Re: Why not 13 months? (Was La Systeme Metrique)

Michael L. Siemon (
Sat, 15 Jul 1995 19:16:21 -0400

In article <3u9fno$>,
(Michael J. Tuvin) wrote:

+The problem is that the length of the lunar month varies rather broadly
+because of the complicacy of Moon's movemements. If I am not mistaken it
+varies in the range of several minutes (4-6?).

The mean length of a month is very close to the figure used in Jewish
calendrical calculations (which figure comes from the Babylonians),
namely 29 days, 12 hours and 793 halakim (a helek is 1/108 of an hour.)
this comes out about 1/3 of a second long.

The moon's motion is extremely complex -- besides the 3-body problem
of sun-earth-moon, there are significant planetary contributions (most
notably from Jupiter.) The actual length of a month can vary about 0.6
days from the average (probably more, theoretically, but a scan of the
mean versus actual new moons over the last 2000 years shows a max-
imum divergence over that period of 0.6 days.)

+Thus it is very
+difficult to get a reasonably correct average length. Astronomy of
+that period is not known to be able to make such measurements.

That turns out not to be the case :-). In fact, Babylonian observation
over centuries comes up with an exceptionally good value for the length
of a month, and that is what's used in the Jewish calendar since Hillel II.

+Go figure how they managed it...

It is known how they managed it -- they borrowed the value from the
Babylonians (along with the month names, the scheme of intercalary
months in the 3rd, 6th, 8th, 11th, 14th, 17th and 19th years of a 19-
year cycle, etc.) This same value was known to the Greeks, from the
same source.

You realize, I hope, that Nisan and Adar and all the rest were the names
of the Akkadian months a millenium before there were any Jews?

Michael L. Siemon (

"Stand, stand at the window, as the tears scald and start;
you shall love your crooked neighbor, with your crooked heart."