The Fool wrote: > >It was my understanding that the Jewish calendar (which is lunar) added a >thirteenth month every couple of years in a periodic fashion. > Not exactly. The actual formula is much more complex. Damn. I had a link to a "Calendar FAQ", but, as usual, the link is dead :-/
Anyway, since the lunar month is about 29.5 days long, there are about 12.4 lunar months per tropic year, so it's not an alternation of years with 12 and years with 13 months. The figure 29.5 [whose exact value I have somewhere, and which is calculated with absurd accuracy in the Jewish calendar] is the mean of the *synodic* period of the Moon, which corresponds to the period between two new [or full or same phase] Moons. IIRC, there's an A4P entry about that. Alberto Monteiro
