Calendars: Jewish and Christian

1. The "Common Era" (aka "Anno Domini") and "Before the Common 
     Era" (aka "Before Christ") // CE=AD, BCE=BC

Due to an error in changing various relative datings (e.g. "in 
the 253rd year of the Seleucid era"; "in the 21st year of king 
X") to a scheme determined by the supposed birthday of Jesus, we 
have the strange situation that Jesus was born before 4 BCE/BC.

Otherwise, the calendar used for determining the length of a 
year in the standardization was the "Julian" calculation 
(identified with Julius Caesar), which proved to be slightly 
innacurate. In the 18th century, most of the western world made 
the adjustment to the "Gregorian" calendar with its increased 
accuracy. Thus Gregorian dates prior to about 1758 do not 
coincide exactly with Julian dates -- e.g. George Washington was 
born on 11 (?) February 1732 as far as he and his parents were 
concerned (under the accepted Julian calendar), but by the time 
he died in 1799, his birthday was adjusted to the Gregorian 
equivalent of 22 February. Genealogists and other people working 
with such details of history need to know such things!

So now we have 365.25 (more or less) days in our SOLAR year 
(determined by the earth's relationship to the sun), and we 
adjust for the partial day by inserting a "leap year" every 4th 
year (plus some nanosecond adjustments every so often).

We also have 12 months of varying lengths between 28 and 31 days, 
and 7 day weeks that total 52 and a fraction each year. Not a 
particularly simple or symmetrical system, but it works for us!

2. The classical Jewish LUNAR calendar, adjusted to SOLAR cycles

The calendar that is recognized by classical Judaism is, on the 
one hand, basically LUNAR, reflecting the cycles of the moon; on 
the other hand, it also has strong agricultural aspects which 
require adjustment to the SOLAR cycles. Thus it can be called 
LUNI-SOLAR. 

The cycle of the moon is approximately 29.5 days (?), and the 12 
lunar months in a consistently lunar calendar would produce a 
year of 354 days, about 11.25 days short of a solar month every 
year, or about a month short every three years. Thus an 
additional 12th month (the "second Adar") is "intercalated" every 
third year. 

3. The ancient Jewish SOLAR calendar and symmetrical year

In the Dead Sea Scrolls, a different calendar reckoning is 
sometimes found (alongside the adjusted lunar). It uses a year of 
364 days, which divides neatly into 52 weeks of 7 days each, with 
no remainder. Thus the new year and other annual holidays would 
always begin on the same day of the week (normally Wednesday, for 
Passover, Day of Atonement, etc.).

The 12 months each contained 30 days (= 360 days per year), but 
between each quarter (3 months), and additional day was 
included, thus providing the symmatry of 91 days per quarter 
(with 30 + 30 + 30 + 1 as the normalized scheme). Neat, but not 
really solar (about 1.25 days short each year), so some sort of 
intercalation would be needed every so often (e.g. every 24 years 
the calendar would be a month out of whack). We don't know how 
this adjustment was determined, but possibly in relation to the 
concept of "jubilee" (every 7th) and "great jubilee" (every 49th 
or 50th) years, which were also a feature in this group.

4. The classical Muslim LUNAR calendar

[to be added]

/end; draft 2 Oct 1997/