Is Easter This Year?"
Ardem A. Tajerian
greatest festival in the Christian Church is Easter, a movable
feast celebrating the resurrection of Jesus Christ. I have always
wondered about the Byzantine blend of mathematics, astronomy,
and folklore that are used to set the date for this holiest
of days. Especially intriguing is the variation from one tradition
to another of the date on which Easter is observed; Eastern
Orthodox churches, for instance, typically use a different date
for Easter than Catholic or Protestant churches. Why this difference?
the beginning, Christians of Jewish origin commemorated Easter
immediately after Passover, which occurs on the evening of a
full moon and corresponds to a fixed date according to the Jewish
(lunar) calendar. Hence, Easter could fall on any day of the
week. However, gentile Christians wanted Sunday to be the day
for celebrating the resurrection, which required that the feast
occur on a moveable date.
325 A.D., the Council of Nicaea decided that Easter should be
commemorated on the first Sunday after the first full moon on
or after March 21st which was the date of the vernal equinox
in that year. Thus Easter was bound to fall between March 22
and April 25. For each annual celebration, they thought to maintain
the same day of the week (Sunday), the same season of the year,
(Spring, shortly after the equinox), and the same relationship
to the astronomical full moon (a few days following the full
moon) that occurred at the time of Jesusı resurrection.
the time of Julius Caesar, the western world had functioned
according to the Julian calendar, whose errors were becoming
apparent even in the 4th century. Also, because the actual astronomical
equinox could occur several days away from the 21st of March,
additional error was introduced since the Easter calculation
as decreed by the Council would still proceed assuming an equinox
on March 21st.
the 16th century, the accumulated error had reached ten days,
enough to be noticeable even to the non-astronomically inclined.
In 1582, Pope Gregory XIII reformed the calendar and approved
the use of the system that bears his name. Also, to bring the
calendar into conformance with astronomical observations, ten
days in October were skipped. Most Catholic countries switched
to the Gregorian calendar in that year, but Protestant Europe
took almost another two centuries to be convinced. Once the
British Isles began using the Gregorian calendar in 1752, all
the western churches celebrated Easter on the same day. Russia,
the Balkans, and the Middle East did not switch to the Gregorian
calendar until the late 19th or early 20th century.
while virtually all secular institutions today are using the
Gregorian calendar, certain parts of the Eastern Orthodox Church
(including the Russian Orthodox) have not accepted it and are
using the Julian calendar to this day in calculating the dates
of Easter and all their feasts. And thanks to the Nicene fathers,
since the Easter calculation is dependent on an arbitrary date
on the calendar (March 21st), Orthodox Easter differs from the
other churches because that date falls on a different day depending
on whether one uses the Gregorian or Julian calendar. Had the
definition been tied to an astronomical event (the equinox)
which was independent of the calendar, the calendric differences
would have been inconsequential.
1920, Soviet Armenia began using the Gregorian calendar. The
Armenian Apostolic Church waited three more years to establish
its use in calculating feast dates, making it one of the first
Eastern Churches to have adopted it. Nevertheless, the Armenian
Patriarchate of Jerusalem is the lone exception, still adhering
to the Julian calendar because of concerns relating to the established
system of sharing rights over key Holy Land shrines. As a result,
Jesusı birth is celebrated in Jerusalem every year on January
19th, which is January 6th on the Julian calendar.
current discrepancy between the two systems is 13 days, but
because of the more complex rules used to define Easter (the
Sunday after the first full moon after the equinox), there is
no methodical pattern between the Julian and Gregorian Easter.
Orthodox Easter occasionally coincides with the western Easter,
but most often it comes later by either 4 or 5 weeks. In the
200 years between 1900 and 2099, Easter will coincide 57 times
(28% of the years). In the Julian calendar, the sequence of
Easter dates is repeated every 532 years; in the Gregorian,
the cycle repeats but once every 5,700,000 years.
year, the date of Easter according to both calendars falls on
the same day: April 15th (April 2nd in the Julian system). Many
saw this coincidence, coupled with the fact that it would be
the first Easter of a new millennium, as an opportunity to try
and convince the worldıs various churches to agree on a common
method of calculation. The intent was to offer the world a more
unified witness and not allow divisions over arcane matters
to dilute the Christian message. Talks were sponsored by the
World Council of Churches aiming towards a reconciliation of
systems. Unfortunately, these discussions are taking longer
than expected to reach fruition.
are reproduced two of the many L) existing algorithms for calculating
the date of Easter; both of these are for the Gregorian calendar,
but others are available for the Julian as well. Julian algorithms
are shorter and simpler. All are variations on the same theme.
Ardem Tajerian is a librarian with the Los Angeles Public Library.
for Further Inquiry:
great deal of information exists on the World Wide Web regarding
calendar systems and methods for calculating Easter and other
holidays. One good introduction with links to many useful tables
of data can be found at:
World Council of Churchesı press release on the talks over unification
of the dating system is found at:
more technical and thorough discussion of calendar systems in
general is found at:
Algorithms for Calculating the Date of Easter
is an integer remainder when Y is divided by X; Y/X is an integer
division where the remainder is discarded. For example, 18%7=4
G = year % 19
K = (century - 17)/25
I = (century - century/4 - (century-K)/3 + 19*G + 15)%30 I =
I- (I/28)*(1-(I/28)* (29/(I + 1))*((21- G)/11))
J = (year + year/4+ I+ 2-century + century/4)%7
L = I-J
Easter Month = 3+(L+40)/44
Easter Day L + 28 - 31*(EasterMonth/4)
Easter Month = (h+1-7*m+114)/31 [3=Mar, 4=Apr]
Easter Date=p+1 (date in Easter Month)
1. According to Ex. 12:2-6, Passover is to
occur at dusk on the 14th day of the first month of the year.
Because the months are based on the lunar cycle and the first
of the month begins at the new moon, Passover always occurs
at the time of the full moon, but on varying days of the week.
2. The date in spring when morning and evening occupy equal
lengths of time, generally around March 23rd, plus or minus
a few days.
3. These result from an inaccurate addition of leap days relative
to the actual solar year. The Julian calendar falls behind the
actual calendar by one day in 128 years.
4. There are, in fact, a number of other more subtle differences,
such as the meridian at which one observes the new moon (traditionally
Jerusalem, but Greenwich is another possible standard).
5. The proposal involved the abandonment of the arbitrary March
21 equinox in favor of accurate astronomical observations of
the equinox and full moon (at the meridian of Jerusalem), as
well as the agreed use of the Gregorian calendar. Naturally,
the impact on Eastern Orthodox churches would be much more significant
than on western churches.