This value for the average lunation cycle was the one used by Maimonides in his detailed presentation of Jewish calendar calculations.
Note that according to the method of construction of the Jewish calendar, the term Molad Tishri here refers not to the astronomical new moon time but rather that determined from the time of the Molad Tishri of year two (or the fictitious Molad Tishri of year one), the number of the lunar conjunction, and the average lunation period (see note 10 below).
Once this time is established, the time for the "beginning of time" and the imaginary Molad Tishri of Jewish calendar year 1 are determined by subtraction of 6 and 12 lunation periods, respectively.
However, the "average" lunation is based not on observation but on the unseen, theoretical conjunction of sun, moon, and earth; it is easily calculable (by tracking lunar eclipses), which is part of why it eventually supplanted the observance of the new crescent in the Jewish calendar.
The contemporary Jewish calendar maintains an uneasy symbiosis between two cycles of time, which run independently of each other: the Sabbath and the lunation, as Heschel admitted.
It is an alien model which exhibits a useful correlation between lunations
, phases of horticultural activity and Gregorian calendar months--but it is based on a comparison that does not arise in Torres people's minds.
The last record to be obtained in which both M5 and M4 could be positively identified in a lunation was that of 1985 Sept
There is no particular reason to suppose that they would not remain so until the beginning of yet another lunation, so the contention that both M5 and M4 may be regarded as Mountains of almost Eternal Light is held to be fully justified.
Any observer willing to systematically monitor specific features from lunation
and year after year - to become familiar with what changes are expected and what might be anomalous - will provide interesting data.
Shaved even more into a waning crescent visible in the dawn, it finally disappears to be reborn a couple of days later in the next lunation
In other words, a lunation can be as much as 6h 13m shorter, or 7h 15m longer, than its average value of 29d 12h 44m.
But the perigee new Moon defining the shortest lunation of the 9-year cycle need not occur near the July aphelion date; it can come as early in the year as May 5th (1636), or as late as July 28th (2337).