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The problem with strictly lunar calendars is that there are approximately 12.4
lunar months in every solar year, so a 12-month lunar calendar loses about 11
days every year and a 13-month lunar gains about 19 days every year. The months
on such a calendar "drift" relative to the solar year. On a 12 month calendar,
the month of Nissan, which is supposed to occur in the Spring, occurs 11 days
earlier each year, eventually occurring in the Winter, the Fall, the Summer, and
then the Spring again. To compensate for this drift, an extra month was
occasionally added: a second month of Adar. The month of Nissan would occur 11
days earlier for two or three years, and then would jump forward 29 or 30 days,
balancing out the drift.
In the fourth century, Hillel II established a fixed calendar based on
mathematical and astronomical calculations. This calendar, still in use,
standardized the length of months and the addition of months over the course of
a 19 year cycle, so that the lunar calendar realigns with the solar years. Adar
II is added in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of the cycle.
The current cycle began in Jewish year 5758 (the year that began October 2,
1997).
In addition, Yom Kippur should not fall adjacent to Shabbat, because this would
cause difficulties in coordinating the fast with Shabbat, and Hoshanah Rabba
should not fall on Saturday because it would interfere with the holiday's
observances. A day is added to the month of Cheshvan or subtracted from the
month of Kislev of the previous year to prevent these things from happening.
Numbering of Jewish Years
The year number on the Jewish calendar represents the number of years since
creation, calculated by adding up the ages of people in the Bible back to the
time of creation. However, this does not necessarily mean that the universe has
existed for only 5600 years as we understand years. Many Orthodox Jews will
readily acknowledge that the first six "days" of creation are not necessarily
24-hour days (indeed, a 24-hour day would be meaningless until the creation of
the sun on the fourth "day"). For a fascinating (albeit somewhat defensive)
article by a nuclear physicist showing how Einstein's Theory of Relativity sheds
light on the correspondence between the Torah's age of the universe and the age
ascertained by science, see The Age of the Universe.
Jews do not generally use the words "A.D." and "B.C." to refer to the years on
the Gregorian calendar. "A.D." means "the year of our L-rd," and we do not
believe Jesus is the L-rd. Instead, we use the abbreviations C.E. (Common or
Christian Era) and B.C.E. (Before the Common Era).
Months of the Jewish Year
The "first month" of the
Jewish calendar is the month of Nissan, in the spring,
when Passover occurs. However, the Jewish New Year is in Tishri, the seventh
month, and that is when the year number is increased. This concept of different
starting points for a year is not as strange as it might seem at first glance.
The American "new year" starts in January, but the new "school year" starts in
September, and many businesses have "fiscal years" that start at various times
of the year. Similarly, the Jewish calendar has different starting points for
different purposes.
The names of the months of the Jewish calendar were adopted during the time of
Ezra, after the return from the Babylonian exile. The names are actually
Babylonian month names, brought back to Israel by the returning exiles. Note
that most of the Bible refers to months by number, not by name.
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