Secrets of the Hebrew Calendar
Two weeks before the Exodus, God instructed Moses and Aaron regarding reform of the Israelite calendar: “This month shall be unto you the beginning of months; it shall be the first month of the year to you.” (Exodus 12:2). Thenceforward, months were to be known by their ordinal numbers. The spring month of March/April was designated as the first of the cycle.
The mention here in the Torah of calendar reform interrupts the natural flow of the text. It is a conspicuous break between the narration of the plagues and the description of the paschal service. Nahum Sarna explained that the Exodus fostered “a wholly new order of life dominated by the consciousness of God’s active presence in history. The entire religious calendar of Israel is henceforth to reflect this reality by numbering the months of the year from the month of the Exodus.” It was desirable that the text highlight the philosophical underpinnings of the Hebrew calendar before setting down calendar-based ritual commandments that brought to mind the salvation wrought by God (for example, the proper times for disposing of leaven and for eating matzah, bitter herbs, and the meat of the paschal lamb). Ibn Ezra remarks that it is indeed appropriate for Nissan to be reckoned as the first month, since the purpose of an overwhelming number of commandments is remembrance of the Exodus.
Another explanation is that promulgating a calendar is the act of a sovereign. The new Israelite calendar represented the official end of Pharaoh’s dominion over Israel -- and the beginning of God’s (Exodus Rabbah 15:13).
The Anchor Bible Commentary observes that, in the Genesis account, God distinguishes the concepts of day, week, and year only; it is not until Exodus 12:2 that He establishes the notions of the months and of the calendar: “The implication may be that the birth of the Israelite nation and the concomitant establishment of a calendar are themselves acts of cosmogony, completing the unfinished creation.” This idea dovetails with the famous first words of Rashi, suggesting that, theoretically, the Torah should have begun with Exodus 12:2.
Rabbinic legend is that Moses experienced great difficulty trying to understand the commandment of the New Moon (Menahot 29a). God had to extend His finger and show Moses a thin crescent, indicating that when the moon reached that particular size a new month should be proclaimed (Mekhilta d’Rashbi 12). But Rabbi Shimon bar Yochai rejected this Aggadic embellishment, and R. Shimon maintained that God spoke to Moses only during the daytime. In contrast, Rabbi Eliezer defended the verisimilitude of the story, asserting that God spoke to Moses immediately before nightfall and performed the visual demonstration immediately thereafter (Mekhilta d’Rabbi Ishmael, Pis’ha 1).
The textual basis for this tale is the word הזה, “this.” The Talmud often interprets זה as requiring visual identification of the object by the subject. For example, the parents of a rebellious son must be able positively to identify, and to point out, their son בננו זה (Mishnah Sanhedrin 8:4). Another example of the halakhic requirement of visual identification is that a blind man is exempt from reciting the Passover Haggadah because the Seder officiant must be able to see the matzah and the bitter herbs בעבור זה (Pesahim 116b).
Yet there was more at stake in this Midrash concerning Moses and the New Moon than simply hyper-analysis of Biblical text centering around the common adjective ha-zeh. What the sages really desired to emphasize were these three key points about the calendar:
(a) Determining when to declare the New Moon is based, ideally, on eyewitness testimony (Rosh Hashanah 20a). For several centuries, during the late Second Temple and early rabbinic periods, the High Court observed a solemn monthly ceremony. On the thirtieth day of the month, the Sanhedrin would wait to hear from witnesses claiming to have seen the new moon. If that testimony was deemed acceptable, the head of the court would proclaim that the new month had arrived and was sanctified. Yet this very procedure has no basis in the Torah. Several Jewish sects in the Second Temple era rejected it. For example, in the Book of Jubilees, a radically different method is put forward. What the Sages embraced about the Aggadic tale of God’s showing Moses a sliver of the moon was that it bolstered the legitimacy of rabbinic calendar practices by tying them to Mosaic times.
(b) The astronomical calculations relating to the New Moon are complex. They are beyond the capabilities of the layman. They are difficult even for those with facility in astronomy and mathematics. While Rabban Gamliel examined unlearned witnesses, he had to use pictures of the moon in its various phases and ask those witnesses whether what they had seen looked like this or that pictorial representation (Mishnah Rosh Hashanah 2:8). Maimonides notes the great intellectual depth of Hebrew calendar law; he maintains that only a select few are able to master it (Hilkhot Kiddush Ha-Chodesh 11:4). Thus, in claiming that even a great man like Moses had trouble with this area of law, the sages were implicitly extolling the genius of those of their own number who had mastered the calculations inherent in creating the Hebrew calendar.
(c) Scripture provides very little textual guidance regarding the calendar; indeed, there is nothing more than what is set down in Exodus 12:2. How does Rabbinic Judaism solve the overall problem that legal topics are often not thoroughly explained within the four corners of the pages of the Bible? The answer given is that the fuller explanation is, and can be, achieved only by resorting to the oral tradition. The Oral Law is traced back to Moses and is to be inculcated into every student of Torah. Yet, according to our Midrash, on the subject of the calendar even Moses lacked essential information – until he was enlightened by Divine intervention and, literally, finger-pointing. Arguably, then, our Midrash is intended to make the subtle point that the rules governing the Hebrew calendar are, as a matter of policy, not to be widely disseminated. Instead, they are intended to be passed along, in unwritten form, and even then, only on a need-to-know basis.
The most challenging task facing those responsible for fixing the Hebrew calendar is reconciling its lunar and solar aspects, which simultaneously exist. The months are determined by the waxing and waning of the moon, but the years must fit the agricultural cycle. Passover must be in the spring; Sukkot must be in autumn. The eleven-day disparity between the lunar and solar years is bridged, in our calendar, by the periodic addition of a thirteenth month, II Adar.
Knowledge of the mathematical procedure for doing this was zealously guarded. Fittingly, it is called the סוד העיבור “secret of intercalation” (Ketubot 112a). The judicial proceeding at which the year was intercalated was attended only by the seven participating sages. Uninvited guests were expelled from the room (Sanhedrin 10b).
Keeping this information secret was considered necessary to prevent maverick Diaspora scholars from establishing their own calendars for their own communities. Reliance upon messengers coming from the Holy Land with news of the High Court’s calendar decisions forced Diaspora communities into discomforting uncertainty regarding the true date. The result was that, to be conservative, Diaspora Jews added an extra day for observance of each Biblical holiday. This burden of additional holiday-days was unwelcome (Song of Songs Rabbah 1). It could be eliminated by using a calendar no longer beholden to the dictates of scholars in Palestine.
However, such a development was considered detrimental to the broader interests of Judaism because: (a) It could lead to competing and divergent calendars, causing schism; (b) It would negate the historic primacy of Eretz Yisrael in determining the calendar, and violate the dictum that “the law shall go forth from Zion” (Isaiah 2:3); and (c) The procedure of sanctifying the new month through eyewitness testimony would be lost.
On several occasions, Diaspora sages nevertheless attempted to create a new calendar. Hananiah, the nephew of Rabbi Joshua, went to Babylonia in the mid-second century. Acting on his own purported authority, he sanctified new months and intercalated years. Messengers from the High Court of Israel instructed him to cease and desist, or to suffer excommunication (Berakhot 63a). He desisted. Samuel, a third century sage of Nehardea, was a brilliant astronomer and claimed to be capable of establishing a calendar for the entire Diaspora (Rosh Hashanah 20b). He was dissuaded from doing so only after a fellow rabbi made him aware of details of the “secret of intercalation” that he had not known.
Conventional Jewish historiography has it that the secret of intercalation was revealed during the era of Patriarch Hillel II, when a fixed calendar was established in 358 CE. This bold step was taken because anti-Jewish persecution made impossible the continued functioning of the High Court and the acceptance of eyewitness testimony. However, this claim is not based upon anything in the Talmud; it is first found in remarks attributed to Hai Gaon and published in Sefer Ha-Ibbur in 1123. Prof. Sacha Stern points to an 835/6 CE letter, found in the Cairo Geniza, that indicates that our modern fixed calendar did not exist even 500 years after Hillel II. Thus, the historical fact appears to be that the evolution of the Hebrew calendar was a slow process that spanned many centuries, beginning in the Tannaitic period and concluding in Geonic times.
The eventual shift to a fixed calendar, and the associated public exposure of the סוד העיבור, was, ironically, designed to achieve the same ends as the earlier efforts to keep that same knowledge secret. The goal was to prevent anarchy and division among Jewry, where members of the same family might observe festivals on different days (Pesikta Zutreta Exodus 12). So long as the High Court was able to function according to the old paradigm of eyewitness testimony, a fixed calendar posed a threat, because it might not correspond exactly to the unpredictable determinations of that Court. With the demise of both the High Court and the eyewitness method, only an officially authorized fixed calendar could guarantee religious unity.
The calendar of Hillel II (or whoever truly deserves credit for the innovation) has proven its effectiveness. Despite the great theological and ideological divisions plaguing Jewry during the past 150 years – especially the denominational breaks starting in 19th century Central Europe and continuing to date – the Jewish calendar remains one of the few things about which there is universal agreement throughout all Klal Yisrael.