Subject: (urth) Re: lunar calendars [Digest urth.v028.n014]

From: John Bishop <jbishop at blkbrd.zko.dec.com>
Date: Thu, 12 Aug 1999 16:08:53 +0000

Mantis says:

> Lunar calendars have thirteen months (13 x 28 = 364).  Period.

Not that I ever read.  The islamic calendar still offically
depends on a witness to the new Moon, the Judaic calendar used
to (before it could rely on calculation), and the Classic 
world's lunar calendars did also (in particular, the calendars
used to calculate possible eclipse times), though in later
times astronomers calculated a sequence of short and long
months that would be predictable (by reference) but never
more than half a day off the real new Moon.

A fixed lunar calendar would pretty quickly cease to be aligned
with the moon, which kind of loses the point, and produces the
kind of "months" we have, which are unrelated to the Moon.

Thus a year would be either 12 or 13 months, depending on whether
it had an extra month or not (Judaic "second Adar"), and the 
months would have either 28 or 29 days.  Trusted witnesses would
look at the sky, searching for a new Moon to start a new month,
and a trusted (priestly) authority would determine when to insert
an extra month depending on astronomical observations (such as
the Pontifex Maximus in Rome).

During the late Classic time, there was a 19-year cycle of long
and short months, long and short years which kept the Moon phase
predicted by the calendar within half a day of the real Moon
phase, and the seasons within half a month of the real season,
so direct observation (and the consequent need for messengers to
spread the news, etc.) wasn't required.

It wasn't perfect (there was a long-term drift), and there was a
57-year cycle which solved most of the drift, but it never became
widely accepted.  The 19-year calendar was useful because the
eclipse cycle is also 19 years (due to the precession of the
moon's orbit, which moves the location of the ascending and 
descending nodes of the moon's orbit along the ecliptic).  You
could put "eclipse" seasons into the 19-year cycle: they marked
the points at which the Earth, Moon and Sun were aligned in such
a way that an eclipse was very likely.

See Abell's astronomy text and various encyclopedias.  I once long
ago programed up a calendar maker which used modern data to construct
the 19 and 57 year calendars, but I don't know if the Classic ones
would have had the same distribution of longs and shorts.

        -John Bishop

*More Wolfe info & archive of this list at http://www.urth.net/urth/

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From: Michael Andre-Driussi <mantis at sirius.com>
Date: Thu, 12 Aug 1999 14:52:29 +0000

John Bishop,

>kind of "months" we have, which are unrelated to the Moon.

Maybe there is trouble caused by the term "calendar"?  That is, if I wrote
"Lunar reckoning" rather than "Lunar calendar," would that clear up some of
the problems?  ("Calendar" carries such imperial trappings, such a sense of
precision, or something written down, at least.)  Because by the "Lunar
reckoning" I'm describing, the situation can =never= cease to be aligned
with the Moon; by the same token, dates are about as fluid as the watches
of the day (where they have no mechanical timepieces, nor do they even
assign sequential numbers to the watches).  And years are numbered by the
reign of the Autarch.

I am aware of the "Lunar" calendars you mention, and frankly I think they
get into the trouble they do because they are trying to match the solar
year (and see next paragraph).  I sense that they are hybrid Lunar
calendars, born from a tradition of simple Lunar reckoning and then
retrofitted to approximate a solar calendar.  (I also believe that the
number of thirteen months to Lunar reckoning is the largest contributor to
the taboo/"unlucky" nature of the number 13--shows how deep these arguments
run!)

I'm also aware that those pesky fractions (29.nnn days per lunar month)
will throw the counting off--after all, that was the impetus to have
alternating long and short months (I've seen Chinese women calculate long
and short months of their Lunar calendar by somehow counting on the
knuckles of their fists--don't ask me how).  Then again, what with the
terraforming and all, the 1.nnn fraction may have been shaved off!  (And 28
is handy because it divides evenly by seven, and . . . uh oh, next we'll be
arguing on how many weeks there are to a year!)

In addition to historical examples I've also seen models for interesting
modern solar calendars (aside from that of the French Republic) that will
fix one problem or another.  One, for example, that has thirteen months
(the new one being "SOL," inserted between JUN and JUL) of exactly 28 days
each (13 x 28 = 364), where the 13th of every month is always a Friday, and
a special off-calendar day for New Year's Day (364 + 1 = 365), plus a bonus
off-calendar day every fourth year.  Whooh!

(Kind of like the "missing time" in the Martian day of Kim Stanley
Robinson's RED MARS--have a quick party, we're outside of time!)

Now then, I think the point of all this is =not= that Wolfe worked out any
sort of Lunar reckoning and/or calendar.  The point is (or so it seems to
me) that in the Commonwealth they are crushed by history; time to them does
not have the precise, mechanistic sense that it does to us--theirs is
"medieval" or even more primitive, yet it suits them fine.  The zinger
being that they are the heirs of a starfaring culture, yet they have sunk
to this level, a deeper, longer dark age, such that they do not have a
calendar (also implying a loss of astronomical skills) . . . the Aztecs
were basically neolithic, but they had a very elaborate calendar (to say
nothing of Mesopotamia).  So the Commonwealth would appear to be less
developed than a neolithic level in this regard.

Plus, hinting at different calendars/time reckoning systems, etc., is a
neat way to imply "otherness," to give the sense of strange-yet-valid (if
not very accurate).  One of the details I look for in sf.

And time proves to be even more fluid and fuzzy, as the story progresses.

=mantis=

---

From: Derek Bell <dbell at maths.tcd.ie>
Date: Sat, 28 Aug 1999 21:15:38 +0000

>more than half a day off the real new Moon.

      The Islamic calendar is also exclusively *lunar* based with a
mean year shorter than the solar year. I once did a
back-of-the-envelope calculation about when the two calendars would
have the same year simultaneously - sometime after the 30th century,
IIRC. 

>would have had the same distribution of longs and shorts.

       I'd also suggest _Calendrical Calculations_ by Dershowitz and
Reingold - a web site with sample material is at
http://emr.cs.uiuc.edu/home/reingold/calendar-book/index.shtml

	Derek Bell