Jewish Calendar System
I'm confused every year when the High Holidays come out at the beginning of September, or the middle of the month, and sometimes in October. How does the Hebrew calendar correspond to the English calendar?
The Aish Rabbi Replies
The Jewish calendar is based on both the solar and lunar cycles. Every Jewish month begins with the New Moon. To ensure that the holidays occur in their proper seasons (e.g. Passover in the springtime), an extra "leap month" is added 7 times every 19 years. Thus, the Jewish calendar is "luni-solar."
This is 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.
(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)