Re: Why 7 day week ?

Michael Siemon (
15 Feb 1995 21:03:22 -0500

In article <>
("Daniel Cohen") writes:

>I had lways assumed that the 28-day month was based on moon cycles (though
>strictly speaking the cycle is nearer 29 days than 28), and the 7-day week
>is a convenient way of breaking the month into shorter periods. If this
>is true, it would suggest that both are ancient and widespread without need
>for importation.

The average month is closer to 30 days than to any other integral value.
It *is* common to find the "month" broken into 10-day periods. 7-day
periods just don't work very well. The reason they seem to show up in
Mesopotamian-influenced sources is a kind of coincidence: If you have
socio-religious "observation" of new moons and full moons, then (before
sophisticated calculations of the invisible moment of conjunction), you
tend to mark the "new moon" at first sighting of the crescent (this is
still the "official" way to mark months in the Muslim calendar), and
it then transpires that full moon is *pretty* well marked as 14 days
later (usually rather less, but full moon is hard to gauge visually.)

Mesopotamian moon observance eventually got to the point of marking
the quarters as well, and for that 7 days is pretty reasonable (the
average lunar quarter is 7.3825 days). By the end of the Babylonian
era, there was a sequence of 7-day observations (new moon->1st quarter
->full moon->3rd quarter, with an 8 or 9 day wait thereafter until the
next new moon. The Jewish (invariable) 7-day week *may* have been a
simplification of this. As far as I know, there is no data on which
the question can be empirically decided, so one is faced with a lot
of more or less plausible speculation from extremely exiguous data.

The Greeks used a 10-day cycle, which (roughly every other month) was
implicitly a day short in the third decan. The Egyptians had some kind
of lunar calendar for religious purposes, and also used decans in some
astronomical contexts, but I don't think they used anything from that
for secular purposes, as the Greeks used decans. It then becomes unclear
whether the Greek usage was influenced by Egypt (quite possible) or
independent (also, in a case like this, quite possible.)

Joseph Needham's volumes on _Science and Civilization in China_ have a
fairly extensive treatment of calendar issues in that culture area --
unfortunately, I don't have the volume in question and I've only seen
brief cross-references to this in his later volumes, so I don't know
whether he treats this issue of month-divisions in China in a compelling

Michael L. Siemon We must know the truth, and we must love the truth we know, and we must
act according to the measure of our love.
-- Thomas Merton