Q. Why isn't Easter on the same date every year - like Christmas, for instance?

A. The short answer is that in the 4th century it was decided that Easter would fall after the first full moon following the vernal or spring equinox. (The equinox is a day in the year on which daytime and night-time are of equal length. This happens twice a year, once in spring and once in autumn.)

A more detailed answer would be this:

We know from the New Testament that Jesus' death and resurrection happened around the time of the Jewish feast of Passover. According to Matthew, Mark and Luke's Gospels, the last meal Jesus shared with his disciples was a Passover meal, while John's Gospel says that Jesus died on the feast of Passover itself. In those days, the Jews celebrated Passover on the "14th day of the first month" in accordance with the Bible's commands (see Lev. 23:5, Num. 28:16, Josh. 5:11). The months of the Jewish calendar each began at new moon, so the 14th day would be the day of the full moon. The first month, Nisan, was the month that began from the spring new moon. In other words, the Passover was celebrated on the first full moon following the vernal equinox and was therefore a movable feast.

Early sources tell us that this very soon led to Christians in different parts of the world celebrating Easter on different dates. As early as the end of the 2nd century, some churches were celebrating Easter on the day of Passover itself, whether it was a Sunday or not, while others would celebrate it on the Sunday that followed it. By the end of the 4th century there were four different methods of calculating the date of Easter. In the year 325, the Council of Nicaea attempted to bring in a unified solution that would retain the link with the date of Passover as celebrated in Jesus' time. Eventually, therefore, Easter's date was established as movable.

Q. So how is the date of Easter calculated?

A. The Council of Nicaea established that the date of Easter would be the first Sunday after the full moon following the vernal equinox.

Q. Why, then, despite the universal rule laid down at Nicaea, do different parts of the Church still celebrate Christ's resurrection on different dates?

A. The first thing to remember is that, even after the Council of Nicaea, differences in the date of Easter remained, since the Council had said nothing about the methods to be used to calculate the timing of the full moon or the vernal equinox.

But the real problem behind the situation we have today arose in the 16th Century, when the Julian calendar, which had been established in 46 BC, was superseded by the Gregorian calendar. It took some time for the new calendar to be adopted by all countries (it did not happen in Greece until the start of the 20th Century!). However, the Orthodox churches still use the Julian calendar to this day to calculate the vernal equinox and the full moon that follows it. This is why they calculate a different date.

Q. Why did the Gregorian calendar reform happen at all? Was it necessary?

A. The calendar reform established by Pope Gregory XIII was necessary because the Julian calendar used in those days had begun to lag behind astronomical reality - which is to say that by the time 21 March came around on the calendar, the actual, astronomical vernal equinox had already happened.

The fundamental problem behind this is that the astronomical year - that is, the time the earth takes to make its journey round the sun - is not exactly 365 days: it's actually 365 days, five hours, 48 minutes and 46 seconds. However, as the year has to be divided into equal portions for practical purposes, leap years have to be introduced to resolve the problem.

Q. What's the difference between the Julian and Gregorian calendars?

A. The difference between the two calendars lies precisely in how they resolve this problem. The Julian calendar's solution was to add a leap day every four years, with the end result that the Julian calendar year was an average of 11 minutes and 14 seconds longer than the earth's actual journey around the sun. This meant that the astronomical facts and the calendar calculations would eventually be out by one day in every 128 years. The real equinox, for instance, would then happen one day earlier than the date given on the calendar. The Gregorian calendar attempted to correct this by shortening the average calendar year. It introduced the additional rule that, in contrast to the Julian calendar's leap-year rule, there would be no leap day in years whose number could be divided directly by 100 but not by 400. Thanks to this reduced number of leap years, the Gregorian calendar comes closer to astronomical reality - although it, too, is not "exact" - but the difference between the facts of astronomy and the calendar date is now only 26 seconds a year. It takes 3,600 years to develop a lag of one day. At present, the Julian calendar is running 13 days "slow" of the Gregorian; by the year 2100, the difference will be 14 days. This means that the vernal equinox, which is established as 21 March and on which the date of Easter depends, falls in the Julian calendar on a day which under the Gregorian calendar is 3 April.

Q. So are the two dates always two weeks apart?

A. No. The gap between the two Easters is different every year. It can be as much as five weeks. Besides the fact that the dates of the vernal equinox lie 13 days apart, we also have to consider when the full moon falls. So, if the full moon falls within the 13 days between the Gregorian and Julian equinoxes, Orthodox Easter will be later.

There's another complication here, which is that, alongside the equinox, the sun and moon have a part to play as well. Under the Julian calendar, the full moon is calculated using the so-called Metonic cycle (a 19-year cycle under which the phases of the moon fall on the same date every 19 years). However, this calculation is not astronomically accurate either, so it, too, leads to the dates shifting out of place. When this is added to the discrepancy between the Julian and Gregorian equinoxes, it can lead to a difference of up to five weeks between the Orthodox and Western dates for Easter.

The Nicaea ruling contains one other provision that is extremely important for the Orthodox churches. It states that Easter should not be celebrated "with" (Greek "meta") the Jews. Today's theologians are no longer entirely certain what was meant by this, but Orthodox Easter still cannot fall on the same day as Passover. If it does, it is postponed by a week.

Q. This year (2007), both Easters are on the same date. When does this happen?

A. The two dates coincide when the full moon following the equinox comes so late that it counts as the first full moon after 21 March in the Julian calendar as well as the Gregorian. This is not a regular occurrence, but it has happened more frequently in recent years - in 2001, 2004 and 2007. In the near future, it will also take place in 2010, 2011, 2014 and 2017, but, after that, not again until 2034.

Q. In that case, though, why do some Orthodox churches celebrate Western Christmas?

A. All churches celebrate Christmas as a fixed feast and all (apart from the Armenian church) hold it on 25 December. However, since the Russian Orthodox Church, the Serbian Orthodox Church, the Greek Orthodox Patriarchate of Jerusalem and the Georgian Orthodox Church follow the Julian calendar, they celebrate Christmas on what, under the Gregorian calendar, is 7 January. The Greek Orthodox Church, the Bulgarian Church, the Antioch and Alexandria Patriarchates and the Romanian Orthodox Church follow the Gregorian calendar (except with respect to the calculation of Easter), and celebrate Christmas at the same time as the Western churches. Only the Armenian Apostolic Church celebrates Christmas on its original date of 6 January and the Feast of the Baptism of the Lord on the same day.

Q. Are there any efforts to bring the two Easters together?

A. Efforts have been and are still being made to achieve this. For various reasons, there were particular efforts to tackle the question at the beginning of the 20th Century. In 1902, Patriarch Joachim III of Constantinople began a discussion aimed at achieving greater unity among Christians.

The decision of the Greek Parliament to introduce the Gregorian calendar in 1923 sparked conflict between Church and State. It was not least for this reason that a pan-Orthodox congress was called in May 1923, which revised the Julian calendar to lend it greater astronomical accuracy. This calendar, known as the Meletian Calendar, is only two seconds longer than the calendar year, which means it takes 45,000 years to develop a lag of one day. Calculations are based on observations from Jerusalem rather than Greenwich. The calendar is thus the most accurate yet. However, its introduction led to divisions within the Orthodox Churches - particularly the Greek and Romanian Orthodox Churches. Since then, the issue has time and again been on the agenda of pan-Orthodox conferences.

At the same time, discussion was getting under way in secular life. The business world was seeking a simpler and more sensible method of calculating the date of Easter. In 1928, the British Parliament passed the Easter Act, calling for Easter to be held on a fixed Sunday - the Sunday following the second Saturday in April. However, the Act stipulated that this should only be introduced with the unanimous agreement of the Christian churches.

As early as 1923, the League of Nations addressed the question and forwarded the matter to the Advisory and Technical Committee for Communications and Transit, which, for its part, wanted to introduce a brand-new calendar across the globe, dividing the year into months of equal length. This would have had the effect of requiring one or two days to be included outside of the normal seven-day rhythm of the week, in order to make up for the time lacking. With regard to the date of Easter, the British solution was proposed. The Committee asked the churches' opinion, and found that the majority of Protestant churches, as represented by the Ecumenical Council for Practical Christianity, favoured a fixed date for Easter. The Ecumenical Patriarchate of Constantinople replied that, although the Orthodox Church would favour a calendar that retained the continuity of the week, it would be open to a fixed date for Easter, as long at it remained a Sunday and all Christian churches were in agreement. The Roman Catholic Church's first response was that the issue could only be resolved by an ecumenical council. Some years later, however, it changed its answer to a definitive "no".

The efforts were taken over by the League of Nations' successor organization, the United Nations, but finally foundered in 1955, after the USA rejected the idea of a new calendar, fearing public opposition on religious grounds.

Nothing changed until the Second Vatican Council, whose Constitution on the Sacred Liturgy stated that the Roman Catholic Church would assent to a common date for Easter - movable or fixed - if all the churches could agree on a solution. The World Council of Churches (WCC) then took up the issue again, surveying its member churches in 1965 and 1967. It found that all the churches would be willing to celebrate Easter on the same day. However, while most Western churches preferred a fixed date, the Orthodox churches wanted a common movable date based on the Nicaea rule. In 1975, the matter was placed on the agenda of the WCC General Assembly in Nairobi, following a request to the WCC from the Roman Catholic Church for the churches to undertake something together on the issue at the General Assembly. Another survey was made of Council's member churches, which echoed the results of the first survey. It became abundantly clear at the General Assembly that a decision could only be reached by the churches themselves, not by the WCC. It was decided that, at that stage, specific proposals would not be helpful, but that work into the issue ought to continue.

Then, at their first pre-conciliar conference in 1976, the Orthodox churches moved to hold a congress as soon as possible. This took place in 1977 in Chambesy. The congress dealt primarily with the pastoral problem that abandoning the Nicaea rule would lead to divisions. This conclusion was repeated at the second pre-conciliar Orthodox conference in 1982 and the revision of the calendar postponed until such time as would, God willing, be more suitable.

The issue was not brought up again at the WCC until 1997. Two of its departments - "Worship and Spirituality" and "Faith and Order" - organized a consultation session on behalf of the executive committee in Aleppo, Syria. This resulted in a concrete proposal keep the Nicaea rule but calculate the equinox and full moon using the accurate astronomical data available today, rather than those used many years ago.

Q. Why has this solution still not been put into practice?

A. The Orthodox church is still grappling with the arguments first brought up at the so-called pre-conciliar conferences in 1977 and 1982.

The problem is that, while the use of the astronomical calculations will mean hardly any change for those churches that use the Gregorian calendar, the Orthodox churches have had painful experiences in the past with schisms resulting from calendar reforms, and are therefore very cautious about them. However, a proposal for the Western churches to move their Easter to coincide with the Orthodox date garnered just as little support.

Tell us your thoughts:

-       Should there be one, unified date for Easter, or should the two current dates be kept?

-       What do you think would be a solution?