Easter-A Festival of Volatile Dates

Easter is an annual festival of Christians throughout the world. The date of Easter shifts every year within the Gregorian calendar. The current Gregorian ecclesiastical rules that determine the date of Easter trace back to 325 AD at the First Council of Nicaea convened by the Roman Emperor Constantine. At that time the Roman world used the Julian Calendar (put in place by Julius Caesar).

The Council decided to keep Easter on a Sunday, the same Sunday throughout the world. To fix incontrovertibly the date for Easter, and to make it determinable indefinitely in advance, the Council constructed special tables to compute the date. These tables were revised in the following few centuries resulting eventually in the tables constructed by the 6th century Abbot of Scythia, Dionysis Exiguus. Nonetheless, different means of calculations continued in use throughout the Christian world.

In 1582 Gregory XIII (Pope of the Roman Catholic Church) completed a reconstruction of the Julian calendar and produced new Easter tables. One major difference between the Julian and Gregorian Calendar is the "leap year rule". Universal adoption of this Gregorian calendar occurred slowly. By the 1700's, though, most of western Europe had adopted the Gregorian Calendar. The Eastern Christian churches still determine the Easter dates using the older Julian Calendar method.

The usual statement, that Easter Day is the first Sunday after the Full Moon that occurs next after the vernal equinox, is not a precise statement of the actual ecclesiastical rules. The Full Moon involved in this statement is not the astronomical Full Moon they are talking abut, but an ecclesiastical Moon (determined from tables) that keeps, more or less, in step with the astronomical Moon.

The ecclesiastical rules are:

(1) Easter falls on the first Sunday following the first ecclesiastical Full Moon that occurs on or after the day of the vernal equinox;
(2) this particular ecclesiastical Full Moon is the 14th day of a tabular lunation (New Moon); and
(3) the vernal equinox is fixed as March 21.

resulting in that Easter can never occur before March 22 or later than April 25. The Gregorian dates for the ecclesiastical Full Moon come from the Gregorian tables. Therefore, the civil date of Easter depends upon whichever tables (Gregorian or Julian) are used for this calculation. The western (Roman Catholic and Protestant) Christian churches use the Gregorian tables; many Eastern (Orthodox) Christian churches use the older tables based on the Julian Calendar.

There are three major differences between the ecclesiastical system and the astronomical system.

(1) The times of the ecclesiastical Full Moons are not necessarily identical to the times of astronomical Full Moons. While the ecclesiastical tables did not account for the full complexity of the lunar motion.

(2) The vernal equinox has a precise astronomical definition determined by the actual apparent motion of the Sun as seen from the Earth. It is the precise time at which the apparent ecliptic longitude of the Sun is zero. (the Sun's ecliptic longitude, not its declination, is used for the astronomical definition.) This precise time shifts within the civil calendar very slightly from year to year. While in the ecclesiastical system the vernal equinox does not shift; it is fixed at March 21 regardless of the actual motion of the Sun.

(3) The date of Easter is a specific calendar date. Easter starts when that date starts for your local time zone. While the vernal equinox occurs at a specific date and time all over the Earth at once.

Inevitably, then, the date of Easter occasionally differs from a date that depends on the astronomical Full Moon and Vernal Equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth.

For example, take the year 1962. In 1962, the astronomical Full Moon occurred on March 21, UT=7h 55m - about six hours after astronomical equinox. The ecclesiastical Full Moon (taken from the tables), however, occurred on March 20, before the fixed ecclesiastical equinox at March 21. In the astronomical case, the Full Moon followed its equinox; in the ecclesiastical case, it preceded its equinox. Following the rules, Easter, therefore, was not until the Sunday that followed the next ecclesiastical Full Moon (Wednesday, April 18) making Easter Sunday, April 22.

Similarly, in 1954 the first ecclesiastical Full Moon after March 21 fell on Saturday, April 17. Thus, Easter was Sunday, April 18. The astronomical equinox also occurred on March 21. The next astronomical Full Moon occurred on April 18 at UT=5h. So in some places in the world Easter was on the same Sunday as the astronomical Full Moon.
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Computing the Date of Easter
The rule is that Easter is the first Sunday after the first ecclesiastical Full Moon that occurs on or after March 21. The lunar cycles used by the ecclesiastical system are simple to program. The following algorithm will compute the date of Easter in the Gregorian Calendar system.

The algorithm uses the year, y, to give the month, m, and day, d, of Easter. The symbol * means multiply and the symbol / means divide.

Please note the following: This is an integer calculation. All variables are integers and all remainders from division are dropped. For example, 7 divided by 3 is equal to 2 in integer arithmetic.

c = y / 100
n = y - 19 * ( y / 19 )
k = ( c - 17 ) / 25
i = c - c / 4 - ( c - k ) / 3 + 19 * n + 15
i = i - 30 * ( i / 30 )
i = i - ( i / 28 ) * ( 1 - ( i / 28 ) * ( 29 / ( i + 1 ) )
* ( ( 21 - n ) / 11 ) )
j = y + y / 4 + i + 2 - c + c / 4
j = j - 7 * ( j / 7 )
l = i - j
m = 3 + ( l + 40 ) / 44
d = l + 28 - 31 * ( m / 4 )

For example, using the year 2010,
y=2010,
c=2010/100=20,
n=2010 - 19 x (2010/19) = 2010 - 19 x (105) = 15, [see note above regarding integer calculations]
etc. resulting in Easter on April 4, 2010.

The algorithm is due to J.-M. Oudin (1940) and is reprinted in the Explanatory Supplement to the Astronomical Almanac, ed. PK Seidelmann (1992). See Chapter 12, "Calendars", by L. E. Doggett.

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Good Friday and 21st March, the Spring start day, coincides only once in millennium.

Ash Wednesday is 46 days before Easter Sunday.

 Easter is a movable feast. It is celebrated on the first Sunday after the Full Moon day on arrival of the Spring season (March 21), so ---

21st March > check for First Full Moon > Look for the First Sunday - That is Easter Day.

The Spring season begins on 21st March. The clocks are advanced to save daylight time. If a child is born during the Daylight Saving Time period, it is better to give the UTC Universal Coordinated Time [GMT] along with the local standard time to cast a birth-chart. The clocks are retarded and advanced not on the same date every year. The UTC will eliminate this confusion. Some countries the Time is slow and some fast on GMT. The Standard time is based on GMT/UTC]. Indian Standard time is 5.5 hrs ahead GMT and US Eastern Time is 5 hrs behind the GMT.

After 22nd of September the day becomes shorter and night becomes longer in the Northern Hemisphere. The Sun's Declination becomes South. If the latitude of a place is greater than the polar distance, the heavenly body will not set. Sun's highest Declination is 23.5 Degrees [appx]. If it is South, it will not set above 66.5 Degrees South latitude and will not rise above 66.5 degrees North latitude. That is why the Poles have six months night and six months day. It takes six moths to come back to zero Declination. When the Declination is zero, the heavenly body is on the celestial equator. Then equal day and night is experienced[ in the case of the Sun.

In higher latitudes the speed of rotation is slower than the lower latitudes. Equator is the longest circle. Its diameter passes through the centre of the Earth. Whereas all other parallel circles of the equator are smaller circles. Both complete one rotation, 360 degrees, in 24 hours. Equator is 360 x 60 x 1.852 kms long. But 60 degree parallel latitude is cos 60 x 360 x 60 x 1.852 kms, ie. it covers only half the distance covered by the equator in 24 hrs.

Some Easter Dates

April  18, 1954
April  22, 1962
March  31, 2013

In which country the bunny rabbit appeared first?
France, US, Spain, Germany, or Italy?

The rabbit first appeared as a symbol of Easter in 16th century Germany; and the first edible Easter bunnies were alo produced in Germany during 1800s. Germans later brought this custom to North America.