How Jewish Leap Years Came to Be Calculated

The Jewish year 5777 has been slow in arriving. In fact, it has pushed things practically to the limit, since the latest date in the solar calendar on which the lunar-calculated first day of Rosh Hashanah can fall is October 5—only two days later than its occurrence this year. That’s because 5776 has been pregnant, as was 5774.

A “pregnant year”—shanah me’uberet—is the Hebrew term for a lunar leap year, and it has been in use at least since talmudic times. The Hebrew verb l’aber means to impregnate, and ubar is a fetus, and it’s easy to see why a year with an extra month is suggestive of a woman with child. The act of intercalation, or adding more days to the year, is known in Hebrew as ibur ha-shanah, “impregnation of the year”; the method of calculating it as ḥokhmat ha-ibbur, “the science of impregnation,” or sod ha-ibbur, “the secret of impregnation.” Both terms reflect the fact that in ancient times, before fixed calendars existed, determining leap years was not a simple matter.

Intercalating a lunar calendar is a necessary measure if one wishes to keep it aligned with the solar calendar. The length of a lunar month—that is, the time between one new moon and the next, or between any phase of the moon and the next exact recurrence of it—is 29.53 days. This means that a lunar year of twelve months has 354.36 days in it, almost 11 days fewer than the 365.25 days of a solar year.