Jewish Calendar System

The Aish Rabbi Replies

The Jewish calendar is based on both the solar and lunar cycles. Every Jewish month begins with the New Moon. However, since the lunar "year" - twelve lunar months consisting of around 354 days - is shorter than the solar of about 365.25, the Sages add occasional "leap months" at the end of the year. This ensures that the holidays occur in their proper seasons (e.g. Passover in the springtime and Sukkot in the fall - see Deut. 16:1 and Exodus 34:22). On such leap years, instead of one Adar (the Jewish month in which Purim falls), there are Adar I and Adar II (with the main Purim observed in the second).

Until late Talmudic times the courts would decide when to add a leap month (based on seasonal factors and the like) - as well as proclaiming each new month based on eyewitness testimony, but at a point it became infeasible to maintain this system. Then, in the mid-4th century, Hillel II created a set calendar system. At its core, it consists of a 19-year cycle, in which 7 leap months are interspersed over that period. This created a continuous cycle of lunar-solar years remarkably parallel in length to actual solar years (see below). His system also addresses other calendrical requirements - such that the holidays fall out on their proper days - for example that Yom Kippur not fall out just before or after Shabbat. (This is set in part by adjusting the length of the months of Cheshvan and Kislev.)

The Jewish calendar thus stands in contrast to the solar civil (Gregorian) calendar, where the months have completely lost their relation to the moon. It is also different from the Muslim calendar, an absolutely lunar system, in which the months (and holidays) wander through all four seasons.

Interestingly, Jewish months are calculated at 29.53059 days, following the cycle of the moon. This practice is based on the Talmud (Rosh Hashanah 25a), according to tradition dating back to Moses and codified by scientists such as Ptolemy.

Incredibly, it took the modern world many centuries to confirm this figure. Only after calculations using solar satellites, hairline telescopes, laser beams and super-computers, did NASA scientists determine that the length of the "synodic month," i.e. the time between one new moon and the next, is 29.530588 days.

Amazing!

(sources: "The New Encyclopedia Britannica," 1990 Micropeadia, Volume 2, p. 740; "Blessing of the Sun" by Rabbi J. David Bleich, ArtScroll-Mesorah Pub., pp. 47-48; "Torah Shleima" by Rabbi S. Kashir 8:7; "Korot Cheshbon Ha'ibur" by Rabbi Z. Yaffe; "Perush" of Rabbi Ovadia Ben Ovadia - Laws of New Month 6:3)