The Jewish Calendar

As we approach Passover, Pesach, the concept of the Jewish calendar comes to mind. In fact, the first commandment the Jews were given as a nation was to celebrate Passover in spring, and to count the months of the calendar from Passover based on the monthly cycles of the moon. So I think this is an appropriate time to talk about Jewish time.

Measuring and marking the passage of time is a universal human preoccupation. At least forty different calendars are in use around the world today.  The most widely used systems are the solar calendar, based on the cycle of the sun, and the lunar calendar, based on the cycle of the moon.  Because of its orbit around the earth, the moon alternates between being fully exposed to sunlight (full moon) and being completely shadowed by the earth (no moon – “a moonless night”).  A lunar month is defined as the time required for the moon to go from a given phase, (e.g. new moon, slim crescent) back to that phase again, a period of about 29 ½ days.  In a lunar calendar, the month begins with the sighting of the new moon, the middle of the month coincides with the full moon and the end of the month with the gradual disappearance of the moon.  The lunar year consists of twelve lunar months, about 354 days, while the solar year is 365 ¼ days long, a difference of about eleven days.

The seasons of the year are dependent upon the earth’s rotation around the sun, its tilted orbit and its distance from the sun.  If the lunar calendar is not adjusted to the seasons of the sun at all, then a particular lunar date will “drift,” until over the course of 33 years, it occurs in each of the four seasons.  Most of the Western world uses the Gregorian calendar, which is solar.  In a solar calendar, the months are not coordinated with the cycles of the moon, and the year is based only on the seasons.  Specific dates consistently occur in the same seasons, but with no relation to the natural cycle of the moon at all (e.g. the first of the month may coincide with a full moon, “no moon” or a quarter moon).

The Jewish calendar is a luni-solar system, combining elements of both the lunar and solar calendars.  The Jewish calendar scrupulously follows the phases of the moon, but it also incorporates features that ensure that the festivals always occur in the same seasons.  Passover, for example, always falls in the spring and Sukkot is always in autumn.  In contrast to the solar calendar, the first day of the Jewish month coincides with the first appearance of the new moon.  Complex formulae regulate the periodic adjustment of the calendar to coordinate the cycle of the moon with the cycle of the seasons. Seven times in the course of nineteen years, an extra month is added to compensate for the eleven day difference between the solar and lunar years.  A Jewish leap year, therefore, has 13 months.  The rules and astronomical measurements used to formulate the calendar are part of the Oral tradition as received on Mount Sinai and passed on from one generation to the next.

In the Torah (as well as in Modern Hebrew), the days of the week are designated by number: i.e. “first day,” “second day.”  Only the seventh day, Shabbat, has a name.  The first chapter of Genesis speaks of  “evening and morning” when describing the creation of the world, indicating that a day is calculated from nightfall to nightfall, so day one begins at nightfall on Saturday and ends at night on Sunday.  Originally, months were also designated only by number rather than by name — beginning with the month in which the Exodus took place.  During the Babylonian exile, which followed the destruction of the First Temple, the Jews began using Babylonian names for the months.  These names are still in use today.

In the times of the Holy Temple in Jerusalem (from the 9th century BCE until 70 CE), the highest Jewish court in Israel, the Sanhedrin, would determine when a new month began.  The beginning of each month was (and still is) marked by a semi-holiday known as Rosh ChodeshEyewitness reports that a new moon had been sighted were critical in declaring a Rosh Chodesh, but astronomical calculations, as well as agricultural, economic and religious considerations were also taken into account. The process of determining the beginning of the new month is called Kiddush Hachodesh, the Sanctification of the New Moon.  Setting Rosh Chodesh ahead or back a day determines when the upcoming festivals will begin; therefore, Kiddush Hachodesh creates the holiness of the festival days.

Every member of the Sanhedrin had authority (called semichah in Hebrew) conferred upon him by someone else who had authority conferred upon him, in an unbroken chain all the way back to Moses, who had given authority to Joshua.  During the period of persecution by the Roman Empire, the Sanhedrin lost much of its power.  Early Roman decrees made it illegal to grant or to receive this authority, and when the Roman Empire became Christian under Constantine, the Sanhedrin was forbidden to convene.  Since Rosh Chodesh can be declared only by the meeting of a Sanhedrin whose members have semichah, it became increasingly difficult and dangerous to sanctify the new moon each month.  By about 360 CE the Rabbis realized that the situation was only getting worse and the time would come when a Sanhedrin with semichah would no longer exist.  Hillel II and his court therefore decided at this time to establish a fixed calendar. He calculated, sanctified and declared all subsequent months and years.  The calendar that we use today is based on these calculations and sanctifications.

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