Rosh Hashana & The Jewish Calendar

Rosh Hashana, the Jewish New Year, is an opportune time to think about time! Time is everywhere, but it’s not just a natural, uniform and absolute entity that we measure. The thousands of calendars in the world indicate that time has cultural dimensions – it is flexible and may be manipulated. Prof. Ely Merzbach of the Department of Mathematics at Bar-Ilan University discusses the calculations and considerations that have shaped the Hebrew calendar for some 1,100 years.

Every nation needs a calendar as part of the common foundation of its spiritual and material life. Calendars are always based on astronomic manifestations of the sun (like the civil Gregorian calendar we use) or of the moon (like the Muslim calendar) because they are cyclical, fixed and may be observed. The Hebrew or Jewish calendar is lunisolar, synchronizing the holidays with the seasons and the moon phases. In the past, the Sanhedrin would decide whether to add a month at the end of the year, that is, at the end of Adar, before Passover. As the calendar became more sophisticated, leap years were determined on the basis of mathematical calculations and formulas alongside certain preconditions relating to the Jewish holidays.

Passover, for example, must occur in the spring, unlike Ramadan, which migrates across the seasons. Rosh Hashana is marked by the appearance of the new moon ushering in the month of Tishrei, but it cannot be celebrated on Sundays, Wednesdays and Fridays, because that also determines the day on which Yom Kippur and Hoshana Rabba will occur. We don’t want Yom Kippur to fall on Sunday because of the difficulty in preparing for it on the Sabbath, nor do we want it to fall on Friday, in order to allow for Shabbat preparations. Rosh Hashana will also be postponed in the case of “molad zaken” – the first appearance of the new moon in the afternoon hours. “Molad,” by the way, is the time when the moon is positioned exactly between the sun and the earth. It always happens 29 days, 12 hours, 44 minutes and 3.3 seconds following the previous “molad.” This number already appears in the Talmud, in the tractate Rosh Hashana.

Once in 19 years, the solar and lunar cycles converge – so that the Hebrew and English birthdays coincide (within a day or two) once every 19 years. In such a cycle, the earth orbits around the sun 19 times, and the moon encircles the earth 235 times. From here are derived calculations related to determining leap years and deciding whether the Hebrew months of Kislev and Heshvan should be 29 or 30 days long. After listening to Prof. Merzbach’s podcast, you too can calculate the specific dates of the Rosh Hashana holidays in the years to come.

To listen to Prof. Merzbach’s Hebrew podcast about the Hebrew calendar, download Bar-Ilan’s free podcast app, Bar-Daat https://www.bardaat-biu.co.il/