Jewish Calendar

Date: 03/28/2002 at 07:38:42
From: Ellie Spitz
Subject: Jewish calendar

If the Jewish calendar has the same number of days each year, why do 
the Holidays fall at a different time each year?  

Thank you.

Date: 03/28/2002 at 08:35:40
From: Doctor Rick
Subject: Re: Jewish calendar

Hi, Ellie.

The Jewish calendar is both a lunar and a solar calendar. 

The months are lunar months: they all start at the new moon, which 
makes them about 29 days long.

But twelve 29-day months would be only 348 days, making a 12-month 
year seventeen days short of a solar year. The Muslim calendar lets 
this discrepancy stand: it is a strictly lunar calendar, so the year 
is about seventeen days short, and the holidays rotate rather rapidly 
around the seasons. (The month of Ramadan can occur at any time of 
year.)

The Jewish calendar doesn't have this problem because an adjustment is 
made. In certain years a 13th month is inserted to bring the months 
back into alignment with the solar year. Thus the holidays do not 
rotate around the seasons: Passover always falls shortly after the 
spring equinox.

The variation that you see in the dates of Jewish holidays is due only 
to the fact that the months don't start on the same days as in the 
Western (Christian) calendar. As I said, each Jewish month begins at 
the new moon. But in the Western calendar, a month can start on any 
day of the lunar cycle.

It is worth noting that the timing of the Christian holiday of Easter 
acknowledges its close association with Jewish holidays. Christ was 
crucified in the season of Passover, and Passover starts on the 15th 
day (full moon) of Nisan, the first month of spring. Therefore Easter 
falls on the Sunday after the first full moon after the spring 
equinox. (That's a loose description that isn't entirely accurate.) 
Most of the time, therefore, Passover and Easter fall at about the 
same time; but occasionally they get out of sync by a month because of 
differences in the details.

Here is one interesting Web site on the Jewish calendar; the same site 
has a page on the date of Easter. I can't vouch for its accuracy 
because I am not an expert in this field.

  Historical Ecclesiastical Calendar: The Jewish Calendar 
  - Karl T. Hagen
  http://ourworld.compuserve.com/homepages/khagen/Jewish.html   

- Doctor Rick, The Math Forum
  http://mathforum.org/dr.math/