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Introduction to the Jewish Calendar

Introduction to the Jewish Calendar


Since Biblical times, various astronomical phenomena have been used to establish uniquely Jewish definitions for the day and its hours, the months and the year.

The length of days and hours vary by the season, controlled by the times of sunset, nightfall, dawn and sunrise. The months and years of the Jewish calendar are established by the cycles of the moon and the sun.

Though the months follow the lunar cycle, the lunar months must always align themselves with the seasons of the year, which are governed by the sun. Thus, the Jewish calendar is "Luni-Solar." The discrepancy between the solar year (365 days) and the lunar year (354 days) was resolved by every so often adding a thirteenth month to the year, to form a "leap year."

In the early times of our history, the High Court (Sanhedrin) in Jerusalem was assigned the tasks of determining the beginning of each month and the balancing of the solar with the lunar years. They relied on direct observation of the New Moon, astronomical data, and other considerations.

In the fourth century after the Temple's destruction, however, when oppression and persecution threatened the continued existence of the Court, a fixed calendar was instituted -- based on the Sanhedrin's closely guarded secrets of calendric calculation. This is the permanent calendar according to which the New Moons and festivals are calculated and celebrated today by Jews all over the world.

Like the original system of observation, it is based on the Luni-Solar principle. It also applies certain rules by which complex astronomical calculations are combined with the religious requirements into an amazingly precise system.

The following pages will provide a brief digest of the factors which control the determination of the Jewish hour, day, month and year.

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Discussion (27)
January 26, 2014
can the jewish calander be used to calulate the end time
January 21, 2013
Hebrew date for Yahrzeit on leap year
Was there a ruling or law or discussion in the past about what Hebrew date to use if the Yahrzeit date was on 30 Adar I and it is now not a leap year?
Los Angeles, CA
December 29, 2012
Why do you spell God as G-d?
August 24, 2012
To Anonymous, Port Vila, Vanuatu
People are free to follow whichever calendar they wish.
Rabbi Menachem Posner
August 24, 2012
Does the whole world need to follow the Jewish calendar?
port vila, vanuatu
August 24, 2012
Isn't the Jewish calendar based on harvest times?

Makes sense to me.
Meira Shana
Vista, California
April 8, 2012
I live in American and I noticed on this site the calendar has the Jewish days and American days all in one. Is there a place where I can purchase one just like the one on this site.
HOPKINS, Minnesota
November 27, 2010
Calendar change
Yaakov you could be right I was working from assumption not a source. I know they moved 11 days, I assumed that therefore they lost a few days of the week.
Sydney, Australia
November 26, 2010
To Anon in Sydney
Where did you get that from? The Gregorian/Julian switch only affected the days of the calendar (which have no connection to the Jewish calendar). They days of the week remained intact. Please provide a source.
Yaakov Shwecky
November 25, 2010
Calendar change
What was the halachic approach to the change from julian to gregorain claedar in the 1700s? The days of the week changed. What did we do about shabbat?
Sydney, Autralia
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