The Drift and What It Does and Does Not Affect

The Julian year is eleven minutes too long, and the error accumulates: the fixed calendar now runs thirteen days behind the sun. But the received Paschalion is a separate reckoning, not read off the fixed calendar, and the drift reaches it by a different route. This file explains the mechanism plainly enough that a reader can see exactly why the two cycles can come apart — which is the technical fact the whole calendar question turns on.

Tradition: Pan-Orthodox

The Arithmetic of the Drift

The Julian year is exactly 365.25 days. The mean tropical year — the period of the sun’s return to the vernal equinox, averaged over its slight variations — is about 365.2422 days.

The difference is about 0.0078 days per year, which is 11 minutes and 14 seconds. That is trivial in a lifetime and decisive over centuries:

one day of error in roughly 128 years;
ten days by the late sixteenth century, which is what Gregory XIII deleted in 1582;
thirteen days at present.

The current figure holds from 1 March 1900 to 28 February 2100, after which it becomes fourteen days. (The offset changes at those boundaries because the Gregorian calendar omits the leap day in 1700, 1800, 1900, 2100 and so on, while the Julian does not.) So: 25 December Julian is 7 January Gregorian now, and will be 8 January Gregorian from 2100.

Equivalently, and more usefully for the Paschalion: 21 March Julian is at present 3 April Gregorian, while the astronomical equinox occurs about 20 March Gregorian.

What the Drift Affects

The fixed calendar

Directly and completely. Every commemoration in the Menaion keeps its Julian date and therefore slides later and later against the sun. The consequences are of two kinds.

The visible one is social: on the Julian calendar the Nativity falls on 7 January in civil reckoning, Theophany on 19 January, the Church New Year on 14 September, the Dormition on 28 August. The Nativity Fast runs through the whole Western Christmas and New Year season (see Orthodoxy: Fasting Periods; Orthodoxy: Calendar and Typikon in ROCOR).

The slower one is seasonal. Several fixed feasts were placed with an eye to the natural year: the Nativity near the winter solstice, the Nativity of the Forerunner near the summer solstice six months earlier, the Annunciation near the spring equinox. That anchoring loosens by a day every 128 years and will not stop loosening. This is a real cost, and this collection does not pretend otherwise; it is the strongest point on the other side of the argument (see The Case on the Other Side).

The Paschalion — but indirectly

The drift reaches the date of Pascha too, but not in the way most accounts assume, and the mechanism has to be stated with care. Because the distinction turns on what the Paschalion does not inherit from the fixed calendar, it is set out below — first negatively, under What the Drift Does Not Affect, and then positively, under How the Drift Reaches Pascha After All. Readers who want only the mechanism should go straight to the latter.

What the Drift Does Not Affect

The counted cycles

Nothing in the daily or weekly cycle is touched. Sunday is the same day everywhere; Wednesday and Friday are the same days; the tone of the week is the same tone. Calendar reforms renumber days, they do not reorder them: when ten days were deleted in October 1582, Thursday was followed by Friday (see The Daily and Weekly Cycles).

The date of Pascha, as read off the fixed calendar

This is the key. The Paschalion is not a rule about the fixed calendar. It is a separate computation whose output happens to be expressed as a Julian date.

Consider what the computation actually consumes and produces:

Inputs: the year’s position in the nineteen-year lunar cycle (its golden number), and an ecclesiastical equinox defined as 21 March. Both are internal to the scheme. The computation never asks where the sun is or where the moon is.
Output: a date between 22 March and 25 April, on the Julian date grid.
Then, and only then: that Julian date is expressed in whatever civil calendar the local church happens to use — as 4 April to 8 May in present Gregorian reckoning.

That last step is a translation, not a computation. And because it is only a translation, a church can change the calendar it uses to express fixed feasts without touching the Paschalion at all.

This is not a theoretical possibility. It is exactly what the churches that adopted the Revised Julian calendar did: they moved the fixed grid and left the Paschalion alone, which is why every Orthodox Church except Finland keeps Pascha on the same day (see The Revised Julian Calendar and Its Reception).

The consequence for how one speaks

It follows that “the Orthodox are thirteen days behind” is true of the fixed calendar and false of Pascha. Orthodox Pascha is later than Western Easter by nothing, seven days, twenty-eight days, or thirty-five days — never thirteen. In 2025 the two fell on the same Sunday, 20 April. Anyone who says the Orthodox keep Easter thirteen days after the West has confused the two systems, and the confusion is very common (see How Pascha Is Reckoned).

How the Drift Reaches Pascha After All

The Paschalion is insulated from the fixed calendar, but not from the astronomy. Two separate errors accumulate inside the computation itself, and both push Pascha later.

1. The solar error, entering through the defined equinox. The scheme fixes the ecclesiastical equinox at 21 March Julian. Because the Julian year is too long, that Julian date has slid to 3 April Gregorian, while the real equinox is about 20 March Gregorian. The computation is therefore looking for its paschal moon in a window that opens roughly two weeks after the astronomical equinox.

2. The lunar error, entering through the Metonic cycle. Nineteen Julian years are 6,939.75 days; 235 lunations are about 6,939.69 days. The cycle over-counts by about 0.06 days each time round, roughly one day in 308 years. Over seventeen centuries the ecclesiastical moon has fallen some four to five days behind the moon in the sky (Mosshammer, The Easter Computus and the Origins of the Christian Era, Oxford: Oxford University Press, 2008, ch. 4).

Both errors delay the selection of the paschal moon; sometimes they delay it past a lunation, which is why the two Easters can differ by four or five weeks rather than one. And it is the combination — not the thirteen-day fixed-calendar offset — that makes Orthodox Pascha late.

The Seam, and Why It Matters

Put the two findings together.

The fixed cycle and the movable cycle are separable: one can be changed without the other.
But the liturgical year contains structures that span both.

The principal such structure is the Apostles’ Fast, which begins on the Monday after All Saints Sunday — fifty-seven days after Pascha, and therefore a movable date — and ends on 28 June, the eve of Ss Peter and Paul, a fixed date in the Menaion (see The Fixed and the Movable Year; Orthodoxy: Fasting Periods).

If the fixed grid is moved thirteen days earlier while the Paschalion stays where it is, the fast’s end moves thirteen days earlier and its start does not move at all. The fast is shortened by exactly thirteen days in every year — and in years when Pascha is late, its end arrives before its beginning, and the fast disappears from the year altogether. The mechanics and the worked examples are given at The Revised Julian Calendar and Its Reception.

Two things should be said about this at once, and the collection means both.

It is a genuine defect, and a checkable one: not an aesthetic complaint but an arithmetical consequence with a datable effect on a fast prescribed by the Typikon.

And it is a defect of the arrangement, not an absurdity. The very separability that produces it is what made the Revised Julian scheme possible in the first place, and the same separability is what preserved a common Pascha for the whole Orthodox world. A reform that had changed the Paschalion instead would have divided Orthodoxy at the one point where it is genuinely united. The churches that changed in the 1920s changed the less dangerous of the two systems. That is a real point in their favour and it is stated here, before the section that argues against them.

References

Bickerman, E. J. Chronology of the Ancient World. 2nd ed. London: Thames and Hudson, 1980.
Declercq, Georges. Anno Domini: The Origins of the Christian Era. Turnhout: Brepols, 2000.
Kuzenkov, Pavel V. Khristianskie khronologicheskie sistemy. 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.