Biblical Calendars

Thinking maturely about the Christian faith
Search

// you’re reading...
Theology

Biblical Calendars

By Rowland Croucher and others ⋅ January 5, 2003 ⋅ Post a comment
SUSAN ERICKSON () wrote:
> If God didn't create this earth and everything on
> it in 7 days where
> do we get the time span of a week? The 7 days of
> creation is where.
> It's not measured by the sun or moon as days and
> months are.

[To be technical, but pedantic, the Genesis account
has six days of creation, followed by a day
of rest.]
-------- end quoted material

The week is about a quarter of a lunar month, but
that "about" is important when looking at the
OT or Jewish calendars. Although in biblical times
the month was done by actual observation, it
would have averaged close to 29.5 days, i.e. close
to alternating 29 day and 30 day months. I
claim this difference from 28 is important since a
28 day month would be a multiple of 7 days,
the period of a week -- the weekly sabbath. As it
is, the week is the calendar unit (day, week,
month, year) that has no direct astronomical basis.

How about different years, i.e. a different
lengths of years to try to explain the extraordinary
lifetimes of some early patriarchs? Ancient Jews
knew, at least approximately, the length of a
solar year (our 365 days) although their years
never had that amount. See below for details of
why the 12 lunar month year was too short and the 13
lunar month year (a "leap" year more
significant than our paltry one extra day kind!)
too long. But by a careful mix of these two, the
correct "average" was achieved.

Note that the really long lifetimes (the multiple
hundred of years) are all for the antediluvian
patriarchs, i.e. those before the flood. The
footnote in the NAB on Gen 5:1-32 includes the
comment:
"The long lifespans attributed to these ten
antediluvian patriarch have a symbolic rather
than a historical value.
Babylonian tradition also recorded ten kings
with fantastically high ages who reigned
successively before the flood."

The reference to ancient Babylonian king lists
bring to mind a memory of something that I read
which had one of these kings reigning for 30,000
years, I think. This makes those Book of
Genesis gray-beards seem just kids.

Once you get to Abraham in Genesis 12 (i.e. more
"historical" times rather than primeval
history) the ages are down further. Abraham dies
at 175 (according to Genesis 25:7). Later
Moses lives to 120. By the time of the historical
books of Samuel and Kings the lifetimes are
essentially ours. There is an interesting psalm,
Psalm 90, which is attributed in its
superscription (heading) to Moses(!). Psalm 90:10
starts "Seventy is the sum of our years, or
eighty, if we are strong." 

I think the original question was more interested in
whether the year involved in the multi-hundred
year lifetimes attributed to some people in the
early part of the Old Testament was the same as
ours. However, since this question does involve the
calendar, I include below a slightly modified
version of something I had previously written on the
subject.

CALENDARS: DAYS, WEEKS, MONTHS, AND YEARS

Edward Pothier July 1999

Of the four calendrical units (day, week,
month, and year) all have astronomical significance
except for the week. And, from the Biblical
perspective, it is this weekly (the Jewish Sabbath or
the Christian Sunday/Lord's Day) aspect which might
be the most
significant, despite its semi-arbitrary origin. 

Remember the story of Creation by God in six
days, followed by a seventh day of rest, in the
first chapter of Genesis. In the Ten Commandment
list in Exodus 20 (but not in the list in
Deuteronomy 5!) the six days of creation followed by
a seventh of rest is given as the
*explanation* of the Sabbath. [The Deuteronomy 5
retelling of the Sinai event, while giving a
Sabbath commandment with slightly different wording,
gives as an explanation that the Israelites
had been slaves in Egypt and were brought out by the
LORD.]

Knowing what we do now, we can describe the day,
month and year in terms of earth, moon
and sun relative motions, i.e. astronomy. 

(1)The DAY is how long it takes the earth to
rotate on its axis one complete rotation with
respect to the sun. There are some
differences in the Bible and other places about how
you measure or define an individual day -- is
it from sunset to sunset, sunrise to sunrise,
midnight to midnight? The main Jewish way in the
Bible was from sunset to sunset. [Remember, the
"evening came and morning followed, the Nth
day" refrain in the Genesis chapter 1 Creation
story.] There are some numbering systems of the
hours of a day which start at sunrise, i.e. more the
hours of "daylight" than "day". 

Our usual modern system of a day from midnight
to midnight is possible with our modern
time-keeping devices, but impossible in biblical
times. Sunrise or sunset is determinable by
observation (weather permitting) with little error,
but how could you tell the middle of the night
(midnight) without clocks. [Of course, the
phenomenological language of the sun going around
the sky in a day, rather than the earth
rotating on its axis, is used in the Bible.] 

(2)The YEAR is the time it takes the earth to
make a complete revolution around the sun. As
we know now, it is very slightly less than 365 and a
quarter days -- the "slightly less than a
quarter" is why we add a leap year day *almost*
every four years.

(3)The MONTH is actually, in a lunar calendar,
the time between complete passages through
the phases of the moon. The words for
"month" and "moon" are related in English and some
other languages. In Biblical times the
month started with the first observation of the new
moon, and it was done by observation! The
months were equally often either 29 or 30 days long,
neatly balancing the actual astronomical
29.53 days. Our modern months of 30 or 31 days
(except for February's depleted-length 28 or 29)
are actually too long to keep in phase with the moon
and are more of an *artifact* to make up a
solar year with twelve months (totalling 365 days or
366 days in a leap year). Our present
months have no astronomical meaning in themselves. 

The biblical months of 29.5 days on the
average (while keeping quite well in phase with the
moon!) give a twelve month year of only 354 days
(29.5 days per month times 12 months). This
is way too short, more than 11 days too short every
year. Feasts set to a calendar date on such
a purely lunar calendar would soon migrate backwards
through the seasons (as the Muslim ones
now do, since they do follow a purely lunar calendar
with no solar year adjustment). You would
eventually start to celebrate Passover (clearly a
Spring festival) progressively
backwards, first in then Winter then in later years
in the Fall and Summer and then back to
Spring -- unacceptable! The Jewish method involved
"leap" years, which involved insertion of a
whole extra month. Although at the time of Jesus,
the determination of when an extra month
would be inserted in a year (as well as when each
month was started by the new moon) was
done by observation and Jewish court decision, later
(several centuries into the Common Era)
this was systematized. And there are 7 leap years
out of every 19 year cycle of years -- 19 years
being the interval in which mathematically every
thing balances out sufficiently well.

In these leap years a whole second month of
Adar is
intercalated before the month of Nisan (the first
month in the
Spring, under one of the counting schemes of
months). There is both a First Adar and a Second
Adar in these years. Note than slightly more than a
third of the years (7 out of 19) are such leap
years with an extra month, this to balance the
more-than-a-third of a month shortage in a regular
year of twelve lunar months. Then the 19 year cycle
would repeat. Calendar dates never get *too
far* out of proper season, never more than a
fraction of a month.

(4)The WEEK of seven days has no astronomical
significance. Seven is the integer closest
to a quarter of the 29.5 days lunar cycle, but not
exact. As mentioned in the very first
paragraphs, the seven day week ending with the
Sabbath rest is given as a religious "given" in
the Old Testament accounts. It is understood as
being by divine command (and in the Book of
Exodus elaboration of the Sabbath commandment by
imitation of the divine work and rest).

= Edward L. Pothier