How Pascha Is Reckoned

The date of Pascha is not looked up but computed, by a self-contained arithmetic — the Paschalion — that combines a nineteen-year lunar cycle with an ecclesiastical equinox fixed at 21 March on the Julian calendar. This file sets out how the computation works, what makes it diverge from both the astronomical moon and the Western Easter, and why almost every Orthodox Church on earth still uses it.

Tradition: Pan-Orthodox

The Rule in Three Clauses

The received rule, as the Orthodox Church applies it, has three parts (see Orthodoxy: The Church Calendar, whose account this file expands and does not depart from):

Pascha falls after the vernal equinox — the ecclesiastical equinox, which the Alexandrian computus fixes at 21 March, Julian. It is a defined date, not an observation.
Pascha falls on the Sunday after the paschal full moon — the first full moon of the ecclesiastical moon on or after that equinox. Again a defined quantity: the moon of the tables, not the moon in the sky. If the paschal full moon itself falls on a Sunday, Pascha is the following Sunday, so that Pascha never coincides with the fourteenth of the paschal moon.
Pascha falls on a Sunday — the day of the Resurrection, which was the substance of the Nicene settlement (see Nicaea and the Independence of the Christian Reckoning).

A fourth clause is very widely repeated and is not part of the rule: that Pascha must fall after the Jewish Passover. The hub file states the correction and this collection follows it: Nicaea decreed that the Christian Pascha be computed independently of the Jewish reckoning, not in dependence on it; the “after Passover” requirement appears as a formal principle in the twelfth-century canonist John Zonaras, commenting on Apostolic Canon 7, and in practice Orthodox Pascha does follow Passover — but as a by-product of the ageing Julian lunar tables, not as a rule of the Paschalion (Patsavos, “Dating Pascha in the Orthodox Church,” Greek Orthodox Archdiocese of America; Kuzenkov, Khristianskie khronologicheskie sistemy, Moscow: Russkii izdatel’skii tsentr, 2014; and see Orthodoxy: The Church Calendar).

The Machinery

The nineteen-year cycle

The moon and the sun do not divide evenly into one another, and the whole problem of a lunisolar calendar is finding a period in which they nearly do. The classical solution, attributed to the Athenian astronomer Meton (fifth century BC), is the nineteen-year cycle: nineteen solar years are very nearly 235 lunations. Each year of the cycle is given a golden number from 1 to 19, and each golden number carries a fixed date for the paschal full moon. Once the golden number is known, the paschal full moon is known; once the paschal full moon is known, Pascha is the Sunday after it.

The Alexandrian church perfected this scheme in the fourth century, tabulating the fourteenth day of the paschal moon for each year of the cycle, with the ecclesiastical equinox fixed at 21 March (Mosshammer, The Easter Computus and the Origins of the Christian Era, Oxford: Oxford University Press, 2008, chs. 3–6). The tables reached the Latin West through the paschal tables of Dionysius Exiguus in 525, and by way of Bede they became the common European reckoning (Declercq, Anno Domini: The Origins of the Christian Era, Turnhout: Brepols, 2000).

The great cycle of 532 years

Because the lunar cycle is nineteen years and the Julian calendar’s cycle of weekdays is twenty-eight years, the whole Paschalion repeats exactly every 19 × 28 = 532 years. This period is the Great Indiction, and it is the reason the Paschalion could be published as a finite table: a Byzantine or Russian service book can simply print 532 years of Paschal dates and the table will serve for ever, since after 532 years it begins again unchanged (Kuzenkov, Khristianskie khronologicheskie sistemy, on the Byzantine Paschal tables).

This is worth pausing on. The Paschalion is not a formula that consults the sky each year. It is a closed, permanently repeating arithmetical scheme, entirely determined once the year is known.

The range

Applying the rule, the earliest possible date of Pascha is 22 March and the latest 25 April, both Julian. In present-day civil (Gregorian) dates that range is 4 April to 8 May. Orthodox Pascha in 2025 fell on 20 April, in 2026 on 12 April, in 2024 on 5 May.

Why the Dates Drift From the Sky

Two independent inaccuracies have accumulated in the scheme, and both push Pascha later.

The solar error. The Julian year of 365.25 days is about eleven minutes longer than the mean tropical year, an error of roughly one day in 128 years. The ecclesiastical equinox is defined as 21 March Julian, and that Julian date now falls on 3 April Gregorian, whereas the astronomical equinox occurs about 20 March Gregorian. The equinox of the tables is therefore some two weeks later than the equinox of the sky (see The Drift and What It Does and Does Not Affect).

The lunar error. Nineteen Julian years are 6,939.75 days, while 235 lunations are about 6,939.69 days — a discrepancy of roughly 0.06 days per cycle, or about one day in 308 years. Over the seventeen centuries since the Alexandrian tables were fixed, the ecclesiastical moon has therefore fallen some four to five days behind the astronomical moon (Mosshammer, The Easter Computus, ch. 4, on the limits of the Metonic approximation).

These are not concealed defects and this collection does not minimize them. They are the reason Orthodox Pascha can fall a full month after the astronomical paschal limit, and they are the substance of the strongest argument on the other side of the calendar question (see The Case on the Other Side).

Why Orthodox and Western Easter Differ — and by How Much

The Gregorian reform of 1582 corrected both errors at once. It restored the equinox to 21 March, adjusted the solar calendar’s leap rule, and applied two corrections to the epacts — a solar equation tracking the suppressed leap days and a lunar equation correcting the Metonic drift — so that the Gregorian Easter uses the same three-clause rule with better-behaved tables.

Because the underlying rule is the same and only the tables differ, the two Easters do not diverge arbitrarily. They coincide, or differ by one week, four weeks, or five weeks. They never differ by two or three (see Orthodoxy: The Church Calendar). The reason is structural: the difference arises from the offset of the two equinoxes and from whether the two schemes select the same lunation or lunations a month apart, and the arithmetic admits only those four outcomes.

This produces one of the most persistent confusions in the whole subject and it is worth killing here. Orthodox Pascha is not “thirteen days later” than Western Easter. The thirteen-day figure belongs to the fixed calendar and nothing else. Pascha is later by nothing, by a week, by four weeks, or by five — and in 2025 the two coincided exactly, on 20 April, in the year of the seventeenth centenary of Nicaea.

Who Uses It

The Paschalion is the point of greatest unity in the Orthodox world. Churches on the Julian calendar and churches on the Revised Julian calendar keep Pascha on the same day, because the churches that changed their fixed calendar in the twentieth century deliberately did not change the Paschalion (see The Revised Julian Calendar and Its Reception).

There is one exception, and it should be stated rather than passed over. The autonomous Orthodox Church of Finland keeps Pascha by the Gregorian Paschalion, and has done so since the early 1920s; its councils have repeatedly resolved to continue the practice (“Finnish Church resolves to continue using Gregorian Paschalion,” OrthoChristian.com, locator per METHODOLOGY §2.3; and see The Current Map). It is the sole Orthodox church that does so.

Proposals for Reform

The question of a common Christian date has been formally studied more than once, and the results are instructive.

An inter-Orthodox consultation at Chambésy in 1977, following the first pre-conciliar conference of 1976, considered the calendar and Paschal questions and concluded that the pastoral cost of any change outweighed the gain: the abandonment of the received reckoning would provoke division, and unity in the date of Pascha was to be preserved. The second pre-conciliar Orthodox conference of 1982 repeated the conclusion (Ecumenical Patriarchate, “Preparation of a Council: A History,” orthodoxcouncil.org, locator).

A World Council of Churches / Middle East Council of Churches consultation at Aleppo, 5–10 March 1997, proposed a different route: keep the Nicene rule exactly as stated — the Sunday after the first full moon after the vernal equinox — but determine the equinox and the full moon by modern astronomical calculation, taking the meridian of Jerusalem as the point of reference. The proposal would have required neither side simply to adopt the other’s tables (World Council of Churches, Towards a Common Date for Easter, Aleppo, 1997, oikoumene.org). It has not been received by the Orthodox churches.

The 2016 Holy and Great Council in Crete did not take up the calendar or the Paschalion.

References

Declercq, Georges. Anno Domini: The Origins of the Christian Era. Turnhout: Brepols, 2000.
Kuzenkov, Pavel V. Khristianskie khronologicheskie sistemy: Istoriia letoschisleniia v sviatootecheskoi i vostochnokhristianskoi traditsii III–XV vekov. Moscow: Russkii izdatel’skii tsentr, 2014.
Mosshammer, Alden A. The Easter Computus and the Origins of the Christian Era. Oxford: Oxford University Press, 2008.
Patsavos, Lewis J. “Dating Pascha in the Orthodox Church.” Greek Orthodox Archdiocese of America, goarch.org.
World Council of Churches / Middle East Council of Churches. Towards a Common Date for Easter. Consultation, Aleppo, Syria, 5–10 March 1997. oikoumene.org.
Ecumenical Patriarchate. “Preparation of a Council: A History.” orthodoxcouncil.org (locator for the Chambésy pre-conciliar process).
“Finnish Church resolves to continue using Gregorian Paschalion.” OrthoChristian.com (locator).