Rosh
Hodesh is the beginning of the new month. The length of a month
in the Jewish calendar is determined by the time it takes for the
moon to make one revolution around the earth as determined by the
conjunction of the sun, moon, and earth in a line. This is called
the molad, i.e., the "birth" of the "new"
moon. Such a revolution is completed in 29 days, 12 hours, 44 minutes
and 3.5 second….
Since
a calendar month does not begin in the middle of the day because
months are counted by days, not hours (B. Meg 5a), it is necessary
to add half a day to one month or subtract half a day from the next.
As a result, the months alternate between twenty-nine and thirty
days in length. The thirty-day month is malei, i.e. full
or long; the twenty-nine day month is haser, i.e. defective
or short. There are some variations, however, due to considerations
[related to the scheduling of certain holidays]. Nisan, Sivan, Av,
Tishrei, Shevat and Adar I (in a leap year) are always full. Iyar,
Tamuz, Elul, Tevet, Adar II (in a leap year) and Adar (in a nonleap
year) are always defective. Heshvan and Kislev are sometimes full
and sometimes defective. When a month is thirty days, the Rosh Hodesh
of the next month is celebrated for two days because the thirtieth
day of the month that has just passed is counted as the first day
of Rosh Hodesh, and the first day of the next month, as the second
day. Consequently Nisan, Sivan, Av and Tishrei (only the first day
of Rosh Hashanah is counted as Rosh Hodesh) always begin with one
day of Rosh Hodesh; Iyar, Tamuz, Elul, Tevet, Adar I and Adar II
always begin with two days.
These
variations result from the solar-lunar structure of the Jewish calendar.
The year in the Jewish calendar consists of twelve lunar months,
but the festivals follow the solar year, since several of them (Passover,
Shavuot and Sukkot) must take place in certain seasons, and the
seasons are determined by the earth annual revolution around the
sun. Since the lunar years is roughly 354.3 days in length, while
the solar year is roughly 365.5, or roughly eleven days longer,
the festival would eventually fall in the wrong seasons if their
occurrence followed the cycle of lunar months. For example, Passover
would be celebrated eleven days earlier in each succeeding year,
and as a result would eventually be celebrated in the winter, and
then in the autumn and summer, in violation of the biblical prescription
that Passover must take in the month of Aviv, i.e., in the spring
(Deut. 16:1). Similarly, Shavuot must occur at the time of the early
harvest and Sukkot in the fall.
To
prevent this difficulty, the lunar calendar is regularly adjusted
to keep it in conformity with the solar year. This is done through
the periodic additional, or intercalculation, of a thirteenth month,
known as Second Adar or Adar II, immediately after the normal month
of Adar, which in leap years is known as First Adar or Adar I. Since
the discrepancy between the solar and lunar years amounts 207 every
nineteen years, the "leap month" of Adar II is added to
the third, sixth, eight, eleventh, fourteenth, seventeenth and nineteenth
year of every nineteen year period.
