Academic journal article Shofar

The Jewish Calendar-A Mix of Astronomy and Theology

Academic journal article Shofar

The Jewish Calendar-A Mix of Astronomy and Theology

Article excerpt

The lunisolar Jewish calendar is an example par excellence of "time engineering." It is based on the relation between the mean period of the lunar motion about the earth and the mean period of the earth about the sun (the tropical year). This relation is that, to six places of decimals, the number of days in 19 tropical years is equal to the number of days in 235 lunar cycles and is known as the metonic cycle. Moreover, the Jewish calendar, unlike our civil calendar, for example, is intertwined with the days of the week by virtue of the first day of Tishri (Rosh Hashanah) being allowed-for religious reasons-only to occur on Monday, Tuesday, Thursday, and Saturday. Because the metonic-cycle relation is not exact, there will eventually be a significant seasonal drift in holidays such as Passover unless the calendar construction is modified appropriately.

I. Introduction

The Jewish calendar plays a central role in the lives of Jews of various affliations and degrees of commitment. However, other than being vaguely aware of the seasons of the year in which the different holidays and festivals occur, many Jews have only a limited acquaintance with the astronomical and historical/theological roots of this calendar. This lack of knowledge is certainly not due to a dearth of authoritative calendrical references. Indeed, there are many easily available sources of information on the Jewish calendar, including monographs, book chapters, lengthy encyclopedia and journal articles, translations of traditional sources with commentaries, and a recent spate of internet websites. For a list of these sources, see Appendix A.

This paper is intended to give a reasonably concise presentation of how astronomy and theology are intertwined in the Jewish calendar. We feel that its mix and depth of discussion of essential elements of the Jewish calendar covered in a short paper is fairly unique. It is mainly intended for readers who would like an initial overview of the subject as a prelude to more in-depth studies of the Jewish calendar.

The dates of all important holidays-Passover, Rosh Hashanah, Yom Kippur, etc.-are fixed by this calendar. The first day of Rosh Hashanah, for example, occurs on the first day of the month of Tishri, and Passover begins on the 15th day of the month of Nisan. Of particular interest here is the fact that the eight days of Passover always occur in the Spring-late March/early April-and Rosh Hashanah invariably begins a half year later in the fall-September/October. In this respect the Jewish calendar mirrors our civil calendar, which is a solar calendar in which the four seasons-spring, summer, fall, winter-recur annually on the same dates.

By contrast to our civil calendar, however, the Jewish calendar is not only a solar calendar but in addition it is also a lunar calendar, in that it closely follows the cycles of the moon. Thus not only does the first night of Passover, on the 15th of Nisan, always occur in the early spring, but what is not as well known nor often as appreciated is that, on that night of the first seder, the moon is invariably full! Similarly, there is always a full moon on the first night of Succoth on the 15th of Tishri and invariably a new (crescent) moon two weeks earlier on the first night of Rosh Hashanah on the first day of Tishri.

To more fully appreciate this important aspect of the Jewish calendar, of its being synchronized with both the solar and the lunar cycles, let us briefly review some features of our civil calendar. Recall that the 21st of March1 always marks the first day of spring (in the northern hemisphere, of course), that the 21st of June marks the first day of summer, and so on for the 21st of September and the 21st of December marking the first days of fall and winter, respectively. Thus, as designed, our civil calendar correlates fully with the four seasons. By contrast, there is no correlation whatsoever between any particular date on our civil calendar and the phases of the moon. …

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