The Molad Drift
by Remy Landau
The value of the period of the molad is an average value. Consequently, the value of the time of the molad, as derived by the fixed calendar method, is not necessarily the actual time of the new moon.
This idea was noted by Maimonides (also known as the Rambam) in his medieval work Hilkhot Qiddush HaHodesh. At chapter VII:VII Maimonides stated that
...these calculations determine the conjunction of the sun and the moon only according to their average rate of progress, and do not reflect their true positions...
The traditional Hebrew value for the period of the molad is given as 29d 12h 793p.
The value has the rather interesting property that the halakim when remaindered by 18 (the number of halakim in one minute) increment by 1 helek from Hebrew month to successive Hebrew month.
This phenomenon can be seen from the molad tables in
Converting to decimal form the traditional Hebrew time of the molad period, it is possible to have it compared to a value
for the time of the mean lunar conjunction as referenced from the
US Naval Observatory web page.
the period of the molad = 29.5305 941 358 ... (approx)
the astronomical mean = 29.5305 888 531 ... (approx)
The difference between the period of the molad and the above given astronomical value of the mean lunar conjunction is 0.0000 052 827 days per mean lunar period.
Hence, the difference between the molad period and
the astronomical period is about
0.456 425 seconds per lunar month, equivalent to 0.1369 parts per lunar month.
Consequently, the traditional Hebrew value for the lunar month differs from the current astronomical value by about 1 day in every ((86,400/0.456425)/235)*19 = 15,304.883 years.
Up to Rosh Hashannah 5759H (Mon 21 Sep 1998g) 71,217 Hebrew months had elapsed. Consequently, the difference between the elapsed moladot and the astronomical periods had widened to 9.029233 hours, equivalent to 9h 1m 13.57p.
Because of the constantly widening gap between the time of the molad and the time of the astronomical mean conjunction, as well as the unknown value of the time of the Hebrew calendar's first molad, it is very difficult to suggest the geographical location over which any molad takes place.
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First Paged 27 Aug 2000
Next Revised 30 Dec 2012