Birkat Hachama - Lerman's Thesis

Lerman's Thesis

Moshe Lerman suggested a background to Birkat Hachama by pointing out a possible connection between the traditional Hebrew dating and the two machzorim ("cycles") that are observed in Jewish tradition—the "small" 19-year cycle which is the basis of the Jewish calendar, and the "big" 28-year cycle which determines the year in which Birkat Hachama is recited. Mathematically, if one knows the position of a certain year in both cycles, one can compute the number associated to the year modulo 532 (19 times 28), given that the starting point of both cycles is year 1.

Because the astronomical year is slightly shorter than 365.25 days, the date of Birkat Hachama shifts away from the spring equinox as history proceeds. A simple astronomical calculation shows that 84 cycles of 28 years before 5769, in the Jewish year 3417, the spring equinox was in the beginning of the night before the fourth day of the week as stipulated by the Talmud. Lerman takes this as a hint that the astronomically astute Jewish sages of the time concluded that the Jewish year 3417 was a first year in the cycle of 28 years. Moreover, Lerman suggests that these same Jewish sages would have reasoned that year 3421 was a first year in the 19-year cycle, in accordance with an ancient tradition that the world was created in the first week of the month of Nissan, and thus the sun was created on the fourth day of Nissan. Since every 19 years the solar and lunar calendars align, and the Spring equinox of 3421 occurred early in the night leading to the fourth day of the Jewish month of Nissan, it follows that 3421 was the first year of a 19-year cycle.

Lerman surmises that the Jewish sages at the time could argue for a determination of the position of their years in both cycles and could therefore compute the absolute year-count modulo 532 years. They were left with a number of options, 532 years apart from each other, and Lerman suggests that they chose the dating closest to what seemed to be the truth according to a literal interpretation of biblical accounts. The sages legally defined future equinox times by instituting the 28-year cycle, to protect the Hebrew dating against future change, and to leave a remembrance to what they had done.