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  • Allen Versfeld
    April 23, 2011
Astronomy‎ > ‎Questions and Answers‎ > ‎

How do you calculate the date of religious festivals?

Cultures around the world set aside special days every year as festivals, or times of remembrance.  Modern examples (Worker's Day, Mother's Day, Independence Day, Valentine's Day and others) all last 24 hours and take place on the same date every year.  But the really old festivals, as observed by world religions, are often a little more flexible.  Their dates can vary from year to year, sometimes differing by as much as a month.  What gives?
Astronomy wasn't always the aesthetically pleasing scientific pursuit we recognise today.  It used to deal with very fundamental and important issues of daily life:  When do we harvest?  When will the winter end?  When will the moon rise, so we can travel or hunt by night, and when will it be too dark.  With astronomy dealing with such daily matters of survival, it makes sense that the heavenly cycles would be used to indicate more ethereal matters:  When do the gods want us to migrate?  Is this a good time to marry, or should I wait for a more auspicious time?  So astrology was born (and remember that until only a few hundred years ago, astronomy WAS astrology), and so religions turned to astronomy to time their festivals.  Some of the more important festivals are described below:

Easter, and related festivals

In the Christian Calendar, festivals are either timed according to a fixed calendar date (Christmas, for example), or in relation to Easter.  Ascension Day is celebrated forty days after Easter, while Lent is a forty day period of fasting ending on Easter.  Here's the first complication:  Since Sunday's are holy days, people celebrating Lent do not need to fast on those days.  Accordingly, the calendar duration of Lent is actually forty days not counting Sundays, which is usually forty six days.  And then there is Good Friday, which is the Friday directly before Easter.  So when is Easter celebrated?  The official formula, as determined by the first council of Nicea in 325 AD, is that Easter shall fall on the first Sunday after the first full moon after the March equinox.  In other words, Easter can fall any time between March 21 and April 25.  But before you go making your own calculations, you need to note that the formula relies on slightly different definitions of astronomical terms.  Full Moon, in this case, means "fourteen days after the New Moon", and the March Equinox is assumed to always fall on 21 March.  Both of these are merely approximations of the true definitions, to prevent errors and disputes.


The Jewish festival of Passover (or Pesach in Hebrew) is an interesting case:  It's date is determined according to the Lunar calendar, which does not correspond to the standard Civil Calendar.  In the Jewish Lunar calendar, a month starts immediately after New Moon, when the first sliver of the Moon's crescent is visible.  Since the phases of the moon follow a cycle of about 28.5 days, each Lunar month lasts either twenty-eight or twenty-nine days.  The problem with this system is that the Lunar Cycle runs 12.4 times per year, so that the Lunar Calendar slips out of sync with the seasons quite rapidly.  For this reason, a thirteenth Leap Month is inserted every few years.
So when is Passover?  By the Lunar Calendar, it is on the same date every single year:  on the 14th of Nisan.  What does that correlate to in the Civil Calendar?  Like Easter, it can fall anywhere from mid-March to mid-April.


Islam also follows a Lunar calendar which, unlike the Jewish calendar, is not synchronised with the seasons.  Accordingly, the Islamic Lunar Calendar needs no correction, and has no leap months.  The Islamic year thus is shorter than the civil year, measuring twelve Lunar cycles and not one Earthly orbit.  As a result, a specific Islamic date will wander all over the civil calendar as the years pass.  Ramadan is actually the ninth month of the Islamic Lunar Calendar, and the entire month is considered a holy festival in which all able-bodies Muslims must fast from sunrise to sunset.

So we see that the Christian calendar is the modern civil calendar, but the dates of festivals are calculated afresh every year, based on astronomical events.  The Jewish and Islamic festivals are on fixed dates every single year, but these dates are from their own calendars, and adherents must refer to tables to determine what days on the civil calendar they coincide with.

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