Leap Year - Additional Month

QUESTION: Why is it that in some years an extra month of Adar is added to the Jewish calendar?

ANSWER: The Jewish calendar is based on the cycles of the moon. A lunar month (from one new moon to the next) is approximately 29 ½ days. Since two lunar cycles are about 59 days, the length of a Jewish month alternates between 29 and 30 days. A lunar year (which consists of 12 lunar cycles) is approximately 354.36 days, while a solar year is about 365 ¼ days. Thus, the lunar year is about 10.89 days shorter than the solar year(365 ¼ - 354.36). If the calendar would not be amended, Pesach (which falls on the lunar date of the 15th day of Nissan) would be progressively earlier on the solar calendar every year. This would be problematic because the Torah states (Shimos 23:15) that Pesach is celebrated “bichodesh ho’oviv”, in the month of spring. To correct the imbalance between the lunar and solar calendars, a leap month is added every 2 or 3 years, for a total of 7 leap years every 19 years. Since the extra month (Adar Aleph) is always 30 days, 210 days are added to the calendar over the course of 19 years (19 X 30). 210 is close to the 207 days (10.89 X 19), which is the number of days that that lunar calendar lags behind the solar calendar over the course of 19 years. To compensate for the extra three days, we shorten Kislev to 29 days (rather than 30 days) three times during the 19-year cycle. In this way, every 19 years the two calendars coincide. The year 5784 (this year) is the eighth year of the 19-year cycle.

____________________________________________________

The Gerald & Karin Feldhamer OU Kosher Halacha Yomis is dedicated to the memory of Rav Yisroel Belsky, zt"l, who served as halachic consultant for OU Kosher for more than 28 years; many of the responses in Halacha Yomis are based on the rulings of Rabbi Belsky. Subscribe to the Halacha Yomis daily email here.