The Jewish Calendar

The Jewish calendar is based on the rotation of the Earth (daily event); the revolution of the moon around the earth (Lunar month / about every 29.5 days); and the revolution of the Earth around the sun (a solar year / about 365.25 days). These events are not directly related to each other. A solar year, therefore, is about 12.4 lunar months.



In the Jewish Calendar, all months begin on the new moon. Thus, each Jewish month has either 29 or 30 days. The problem with a strictly lunar calendar is that a 12-month lunar calendar is about 11 days shorter than a solar year. Also, the months would cycle around the year, falling at different seasons of the year.

 To compensate for this drift, the Jewish calendar uses a 12-month lunar calendar with an extra month added every few years. This allows all seasonal months, with their corresponding holidays, to always fall during the correct season. Chanukah, for example, begins on the 25th day of the Jewish month of Kislev. The 25th of Kislev always falls during the secular calendar month of December, but not always on the same secular calendar day.



Hillel II established the Jewish lunisolar calendar, which is still used to calculate Jewish holidays. Under this fixed calendar, which is based on mathematical and astronomical calculations, a 13th month is added 7 times in 19 years. The additional month is Adar II.