The Thirteenth Month

This week we will be marking the beginning of the month of Adar One, which is the additional month added to a Jewish leap year, making it 13 months. 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. (19 yrs. X 11 day difference = 209 days over 19 years. 209 days divided by 30 = 6.97 months difference every 19 years. The duration of 19 solar years is equal to 235 lunar months (12 months X 19 years) + 7 (added leap months) = 235 months)   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.