Why does Easter move around our calendar so much?

Got this from someone in the cellgroupā€¦

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Why does Easter move around our calendar so much?
– By Quek Tze-Ming
Easter is the time when we celebrate Christ’s Resurrection. Unlike Christmas
(the other important day in the Christian calendar), which is fixed on
December 25, Easter seems to float around the Sundays of April randomly,
occasionally making an appearance in March (it fell on March 23 in 2008) or
as late as April 25 (this will happen in 2038).

Why is this? The answer is found in the difficulty of calculating a date
based on both the solar and lunar calendars.

According to the gospel accounts, Jesus was killed on or around the time of
the Jewish Passover. He was raised three days later, on the first day of the
Jewish week (Sunday).  So Christians wanted to have their feast day around
the same time as Passover, which was fixed to begin on twilight on the 14th
day of the month of Nisan (Heb: Aviv; see Lev. 23.5).

Got this so far? Now come the disputes.
First, in the mid-second century, Christians in the province of Asia
celebrated Easter beginning on Nisan 14 itself, rather than the Sunday
following, which was the practice of most Christians outside of Asia. This
became known as the Quartodeciman controversy (in the Latin Vulgate of Lev.
23.5, quarta decima = 14). The early father Melito of Sardis was a notable
Quartodeciman. This difference in practice led predictably to bouts of
name-calling, with threats of excommunication and harassment.
Quartodecimanism died out, but it was the first of a whole series of Easter
controversies.

Second, by the third century, some Christians were becoming unhappy with
relying on the Jewish community to determine the date of Easter. The First
Council of Nicaea (325 CE) decided that the calculation for the date of
Easter would be "independent" of Jewish calculations of the Passover.
Because the Council came up with little that was actually useful in
computation, it took centuries for a common method to find acceptance
throughout Christendom. Thus, while the Alexandrian tables eventually became
normative, a Roman 84-year cycle introduced at the end of the 3rd century
continued to be in use in the British Isles as late is 931.

So what is the rule now? What eventually came to be accepted was this:
Easter falls on the Sunday following the first full moon after the vernal
(or spring) equinox, which is the spring day when the length of the day and
the length of the night are exactly the same.

This is why Easter moves around our Gregorian (solar) calendar so much,
because the calculation depends on both the solar and lunar year. A solar
year (the length of time it takes the earth to move round the sun) is 365
days, 5 hours, 49 minutes, and 12 seconds. But a lunar year is 354.37 days.
Even for those who are mathematically inclined, calculating one (solar:
equinox) against another (lunar: full moon day) is seriously complicated.
You would think this is complicated enough, but that didn’t stop even more
fiddling around!

You see, the problem was that BOTH the astronomical full moon and the
astronomical equinox were not days but moments in time, and they vary. The
astronomical equinox can occur on either March 20 (as it did at Nicaea) or
Mar 21 (so 2007). The astronomical full moon can be observed happening on
different days depending on where you were standing on earth. All this
reliance on observation made planning ahead well nigh impossible.

So the church fathers decided that the full moon used to determine Easter
would not be the astronomical full moon, but an Ecclesiastical Full Moon
(EFM), calculated as the 14th day of the lunar month (determined by
formula). Easter was defined as the Sunday after the first EFM that falls on
or after March 21 (whether or not that date is the astronomical equinox).
Got it? So Easter is the Sunday after a notional full moon following a
notional equinox!

Using the formula and tables, Easter can fall on 35 possible dates between
March 22 and April 25 inclusive, with the cycle of dates repeating itself
after exactly 5,700,000 years. The most common date for Easter is April 19,
occurring 3.9% of the time.

In practice, we get our Easter dates from the Catholic church, which has
compiled tables based on the ecclesiastical rules. All the Easter observant
churches in Western Christendom use this table, or the calculation behind it
(the Eastern Churches have liturgical calendars based on the Julian
calendar). The calculation of Easter is one practice that the Protestant
church still gets its cues from the Roman Catholic church. It is a practice
based on conciliar decision and church tradition that the Reformation did
not touch. Perhaps it was too difficult to understand, reform, or offer an
alternative!

In this period leading up to our Resurrection-celebration, it is appropriate
for us to end by meditating on these lines from the old Quartodeciman,
Melito of Sardis:

I, he says, am the Christ. I am the one who destroyed death, and triumphed
over the enemy, and trampled Hades under foot, and bound the strong one, and
carried off man to the heights of heaven, I, he says, am the Christ.

Therefore, come, all families of men, you who have been befouled with sins,
and receive forgiveness for your sins. I am your forgiveness, I am the
passover of your salvation, I am the lamb which was sacrificed for you, I am
your ransom, I am your light, I am your saviour, I am your resurrection, I
am your king, I am leading you up to the heights of heaven, I will show you
the eternal Father, I will raise you up by my right hand.
– Peri Pascha 102-103