam (Kiddush Hachodesh 5:2) rules that the Sanhedrin did not determine the months with the rules of our present calendar but through visual sightings:

“This matter is a halacha l’Moshe miSinai that when there is a Sanhedrin we establish [Rosh Chodesh] through visual sighting, and when there is no Sanhedrin we establish according to this calculation that we calculate with today and we do not need a visual sighting.”

You might ask, to what extent is our calendar different from the old system where the Sanhedrin established each Rosh Chodesh individually? Do we still need to establish each month individually as the Sanhedrin used to, or did Hillel II sanctify all the months in advance? This seems to be an argument between the Rambam and the Ramban. In Hilchos Kiddush Hachodesh (6:13), the Rambam writes, “Nowadays, when every person in his town calculates and says that Rosh Chodesh will be on day so and so, and Yom Tov will be on day so and so, we are actually not establishing [Rosh Chodesh] according to our calculation, and not relying on it, because one cannot make leap years or establish a month in the Diaspora. We rely solely on the calculation and establishment of the people in Eretz Yisroel. And that which we calculate is only to reveal the matter, since we know that they rely on this [same] calculation, we calculate to know which day the people in Eretz Yisroel have established…”

In Sefer Hamitzvos (153) the Rambam goes further, drastically stating that due to this principle, “if we imagined that all the people in Eretz Yisroel left from there, chalilla that Hashem would do this as he promised that the signs of nationhood would never be utterly destroyed, and there was no Beis Din there, and [also] no Beis Din in the Diaspora that had semicha from them, our calculation would not help at all, because we cannot calculate months and make leap years in the Diaspora except under the conditions we mentioned…”

In other words, according to the Rambam, even though Hillel II calculated the calendar in advance, the Jews in Eretz Yisroel need to calculate and establish each individual Rosh Chodesh as it arrives, and without that, our calendar would cease to exist.

The Ramban (ibid) raises a number of objections against the Rambam’s approach, including the following. The Rambam writes (5:1) that the month can only be established by “a Sanhedrin in Eretz Yisroel or a Beis Din of semuchim in Eretz Yisroel to whom the Sanhedrin gave permission.” If so, what power do the Jews living there nowadays have to establish months when semicha no longer exists? (For an answer to this question, see Mahari Beirav in Kunterus Hasemicha and Meshech Chochma parshas Bo).

Because of his objections, the Ramban learns that our calendar is running on automatic pilot from the time of Hillel II and does not need any input from us at all.

“Today we have no such Beis Din [of semuchim] in the whole of Yisroel,” he writes. “But the cure for this great difficulty is that Rav Hillel… who established the calculation of our calendar, sanctified the months and leap years according to his calculation until Eliyahu z”l comes, when we will once more return to visual sightings of the great and holy Beis Din, amen, may it be soon in our days. Because he saw the Yomim Tovim might become null and void due to the disappearance of semicha… and he rose up and enacted the [permanent] calculation, and with it he sanctified months and made leap years until the Bais Hamikdash is built. This indeed is why visual sighting no longer exists with us, because there is no Beis Din fit to receive witnesses and sanctify [months] by their testimony.”

According to this approach, we will continue sailing on automatic pilot until the mitzvah of Kiddush Hachodesh returns to the jurisdiction of the Sanhedrin. May this be soon!


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