1. Disputes

   
   

When the Queen Was Fasting and the King Was Feasting

In the spring of 665 CE, a curious domestic drama unfolded at the royal court of Northumbria. Queen Eanflæd was observing Palm Sunday with solemn fasting, while her husband, King Oswiu, was joyfully celebrating Easter, on the very same day. The queen followed Roman custom, the king the Celtic tradition. Two Easters, side by side in the same royal household, each with its own cycle of feasts and fasts, each grounded in a different way of calculating time.

As Bede put it:

Oswiu, a powerful king of the Anglo-Saxon kingdom of Northumbria, had been raised in the Irish Christian tradition, having spent his youth in exile among the monks of Iona. His queen, Eanflæd, came from the southern kingdom of Kent, where Roman missionaries led by Augustine had brought the customs of the continental Church. Their marriage was a political alliance, uniting two major strands of early English Christianity, yet their household embodied a broader cultural clash. Even the monks in their chapels couldn’t agree when to begin the Lenten fast, when to break it, or when the Resurrection should be celebrated.

The awkwardness of the royal Easter mismatch, recorded with bemused frustration by the Venerable Bede, revealed a deeper tension: how do we pin down the sacred within the shifting cycles of sun and moon? And whose calendar do we trust, Rome’s, or Iona’s? Astronomers, priests, and kings all had stakes in this cosmic arithmetic. The debate culminated in the Synod of Whitby, to agree on a way of calculating Easter. Underneath the politics lay something older and stranger: the age-old challenge of aligning solar years and lunar months, a task that had perplexed and inspired civilisations for millennia.

The Dispute in Britain

   Easter may be all about redemption and hope, but the history of Easter is about disputes: disagreeing with the Pagans, disagreeing with the Celtic Church in early Christian times, and disagreeing with the Jews, despite Easter being based on Passover

   In the Historia Ecclesiastica, Bede mentions the mission of Aidan, the Irish bishop, whom he admires for his Christian virtues, even though he disagrees with Aidan’s view on the observance of Easter. Bede states that he does not approve of Aidan's practice of observing Easter at a time different from the canonical Easter (i.e., the time set by the universal Church). However, he praises Aidan for his love of peace, charity, humility, and commitment to God’s commandments. Aidan’s actions were in line with Christian virtues, but his deviation on the Easter controversy is something Bede does not condone.

   Bede relates that at one point, Peada, son of King Penda of Mercia, was given the kingdom of the Southern Mercians. The land he ruled over was reported to consist of 5000 families, with the Northern Mercians being separated by the Trent River. Peada was killed treacherously by his wife, during the celebration of Easter.

   Following the death of King Penda, Immin, Eafa, and Eadbert (Mercian generals) rebelled against King Oswy of Northumbria. They set up Wulfhere, son of Penda, as king and expelled foreign officers from their lands. This was the political backdrop to the widespread debate over the correct observance of Easter. The followers of Kent and France argued that the Scots (from Ireland and northern Britain) were observing Easter incorrectly, according to universal church customs. Among the most prominent defenders of the true Easter was Ronan, a Scotsman who had been educated in ecclesiastical matters, likely in France or Italy.

Despite his arguments, Ronan was unable to convince Finan, a Scots bishop, who remained obstinate in his views. The conflict around the correct date for Easter resulted in two Easters being observed in some regions, with one group keeping Easter earlier (following Kent's tradition) and the other observing it later (according to the Scots tradition).

   The controversy about the observance of Easter became even more prominent after the death of Finan and the ascension of Colman as bishop. Colman, also a Scots missionary, continued to defend the traditional Scottish observance of Easter, leading to further disputes, especially with King Oswy and his son Alfrid. Oswy, who had been converted by the Scots, was skilled in their language and initially favoured their practices. However, his son Alfrid, influenced by the teachings of Wilfrid (who had been educated in Rome and France), believed the Roman practice of Easter was more correct.

   Wilfrid, a key figure in this dispute, was instrumental in spreading the Roman observance of Easter. He argued that Rome's practice had been followed by the apostles Peter and Paul, and was the correct one for all Christians to observe. He also contended that the Roman practice was in line with the Gospel and the law, as it did not conflict with the teachings of Christ. He ridiculed the Scottish practice, which he argued was inconsistent with both the law and the teachings of the apostles.

    In the Synod of Whitby (664 AD), King Oswiu declared that the Roman practice would be followed in his kingdom, influenced largely by the arguments presented by Wilfrid. The synod also addressed disputes over the tonsure (the style of shaving monks' heads), which was another point of contention between the Celtic and Roman traditions. Oswiu made it clear that those serving the one God should follow the same rules, and he chose to align with the practices established in Rome and advocated for uniformity in the observance of Easter.

   In the synod, Colman defended the Scottish tradition, citing St. John the Evangelist as a key example of following the early Christian customs. However, Wilfrid challenged this by emphasising that John’s practice was consistent with the Mosaic law, and that the early church adapted its practices to avoid the Judaising tendencies. Wilfrid argued that the apostles eventually adopted a more universal practice, which was in line with Peter’s tradition in Rome.

Oswiu, seeking unity and order for his kingdom, sided with the Roman method of calculating Easter. Queen Eanflæd was aligned with the more powerful side in the dispute. And so, after two Easters were celebrated one year, in Britain, the queen of Northumbria got her way.  

Another Dispute: Patrick and King Loíguire

Saint Patrick was a fire starter. This is how the story of Patrick lighting the Paschal fire on the Hill of Slane in Ireland is told, by Muirchú, in his Vita sancti Patricii:

The fire that Patrick lights on the Hill of Slane is no ordinary flame, it is described as divine, blessed with bright light that shines in the night. The fact that the fire shines before the royal fire is lit at Tara, the symbolic heart of pagan power, suggests an assertion of Christian supremacy over the pagan practices of the time.

The story also invites comparisons to the Paschal fire that is lit in many Christian traditions at Easter, symbolising the Resurrection of Christ. The Paschal fire is typically kindled at Easter Vigil and is seen as a symbol of new life. In this story, the fire is described as occurring on the night before a pagan celebration. This alignment of dates and symbolism further underscores Patrick’s role as a figure who bridges the gap between old and new faiths, bringing the light of Christianity into a land steeped in pagan tradition.

The question of why Patrick would have performed such an audacious act is key to understanding the tension between Christianity and paganism in Ireland. While the Vita Sancti Patricii does not provide an explicit explanation for Patrick’s actions, there are a number of possible reasons rooted in theological symbolism and practical strategy:

1. Defying Pagan Authority: The fire is directly set against the royal edict that no fire should be lit until the king’s fire was first kindled. This act of disobedience was a clear challenge to the pagan rulers, and it showcased Patrick’s boldness in confronting the old order. This fire, lit by Patrick, is close in nature to the practices that were being celebrated at the pagan feast, in that a beacon on a hill top was going to be set alight at a specific time. However, by lighting it himself, Patrick was asserting the supremacy of the Christian God over the druidic powers and the pagan rituals that were still very much alive in Ireland at the time.

2. Theological Symbolism: As mentioned, the act of lighting a fire at Easter time can be seen as a theological proclamation. In Christian tradition, the Resurrection of Christ is the ultimate victory over death and darkness, and it may be that the pagan traditions Patrick was standing up against held similar views, only presumably involving different deities.

3. A Form of Evangelism: Patrick’s choice to defy the king’s order can also be seen as an evangelical move, signalling his desire to spread the Christian message in a dramatic and visible way. By performing this act, he attracted attention not only from the king but also from the people of Ireland, signalling that Christianity had arrived in force and was capable of challenging the established religious order.

The question of how Patrick survived the king's threats is a tricky one. According to Muirchú’s account, King Loíguire orders the death of the person responsible for lighting the fire. The druids, however, seem to recognise the supernatural significance of the fire and its symbolism, warning the king that if it is not extinguished, it will spread throughout the land and overwhelm both the pagan practices and the king’s own power. Or at least this is how Muirchú’s version of the story goes. Still, the fire may have been seen as an omen, that Christian faith was set to take over.

One interesting detail in the Vita Sancti Patricii  is that we are told: "They held and celebrated their pagan feast on the same night on which holy Patrick celebrated Easter." Clearly, the date of the Paschal fire being lit corresponds to the eve of a Pagan festival. We have to assume that the fire was set on Easter Sunday, the date arrived at by some ancient computation, and we can even speculate that it could possibly have been the same as Passover. or another significant day, what matters is that the act took place during a moment of cultural transition, when the old ways were being replaced by the new faith. However, in this anecdote, it is clear that the timing of this ancient festival, be it the Christian Easter or the feast celebrated by King Loíguire, was paramount. Saint Patrick clearly believed it was worth risking his life in defiance of the king to assert his belief in the correct timing for the celebration. While the Christian narrative of resurrection must have dominated Saint Patrick's beliefs, perhaps the conquering of darkness by light, and the triumph of spring over winter dominated beliefs in the pagan tradition.

The mention of the druids in the story is significant as well. The druids were the spiritual leaders of pagan Celtic society and were closely associated with the old religious practices that Patrick sought to replace. The druids' warning that the fire could not be extinguished and would ultimately overwhelm the king and his regime can be interpreted as a recognition of the power of Christianity, that it would spread throughout the land and eventually replace the pagan system. This idea of a divinely-ordained spread is a recurring theme in Patrick’s mission and highlights the belief that Christianity was destined to take root in Ireland.

Long before Patrick made the Hill of Slane famous, the site had been considered important. The warrior and Fir Bolg King Slaine met his death upon this hill, and is probably buried here. There is a mound, known as "The Motte" , which is inaccessible to visitors and blocked from view by trees which may be the site. East of the mound is a ring-barrow which could also be the burial place of Slaine. (5)

The story of Saint Patrick lighting the Paschal fire at the Hill of Slane is a deeply symbolic moment in the Christianisation of Ireland, rich with theological significance and dramatic tension. Patrick's fire represents the arrival of Christian faith in Ireland, and appropriates the already ancient symbolism of the triumph of light over darkness at a time chosen for the symbolic harmonisation of the sun and moon cycles. While the Christian narrative of resurrection must have dominated Saint Patrick's beliefs, perhaps the conquering of darkness by light, and the triumph of spring over winter dominated beliefs in the pagan tradition. Lighting the Paschal fire, as St. Patrick famously did on the Hill of Slane within view of the pagan kings at Tara, was both a bold Christian statement and a deeply symbolic act. In many ancient cultures, fire rituals marked the transition from winter to spring, especially at times tied to lunar calendars. In India, for example, the festival of Holika Dahan, held on the eve of Holi, involves lighting bonfires on the full moon of the spring month Phalguna to symbolise the triumph of light over darkness and the renewal of life. In China, the Lantern Festival, which concludes the lunar New Year period, fills the night with firelight and moon symbolism, celebrating reunion and harmony. These practices suggest that fire, moon, and spring have long been linked in the human imagination, marking thresholds between darkness and light, old and new. St. Patrick’s fire, kindled at Easter’s approach, fits into this wider heritage, proclaiming resurrection, but also echoing far older seasonal rites, where light is born anew under the gaze of the moon.

The Dispute Over links to Passover

   The determination of the date of Easter, and whether it should be linked to Passover, became a significant issue in the early centuries of Christianity. There were theological and practical disagreements over whether Easter should be celebrated on the same day as Passover, or whether it should be observed separately as a distinct Christian feast.

   In the early centuries, some Christian communities continued to celebrate Easter on the same day as Passover (the 14th of Nisan in the Jewish calendar), regardless of the day of the week. This practice was known as the Quartodeciman practice (from Latin quartodecima, meaning "fourteenth"). These Christians believed that the Passover feast was deeply connected to the Passion and Resurrection of Jesus, as Jesus' Last Supper was traditionally seen as a Passover meal. Therefore, they felt that Christians should celebrate Easter on the same day that Jews celebrated Passover, maintaining the connection between the two feasts.

   However, this practice created a problem. Because the Jewish calendar is based on the lunar cycle, the date of Passover changes each year, and some Christian communities were keen to anchor Easter's date more firmly into the solar year. Furthermore, early Christians were eager to distinguish themselves from Judaism and to assert that Christianity was a new faith with its own distinct identity. The idea of celebrating Easter on the same day as Passover, which was seen as a Jewish festival, was contentious.

Passover, or Pesach in Hebrew, is one of the most important festivals in the Jewish calendar. It commemorates the Exodus, the foundational story in which God liberates the Israelites from slavery in Egypt, as told in the Book of Exodus. Central to this narrative is the figure of Moses, who confronts Pharaoh and leads his people out of bondage through a series of dramatic events, culminating in the parting of the Red Sea. The name "Passover" refers to the tenth and final plague, when the angel of death "passed over" the houses of the Israelites marked with lamb’s blood. Yet Passover is not only a historical remembrance; it also preserves older roots, connected to ancient springtime festivals of unleavened bread and the sacrifice of the lamb, symbols of renewal, migration, and seasonal change. It’s a time of reflection on freedom, justice, and divine deliverance, observed with a ritual meal called the Seder, filled with symbolic foods, readings, and songs.

   There was clearly a sentiment among some early Christians that it would be desirable to define their own practices in ways that were different to those of Jewish tradition. The Council of Nicaea, convened by Emperor Constantine in 325 AD, became a critical turning point in the dispute over the date of Easter. The council aimed to resolve various theological disagreements within Christianity, including the question of Easter’s timing. A significant outcome of the Council of Nicaea was the establishment of a formula to calculate the date of Easter that separated it from the Jewish calendar.

The council decided that Easter should be celebrated on the first Sunday following the first full moon after the vernal equinox. This formula, known as the "ecclesiastical" calculation of Easter, was designed to ensure that Easter would always fall on a Sunday, the day of the Resurrection. The decision was made for a number of reasons. Firstly, the separation from Judaism: The Council of Nicaea sought to distance Christian observances from Jewish practices. This was consistent with the growing tendency within early Christianity to establish itself as a distinct religion, separate from Judaism. Secondly, a universal method for calculating Easter was needed to prevent confusion and disagreement among Christian communities across the Roman Empire. Prior to Nicaea, different regions calculated Easter differently, leading to a lack of uniformity in observance. Thirdly, the choice of Sunday as the day for Easter was also theologically significant for Christians, as Sunday was the day of the Resurrection. Celebrating Easter on the first Sunday after the full moon was a way to honour this event in a way that was not tied to the Jewish Passover calendar. The way in which this theological and political distancing from the Jewish traditions was played out was through astronomy, and number.

2. Astronomy

   
   

The Movable Feast

The date of Easter changes each year, unlike Christmas, for example, which is the first day after the three day period of the winter solstice, when at last the days start to become longer. Christmas is clearly a solar feast day, marking the start of the approach of spring. Instead, Easter moves mysteriously through the weeks of March and April, arriving sometimes early, sometimes late. The reason lies not in theology but in astronomy, specifically, in the age-old interplay between the cycles of the sun and the moon.

The rules for Easter’s calculation sound deceptively simple: it is celebrated on the first Sunday after the first full moon on or after the vernal equinox. But this formulation is less a rule than a compromise, a centuries-old attempt to reconcile the solar year (which governs the seasons) and the lunar month (which governs the moon’s visible phases). It also reflects a deeper historical and religious paradox: Easter is a Christian feast that emerged from the Jewish Passover, yet it does not always coincide with it. At the same time, it honours a resurrection that took place in history, while being bound to three cycles: the solar year, the lunar year, and the week of seven days.

This makes Easter a “movable feast” in more than one sense. It moves not just across the calendar but through time itself, linking ancient rituals to medieval councils, Egyptian star-priests to Roman emperors, and Babylonian timekeepers to Christian bishops. The feast is pinned to a celestial logic older than Christianity, yet it's also tied to a uniquely Christian event: the resurrection of Jesus. This gives Easter a dual nature: earthly and divine, cosmic and historical, fixed in faith but fluid in time.

Even more curiously, the date we celebrate Easter does not always match the actual astronomical full moon or the true equinox. The Church uses an ecclesiastical approximation of these celestial markers, based on cycles worked out in antiquity and refined in the Middle Ages. The moon that governs Easter is a calculated moon, not the moon in the sky.

And yet, behind all this liturgical complexity lies a deep human instinct: to seek order in the heavens, to harmonise what we see above with the rhythms of life below. Ancient civilisations, from Mesopotamia to Egypt, understood the moon’s shifting phases and the sun’s journey across the sky as part of a divine order. The very act of calculating Easter is an inheritance of that tradition. Through the prism of this lunisolar feast day, we can glimpse the paradoxes of calendars, the elegance of celestial cycles, the politics of religious timekeeping, and the lingering echoes of pre-Christian spring festivals, when the sun and moon were not just timekeepers, but deities whose dance governed life, death, and renewal.

Easter is like an anchor point for many other feasts in the liturgical calendar.

• Ash Wednesday – 46 days before Easter (marks the start of Lent; 40 fasting days, not counting Sundays).

• Palm Sunday – 1 week before Easter (celebrates Jesus’ entry into Jerusalem).

• Holy Week – the week leading up to Easter, including Maundy Thursday, which commemorates the Last Supper, Good Friday, which commemorates the crucifixion, and Holy Saturday, the vigil before Easter Sunday.

• Eastertide – the 50-day season starting on Easter Sunday.

• Ascension Day – 40 days after Easter (commemorates Jesus' ascension to heaven).

• Pentecost – 50 days after Easter (celebrates the descent of the Holy Spirit).

• Trinity Sunday – the Sunday after Pentecost.

• Corpus Christi – Thursday (or Sunday) following Trinity Sunday.

The earliest possible date for Easter is March 22nd, the day after the equinox. However, this is quite rare, last occurring in 1818, and it won’t happen again until 2285. The latest possible date is the 25th April, and for this to happen the full moon must fall on the 20th March, the day before the equinox. Again, this is quite unusual, and last occurred in 1943. It will occur next in 2038.

The quarter days of the year, equinoxes and solstices, divide the solar cycle into four, but they are complemented by the cross-quarter days of Celtic tradition: Imbolc, Beltane, Lughnasadh, and Samhain, each a seasonal hinge with roots deep in agricultural and spiritual life. Against this solar framework, the lunisolar feasts radiating out from Easter—Pentecost, Ascension, Lent, and more, add another intricate layer, binding religious observance to the moving mirror of the sky, where sun and moon trace out the sacred calendar together.. At the heart of it all is a simple question with a surprisingly complex answer: When is Easter, and why?

One of the most striking features of both Easter and Passover is their role in reconciling the lunar and solar calendars, two ancient systems of timekeeping that don’t naturally align. The solar year, with its steady cycle of equinoxes, solstices, and cross-quarter days, forms the backbone of many pagan agricultural calendars, tracking the return of light, the ripening of crops, and the seasons of labour and rest. The lunar calendar, on the other hand, moves in a different rhythm, with months that follow the moon’s waxing and waning, drifting out of sync with the sun unless adjusted. What festivals like Passover and Easter do, each in their own tradition, is enact a kind of ritual reconciliation between these two cycles. Both are fixed in spring, a season of renewal and emergence, when the light is returning and the earth is stirring. This placement is not accidental: it speaks to the desire to ground human time in the greater cosmic pattern. One could even say that these festivals reflect an effort to tame chaos, to bring lunar mystery into solar order, to align heaven and earth, and to affirm that life, light, and liberation will prevail over darkness and disorder. Whether called Eostre, Pesach, or Easter, these spring rites hint at an ancient human yearning for cosmic harmony, encoded in the very structure of the calendar.

The Ancient Science of Time

For something so deeply tied to ritual and tradition, the dating of Easter is fundamentally a scientific problem. Specifically, as with Passover, it’s the problem of reconciling two (almost) irreconcilable clocks: the lunar month and the solar year. The moon completes its visible cycle, new, full, and back again, in about 29.53059 days (the synodic month), while the earth completes its orbit around the sun in about 365.242199 days. Twelve lunar months add up to just over 354 days, leaving a gap of about 11 days per year between a lunar calendar and the actual seasons.

Left unchecked, this gap would cause lunar months, and therefore festivals tied to them, to drift steadily across the solar year. Passover, for instance, would soon end up in winter, then autumn, then summer. But ancient cultures recognised this long before the advent of telescopes or atomic clocks. They knew the moon’s phases, but they also knew the sun’s seasons. And they wanted both.

The Hebrew calendar, from which Easter partly derives its logic, is a marvel of early scientific ingenuity. It is not purely lunar, nor purely solar, but lunisolar, built to honour the moon’s cycle while staying tethered to the solar year. It does this by adding an entire leap month seven times in every 19-year cycle. This is no accident. This 19-year cycle, known as the Metonic cycle (after the Greek astronomer Meton, who likely inherited it from Babylonian sources), comes astonishingly close to synchronising the two clocks:

To be precise, 19 tropical years (of about 365.2422 days each) equals 6,939.601781 days. 235 synodic (lunar) months (of about 29.53059 days) equals 6,939.68865 days.The difference over the entire cycle? Less than two hours.

The Hebrew calendar uses this pattern to decide which years are “pregnant”, shana me'uberet, with an extra month, and which are not. The rule is elegant: years 3, 6, 8, 11, 14, 17, and 19 of each 19-year cycle get an extra month, Adar I, before the usual Adar II. This system keeps Passover, celebrated on the 15th of Nissan, the first full moon of spring, locked to the season of rebirth, even as the moon waxes and wanes on its own, unsynchronised terms.

This solution wasn’t just clever, it was empirical. It required centuries of careful, naked-eye observation of the skies. Ancient Babylonian and Jewish scholars kept detailed records of lunar risings and settings, equinoxes, and star alignments. They weren’t just priests or sages, but also sophisticated astronomers. The Egyptians, too, observed time with extraordinary care. Their civil calendar of 365 days was linked to the heliacal rising of Sirius, a star they noticed coincided with the Nile’s flooding, a vital event in their agricultural year. Though their calendar drifted with respect to the seasons, the Egyptians were well aware of this. Their solution was not to fix the year with leap days, but to track long cycles, like the Sothic cycle of 1 460 years, over which the drift would realign. These weren’t errors, they were systems built to contain error and absorb it gracefully over time.

In ancient Greece, astronomers like Cleostratus and Meton proposed calendar cycles of 8, 19, or 76 years that would bring lunar months and solar years back into step. The Greeks weren’t inventing these cycles so much as inheriting and reworking them from Mesopotamian and Egyptian traditions. So when early Christian scholars set out to calculate Easter, they weren’t building something from scratch. They were drawing on an ancient lineage of timekeeping that stretched back thousands of years, across cultures, languages, and belief systems. However, added to the difficulty of determining a calendar that keeps track of the solar and lunar cycles as accurately as possible was the need, felt by many, to celebrate Easter on a Sunday.

Initially, some, particularly in Asia Minor, followed the Quartodeciman tradition: celebrating Easter on the 14th of Nissan, regardless of the day of the week, in direct alignment with the Jewish calendar. But others insisted Easter must always fall on a Sunday, the day of the Resurrection. By the second century, this disagreement had grown sharp. Polycarp, the bishop of Smyrna and a disciple of the Apostle John, travelled to Rome around 155 CE to debate the date with Anicetus, the Bishop of Rome. They agreed to disagree, but the seeds of conflict were planted. The Quartodeciman controversy began. Christians in Asia Minor (e.g., Polycarp of Smyrna) celebrated Easter on 14 Nisan (like Passover), regardless of the day of the week. However, Rome and the West preferred to celebrate Easter on the Sunday after Passover, symbolising the resurrection. Bede is critical of those who "contend that the same should be kept on the fourteenth moon with the Hebrews”(7), adding that "this manner of keeping Easter continued among them for the space of 150 years, till the year of our Lord’s incarnation 715."(8)

By the early 4th century, the Christian world was fractured not only by theology but by calendar wars. Churches in Alexandria, Rome, Antioch, and Jerusalem all used different systems to compute the date of Easter. Some still followed the Jewish calendar; others rejected it, often for anti-Jewish reasons, preferring to calculate their own paschal full moon. The result was ecclesiastical chaos: in some years, neighbouring Christian communities celebrated Easter on different days.

Enter Constantine. At the Council of Nicaea in 325 CE, the same council that produced the Nicene Creed, Constantine summoned bishops from across the empire to resolve theological and administrative disputes. One of their goals was to unify the date of Easter.

The council’s decision was as political as it was astronomical. Easter would no longer be tied directly to the Jewish Passover. Instead, the council decreed that Easter would fall on the first Sunday after the first full moon following the vernal equinox, a formula that seemed both symbolic and cosmically sound. However, the details were left vague. There was no agreed-upon definition of “vernal equinox” (should it be March 21st? the actual equinox?). Nor was there a unified method for finding the “paschal full moon”, a theoretical moon, not necessarily the astronomical full moon. The Alexandrians, with their advanced astronomical tables and centuries of observation, soon took the lead in devising a system. But Rome developed its own version. And so, ironically, the attempt to unify Easter simply spawned new methods of division.

One result was that the Church, for the next thousand years, relied on computed moons, not observed ones. Easter was no longer tied to the sky directly, it was bound to ecclesiastical astronomy, an abstraction built on old cycles, tables, and approximations. The full moon that determined Easter wasn’t necessarily the one shining above your head. It was the moon as the Church calculated it to be.

The entire process became known as the computus, Latin for “computation”, and it would evolve over centuries, absorbing errors, corrections, reforms, and politics along the way. And yet the formula from Nicaea, however ambiguously framed, still stands:

A simple rule, concealing a vast depth of celestial mechanics and theological compromise.

The Computus: How to Calculate a Resurrection

After Nicaea, the Church faced an unusual task: to predict a full moon that didn’t exist, at least, not in the sky. This “paschal full moon” was a construct, not a literal moon, and had to be defined using cycles and calculations that would remain valid year after year, ideally forever. The tool for this was the computus, a set of mathematical rules that allowed anyone (in theory) to determine the date of Easter from a given year.

Determining the date of Easter has always been a complex balancing act, involving three overlapping cycles: the moon, the sun, and the week. First, Easter must fall in the first lunar month of spring, tying it directly to the phases of the moon, specifically, the first Sunday after the first full moon following the spring threshold. But what defines "spring"? Some early Christian communities used the vernal equinox (around March 21) as the marker, while others followed older astrological logic, placing the spring point when the sun entered the constellation Aries. Second, Easter must fall on a Sunday, a seven-day rhythm unique to Judeo-Christian tradition, making the festival both lunar and weekly. This blend of lunar, solar, and weekday cycles made the computus, the system for calculating Easter, one of the most intricate intellectual efforts of the early Church.

Easter is a lunar-solar festival, determined not just by the time of year, but also by the phases of the moon. It must fall on the first Sunday after the first full moon following the spring equinox, which means it depends on three overlapping cycles: the solar year, the lunar month, and the seven-day week. The problem is, 12 lunar months (about 29.5 days each) total roughly 354.36 days, which is about 11 days shorter than the solar year of 365.25 days. This mismatch accumulates quickly and must be corrected by occasionally inserting extra lunar months, known as embolismic months, to keep the calendar in sync with the seasons.

Ancient astronomers and theologians developed several cycles to manage this. If you look at the lunar excess of 0.3683 months per solar year, it can be approximated by various rational fractions:

• 3/8, meaning 3 intercalated lunar months every 8 years

• 4/11, or 4 embolisms every 11 years

• 7/19, the famous Metonic cycle, used by the Babylonians and later the Greeks and Christians

• 31/84, a less common but surprisingly close approximation

Each of these was used in different calendrical traditions, but the 19-year Metonic cycle proved the most reliable for aligning solar years with lunar phases, without requiring too many corrections. Even so, the Metonic cycle runs slightly long, off by about one day every 312 years, so the Christian computists introduced an occasional correction known as the saltus lunae ("leap of the moon"), skipping one day in the lunar cycle every 19 years.

Then there’s the weekly cycle to consider. Easter must fall on a Sunday, so any full calendrical cycle that aims to repeat the pattern of Easter dates must also be divisible by 7.

   The Solar Cycle, a 28-year system that tracks the recurrence of weekdays with calendar dates, solved the problem of the weekly cycle. Within this cycle, each year was assigned a Dominical Letter, from A to G, corresponding to which day of the week January 1 falls on. Using the Dominical Letter, one could determine the Sundays of the year, and then pair that with the lunar tables to find the first Sunday after the paschal full moon. Let’s say you’re trying to compute Easter for the year 1204. First, you find its Golden Number: 1204 ÷ 19 = 63 remainder 7 → Golden Number = 7.

Then, using a table, you find the paschal full moon associated with GN 7, let’s say it's April 5th. Now find the first Sunday after April 5. In 1204, that was April 6.

So, Easter in 1204 was April 6th.

Because of the way leap years shift the days of the week, calendar dates and weekdays only repeat in the same pattern every 28 years in the Julian calendar. Thus, a true Easter cycle must be divisible by both 19 (for lunar alignment) and 28 (for weekdays), giving us the classical 532-year Paschal cycle (19 × 28), within which Easter repeats identically.

As Bede explains:

However, before this Roman system became standard, other approaches were tried. The early Celtic Church in Ireland used an 84-year cycle. This was a clever attempt to harmonise solar and lunar time. Over 84 solar years (about 30,681 days), there are roughly 1039 lunar months, which is very close to 84 × 12.3683, the number of lunar months in 84 years. The discrepancy between 84 solar years and 84 lunar years is about 924 days, or 11 days per year × 84 years. That’s equivalent to 30 intercalated lunar months plus 24 days, or 31 embolisms minus 6 days, close enough to make the 84-year cycle usable. The Celtic system dealt with lunar drift by inserting a saltus lunae every 12 years to re-align the calendar.

Still, the 19-year Metonic cycle eventually won out due to its greater long-term accuracy. The shift from the 84-year cycle to the Roman 532-year computus marks a turning point in ecclesiastical alignment and theological standardisation.

The Celtic Easter Computus vs. The Roman Method

   The Celtic Church used a distinct method for calculating the date of Easter, which was different from the one adopted by the Roman Church. This difference was not merely a matter of local preference; it became a major theological and political point of contention, culminating in the Synod of Whitby in 664 CE. While the Roman method relied heavily on the Metonic cycle (a 19-year cycle to track the phases of the moon and align them with the solar year), the Celtic method was based on an 84-year cycle, a system that was not as mathematically sophisticated as the Roman approach but still rooted in lunar-solar harmonics. Bede writes in The Ecclesiastical History Of The English Nation:

   The Metonic method was more precise in that it harmonised the lunar phases with the solar calendar every 19 years, ensuring that the Paschal Full Moon (the first full moon after the spring equinox) fell on the same date in the solar year. This, in turn, fixed Easter to a consistent period each year.

   By contrast, the Celtic Church’s 84-year cycle was rooted in simpler calculations, and though it may not have been as scientifically accurate as the Roman method, it still sought to align Easter with the lunar-solar cycles. It was based on the observation that every 84 years, the Paschal Full Moon and Easter Sunday would fall on the same dates and days of the week, aligning with older systems of timekeeping that combined solar and lunar rhythms.

   The 28-year cycle, known as the solar cycle, is a practical tool for understanding how calendar dates and days of the week realign over time. In a standard calendar year of 365 days, each date advances by one weekday every year (for example, if April 1 falls on a Monday one year, it will fall on a Tuesday the next). But every four years, a leap year adds an extra day, February 29, which causes the calendar to skip ahead by two weekdays instead of one. Because of this mix of one-day and two-day shifts, it takes exactly 28 years for the same calendar dates to fall on the same weekdays again in the Julian calendar, which was widely used in early Christian Europe. This made the 28-year cycle a helpful way to predict weekdays for future years, and it was one of the early building blocks for Christian calendar-making, especially when calculating moveable feasts like Easter. But how could this be aligned with the lunar year?

The Metonic Cycle: A Lunar Shortcut

The key to the computus is the Metonic cycle, a discovery from ancient Babylon that was formalised by the Greek astronomer Meton of Athens in the 5th century BC. Meton found that 19 solar years is nearly equal to 235 lunar months, which means that after 19 years, the phases of the moon return to the same dates in the solar calendar. This cycle became the foundation for predicting lunar events without observing the sky.

In the computus, each year in this 19-year cycle was assigned a Golden Number, from 1 to 19. If you knew the Golden Number of a given year, you could find the approximate date of the paschal full moon using precomputed tables. The Church developed and updated these tables over time, most famously in the work of Bede the Venerable in the 8th century.

Layer Upon Layer of Approximation

These cycles weren’t perfect, of course. The Metonic cycle is off by a little over two hours every 19 years. The Julian calendar, which was still in use, drifts by one day every 128 years compared to the solar year. But for medieval Europe, where precision meant staying within a day or two over centuries, these systems were astonishingly effective.

To make things easier for clergy and monks, tables were developed, dense, coded, and deeply symbolic. A typical medieval Easter table would include the Golden Number, Dominical Letter, Epact (a number that tracked the age of the moon on January 1), and the date of the paschal full moon. From these, one could find Easter. All this was done without telescopes, without clocks, without even Indian / Arabic numerals. The computus was a triumph of symbolic logic, coded in Latin, cloaked in religious significance, and passed down through manuscripts with painstaking care.

The southern Irish Church adopted the Roman cycle by 631, followed by the northern Irish (excluding Iona) by 686, the Picts by 710, and finally Iona in 715 or 716.

Fixing the Heavens: The Gregorian Reform and Its Aftermath

By the 16th century, the Julian calendar had drifted out of sync with the solar year by about 10 days. That might not seem like much, but it was enough to throw the spring equinox, and therefore the date of Easter, off schedule. According to the Church, the vernal equinox was supposed to happen on March 21, the benchmark set at the Council of Nicaea. But in 1582, it was already happening around March 11, not just an inconvenience, but a cosmic embarrassment.

The Papal Fix

Pope Gregory XIII launched a commission to correct the drift and re-anchor the Church's calendar to the heavens. Led by Jesuit mathematician and astronomer Christopher Clavius, the solution was simple: Skip 10 days: In 1582, the calendar jumped directly from October 4 to October 15. Tweak the leap year rule: Instead of having a leap year every 4 years without exception (as in the Julian calendar), the Gregorian calendar skips leap years in century years not divisible by 400. So 1700, 1800, and 1900 were not leap years, but 1600 and 2000 were. This new system slowed the drift to about 1 day every 3,300 years.

Easter in the New Order

The computus wasn’t entirely scrapped. The Gregorian reform included a revised computus that recalculated the Epacts (those lunar age numbers) to reflect the new calendar rules. This adjusted the paschal full moon tables to keep Easter in line with the spring equinox.

So even in this modernised system, Easter is still based on:

• The Golden Number (still calculated from the 19-year Metonic cycle)

• The Epact, now tweaked for Gregorian precision

• The Dominical Letter, recalibrated for the new leap year rules

In short, the system was updated but not replaced, it was renovated, not demolished.

Not Everyone Got the Memo

While Catholic countries adopted the Gregorian calendar almost immediately (Spain, Italy, Portugal, Poland), others lagged behind for decades, or centuries. Protestant regions like England didn’t adopt it until 1752, by which time they had to skip 11 days instead of 10.

Orthodox churches still use the Julian computus, which is why their Easter can be one to five weeks later than the Western Easter. Astronomers, meanwhile, began to prefer astronomical Easter, based on actual lunar phases and equinox dates, not computus approximations. But the Church has never used this version.

This divergence means that even today, Christians who celebrate the same resurrection do so on different days, using different moons, in calendars that disagree by centuries.

While the Gregorian reform was a pan-European event, the British Isles had long been a battleground for calendar disputes, especially when it came to Easter. As early as the 7th century, the Synod of Whitby (664 CE) saw the Celtic Church, with its own method of Easter calculation, overruled by Roman practice in a pivotal decision to unify Christian observance under Roman authority. This wasn't just a technical matter, it was a question of allegiance and identity, with Easter as the fault line.

Why the Sun and Moon Matter

It might be tempting to dismiss all this calendrical wrangling, the epacts, the intercalations, the saltus lunae, as arcane or pedantic. But for early Christian thinkers like the Venerable Bede, the effort to properly reckon Easter was anything but trivial. It was a sacred task, linking the temporal world to divine order, and embodying rich theological symbolism in the very structure of time.

In his Reckoning of Time, Bede explains the intricate movement of the moon across the solar year, showing how each year's moon phases are displaced by 11 days, only to come back into alignment every 19 years. This is the foundation of the Metonic cycle and the system of epacts, which indicate the moon’s age on a fixed date, March 22, the earliest possible Easter date. Bede writes:

For Bede, this isn’t simply an astronomical formula, it’s theology inscribed into the heavens. The rising of the sun over the equator at the spring equinox and the full moon that follows it are not arbitrary natural events, but symbols of Christ and the Church:

This idea, that the Church, like the moon, reflects the light of Christ, the Sun of righteousness, was not just poetic; it was liturgically and cosmologically central. Timekeeping was theology. To get Easter wrong wasn’t just a calendar mistake, it was a failure to mirror the divine order.

So when we speak of aligning the feast of Easter with the first full moon after the equinox, it’s not just technical compliance. It's about echoing the moment of creation, when God placed the "greater light" and the "lesser light" in the sky to rule the day and the night (Genesis 1:16), and when Christ, in rising from the dead, brought new light to the world. This alignment, sun and moon, solar and lunar, heaven and earth, is what the computus strives to preserve.

The seven-day week, far from being arbitrary, reflects a cosmic structure inherited from ancient astronomy, in which each day was associated with one of the seven classical "planets": the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. Early Christians embraced this system not merely out of convention, but to imbue sacred time with symbolic depth. Sunday, dies Solis, the day of the Sun, was quickly identified with the resurrection of Christ, the "Sun of Righteousness" (Malachi 4:2), whose rising brought light and renewal to the world. Church Fathers like Justin Martyr and Tertullian saw no contradiction in this association; rather, they claimed that the symbolism of the rising sun was providential, pointing to the risen Christ. Just as the sun governs the day, so Christ governs time and salvation. Meanwhile, the Moon, which reflects the sun’s light and undergoes visible phases, became linked with the Virgin Mary and, by extension, the Church, ever receptive to divine illumination. In this way, Easter becomes a cosmic moment of reconciliation: the solar year and lunar month are brought into harmony, the male and female principles (Sun and Moon, Christ and Church) are aligned, and all of time, weekly, monthly, and yearly, is orchestrated around the resurrection. As Bede wrote in De Temporum Ratione, “the perfection of solar splendour... ought to go before the lunar, which is illuminated”, a reminder that the calendar's intricacies are not just technical, but deeply theological.

3. Musings on Eostre

   Beneath the theological disagreements may lie something older. The British Isles are steeped in pre-Christian traditions of equinoxes and solstices, with sacred sites aligned to solar and lunar events, nowhere more evocatively than in the Boyne Valley of Ireland. The Hill of Slane, where St. Patrick is said to have defiantly lit the Paschal fire in view of the pagan High King at Tara, is surrounded by megalithic monuments such as Newgrange, Knowth, and Dowth. These structures predate Christianity by millennia, yet encode a deep understanding of solar cycles and possibly lunar rhythms as well.

   It is not far-fetched to wonder whether these ancient sites hosted their own spring rites, rites that similarly honoured the return of light and life. The timing of the Paschal fire, lit in spring on a hill of ancestral power, might have been a calculated act: not only a Christian proclamation, but a deliberate echo of older seasonal observances, re-framed through the gospel lens. Easter, in this view, becomes not just a celebration of resurrection, but a moment of cultural synthesis, in which the solar and lunar, the old and new, the local and universal are all brought into alignment.

A pre-Christian echo

Deeper than that lies a tantalising pre-Christian echo: Bede, the 8th-century English monk and historian, tells us that the month of Ēosturmōnaþ (April) was named for a goddess called Eostre, worshipped in springtime fertility rites before the coming of Christianity.

In Chapter 15 of De Temporum Ratione (The Reckoning of Time, c. 725 AD), he writes:

Though historical evidence for Eostre is scarce outside of Bede, the association suggests that Easter may carry with it the lingering hint of older, seasonal celebrations, woven into the fabric of a Christian resurrection festival that still dances to the rhythm of the moon.

The earliest explicit mention of Easter by name comes from (8th century). This is the only known historical reference to Eostre, the supposed Anglo-Saxon goddess. It's worth noting that Bede doesn’t mention "Easter" as a Christian celebration in this context, but rather explains the origin of the name of the month in which Easter usually falls, which English Christians had repurposed. In most other languages, the Christian festival is named after Passover (Hebrew Pesach), such as Pascha in Latin and Greek, or Pâques in French.

   While the link between the Mesopotamian goddess Ishtar and the Anglo-Saxon figure Eostre is often suggested, it remains speculative. Eostre is mentioned only once in surviving historical sources, by Bede, and there is no solid evidence connecting her to a widespread ancient tradition. Nonetheless, both Ishtar and Eostre are associated with spring, fertility, and renewal, and the symbolic logic of a goddess descending into darkness and returning in glory resonates across cultures. Whether or not there is a direct historical lineage between them, the thematic parallels are striking.

The motif of a deity descending into the underworld and returning after three days is strikingly echoed in the Christian story of Jesus’s death and resurrection. In the ancient Mesopotamian myth of Ishtar (or Inanna), the goddess of love, fertility, and war, she descends into the realm of the dead, stripped of her powers and ultimately slain. After three days and nights, she is restored to life and ascends again, bringing renewal and fertility with her. This imagery of descent, death, and triumphant return resonates with the three days Jesus lay in the tomb, culminating in the resurrection on Easter Sunday. While Christianity emerged in a distinct theological and cultural context, the parallel structure of these narratives suggests that such patterns of death and rebirth, especially tied to the seasonal cycles of spring, were widespread in the ancient imagination. Whether by cultural inheritance, theological resonance, or symbolic convergence, the Easter story may carry an ancient echo of the sacred drama of returning light and life.

   Whatever the history, the name Easter, in English, carries the tradition of the mysterious Eostre, and of this ancient continuity of watching the sun and the moon closely, over long periods of time, and celebrating renewal and spring. Though Bede alone records her name, the idea she represents is timeless. In every new spring, there is something old stirring, a rhythm, a reckoning, a return. Easter, then, is not just a Christian feast or a calendrical calculation. It is a reminder that we live within the cycles of the heavens, and that each year, as we chart the moon and wait for the equinox, we are also reconnecting to the ancient world, to its science, its symbolism, and its sense of wonder.

As for the Christian celebration itself, it predates Bede by centuries. By the mid-2nd century, Christians were already observing an annual celebration of the resurrection, usually on or near the Jewish Passover (14th of Nisan). The earliest known reference to this celebration comes from Melito of Sardis (c. 160 AD), who wrote a homily titled On the Pascha. The Quartodeciman controversy in the 2nd century, where churches disagreed on whether to celebrate Easter on 14 Nisan or the following Sunday, shows that the practice was already well established and hotly debated.

Bede and Stellarium: The Celestial Logic of Easter

Is there an ancient spring festival behind Easter and Passover, one that not only honours the turning of the year at the equinox, but also demands the presence of the full moon? While many cultures have marked the vernal equinox as a moment of cosmic balance and renewal, the idea of a festival timed by the first full moon after the equinox is far less common, and far more intriguing. It hints at something more complex: a sacred dance between solar and lunar rhythms, one that requires both light and darkness, both motion and opposition.

In Christian tradition, Easter celebrates the resurrection of Jesus. In The Reckoning of Time (8th century), the Venerable Bede explains with great care how the sun and moon relate to one another throughout the seasons. When the moon is full, it rises in direct opposition to the sun, and thus, as Bede notes:

In other words, the full moon is the mirror of the sun, placed opposite it in the great wheel of the zodiac. This mirroring reveals something deeper: when the sun rises in Aries at the spring equinox, the full moon rises in Libra or Virgo, a perfect cosmic opposition.

Bede continues:

The full moon, then, becomes a kind of prophecy, a glimpse into where the sun will be six months in the future. This idea may have deep roots. In Babylonian myth, the goddess Ishtar’s journey from the underworld is celebrated at the full moon, perhaps anticipating the return of the sun to her realm in autumn.

The Christian tradition inherited not only this lunar awareness, but also the complexity of a luni-solar calendar, where months are lunar but years are solar. As Irv Bromberg notes, the Passover lamb was traditionally sacrificed on the 14th of Nisan, just after the full moon, at the moment closest to the equinox. This careful balancing act led to the need for periodic leap months, a 13th month added every few years to keep the calendar in sync with the seasons.

Bede also affirms that Easter must come after the equinox:

Today, we can visualise these celestial events using tools like Stellarium, a digital planetarium. In 2025, Easter and Passover fall late in the season: the sun rises in Pisces, the sign of the fish, an early Christian symbol, and the full moon rises in Virgo, associated with Mary. Two thousand years ago, the sun rose in Aries, the ram, and the moon in Virgo. Six months later, the sun would rise in the claws of Scorpio, just below Ophiuchus, the so-called 13th zodiac sign, long ignored but now increasingly recognised.

The sun rose in Aries, the ram, at Passover time, two thousand years ago, and the moon rose in Virgo, and six months later, the sun would be rising in the claw of Scorpio, just below Ophiuchus. The connection with the ram or lamb would make the sacrificial paschal lamb extra significant, for Hebrew and Christian traditions alike, especially when one remembers that in the Hebrew story of the twelve plagues of Egypt, the blood of the sacrificed lamb, painted over the front doors, was to protect the houses from the wrath of the Lord, allowing the plagues of Egypt to 'skip' over, or pass over them. Today, some Christian traditions in the east have a red egg at Easter which they keep in their house for the year until the following Easter, symbolising the blood of Christ, to protect the household. This alignment lends special significance to the paschal lamb in Hebrew and Christian traditions alike.

In the development of Roman Christianity, a striking dual tension emerges: a rigorous effort to distance itself not only from paganism, but also from Judaism, and, perhaps more surprisingly, from alternative forms of Christianity itself, such as the Celtic tradition. This triple rejection was not just theological, but also symbolic and liturgical. The early Church was carving out its identity in a crowded spiritual landscape, and did so by opposition, by what it was not, as much as by what it was.

This distancing is nowhere more apparent than in the long controversy over the date of Easter. Early Christians in places like Asia Minor and the Celtic lands followed traditions closely linked to the Jewish Passover, celebrating the resurrection on or near 14 Nisan, according to the lunar calendar. These were not heresies , they were older practices, rooted in the context of the actual events described in the Gospels. But Roman Christianity rejected this, insisting on a uniform Sunday celebration, separate from the Jewish calendar. This was more than a calendar quarrel. It was a move to define Christian time apart from Jewish time, to erase any reliance on a tradition now seen as obsolete or even hostile.

And yet, the irony is unmistakable. Jesus is called the "Lamb of God", a title that only makes sense in light of Passover, when a lamb is sacrificed to mark liberation from bondage. The Gospel writers intentionally frame Jesus' death during Passover week, reinforcing the connection. Even the stars align: on the Passover of the year 30 CE, Stellarium shows the sun rising in Aries, the Ram, a cosmic echo of the sacrificial lamb. These symbols are deeply intertwined, cosmically and ritually. To reject the Jewish root of the tradition is to reject the soil in which the entire Christian mythos was planted.

Simultaneously, Roman Christianity was distancing itself from paganism and local expressions of faith like those practiced in the Celtic Church, often more ascetic, mystical, and deeply tied to nature and monastic independence. Councils like Whitby in 664 were less about doctrinal purity than they were about power, conformity, and ecclesiastical centralisation. The Roman Church demanded not just orthodoxy, but uniformity. There was little room for pluralism, whether in dates, tonsures, or theology.

Window to the ancient world

As we trace the long and winding path that leads to the modern Easter, through cycles of moon and sun, through tables and disputes, through equinoxes and ecclesiastical decrees, we begin to see not just a religious observance, but something older and more universal. The union of solar and lunar timekeeping is not merely a technical achievement, it is a ritual act of reconciliation between two celestial forces that govern life on Earth. In ancient mythologies, this duality was often made divine: the sun and moon as the twin eyes of a god, gazing down from the heavens. In Egypt, the sun and moon were the right and left eyes of Horus, though others claimed they belonged to Ra or even Thoth. One eye that sees clearly by day, the other that watches subtly by night. To reconcile the two, to make them dance together in cycles and calendars, was a way of keeping harmony not just in time, but in the cosmos itself.

That Easter and Passover both attempt, in their different ways, to honour both the solar and lunar cycles, is no coincidence. These feasts mark not just a moment in sacred history, but a celestial choreography that reflects a deeper truth: that new life begins when old patterns are reawakened. The full moon after the equinox is not just a date, it is a symbol of balance: light and dark in equal measure, the moon at its fullest, the earth turning green again.

Conclusion

The history of Easter’s calculation, from its roots in early Christian disputes to its eventual harmonisation in the Gregorian reform, reveals a complex tapestry of theological, cultural, and astronomical considerations. While the Gregorian calendar reform was a pan-European effort to standardise the observance of Easter, the British Isles remained a battleground for much longer, with the Celtic Church’s distinct method of calculating the feast day standing as a symbol of religious independence and cultural identity. The Synod of Whitby in 664 CE marked a turning point, when the Roman Church overruled the Celtic computus, bringing the British Isles in line with Roman practice, not just for practical reasons, but as a powerful assertion of ecclesiastical authority. The Celtic Church, with its roots in ascetic monasticism, was deeply tied to the ancient rhythms of the land and its own distinctive traditions, which included a method for determining Easter based on an 84-year cycle, a number that seemed to resonate with the lunar-solar patterns of timekeeping.

The computus is complex, but also quite simple in a way. Three cycles must be included in one greater cycle: the solar year, the lunar year, and the days of the week, which reflect the seven planets. Number was clearly very important to early Christian theologians. In praise of computus, Isidore of Seville says:

The disagreement, however, was more than a simple matter of miscalculations. It was a matter of identity and allegiance, with Easter acting as the fault line between two divergent expressions of Christianity, one shaped by the Roman imperial system, and the other deeply rooted in local, indigenous traditions. Figures like St. Columba, who spread Christianity through Ireland, Scotland, and northern Britain, embodied this spiritual authority, which often rivalled Rome’s influence. The division within royal households, such as that of Queen Eanfled and King Oswiu of Northumbria, exemplified the very personal and political stakes of the Easter dispute. The Synod of Whitby resolved this theological divide in favor of Roman practice, but not without leaving a lasting legacy of tension between the Celtic and Roman traditions.

However, beneath these ecclesiastical debates, there may lie a deeper and older connection to the land and its ancient rhythms. The British Isles, particularly Ireland, have long been centres of pre-Christian solar and lunar observance, with sacred sites like the Boyne Valley, Newgrange, Knowth, and Dowth revealing an ancient understanding of equinoxes and solstices. These megalithic structures, older than Christianity by thousands of years, are aligned with celestial events in ways that suggest a deep knowledge of both solar and lunar cycles. St. Patrick’s defiant lighting of the Paschal fire on the Hill of Slane, in direct view of the pagan High King of Tara, may not have been merely a Christian proclamation, but a calculated and deliberate act of cultural synthesis. By aligning the Paschal fire with the spring equinox, a time sacred in earlier traditions, Patrick may have been reinterpreting and repurposing pre-existing seasonal rites, rather than erasing them entirely.

In this view, Easter becomes not only a celebration of Christ’s resurrection but a reflection of the ancient tension between the old and the new, the solar and the lunar, the local and the universal. The merging of Christian observances with earlier Celtic festivals shows how Christianity did not simply replace pre-Christian beliefs, but often absorbed and re-contextualised them. This process of cultural synthesis gave rise to a unique form of Christianity in the British Isles, one that was as much about aligning the cosmic rhythms of the sun and the moon as it was about celebrating the resurrection of Christ. The timing of Easter, rooted in astronomical computations and ancient rites, thus becomes a living testament to the enduring power of both tradition and transformation, a reminder that the cycles of the heavens, much like those of human history, are not linear but deeply intertwined.

Appendix

The Solar and Lunar Roots: 28, 19, 84, and 532

To fully understand how the 84-year cycle of the ancient Celtic computation system fits within broader timekeeping systems, it's important to explore the 28-year solar cycle and the 19-year Metonic cycle, as these cycles together form the 532-year Paschal cycle, which was crucial in determining the date of Easter in the Julian calendar. 84 is 3 x 28, or 19 x 4 + 8.

Solar year ≈ 365.242199 days

Synodic lunar month (lunation) ≈ 29.53059 days

12 lunar months ≈ 354.36708 days

Lunar year is 10.875119 days (almost 11 days) shorter than the solar year, which is why you need to add extra lunar months over time to realign.

With the 84 year computus: 84 lunar years = 84 × 354.37 = 29 768.88 days

84 solar years = 84 × 365.2422 = 30 678.35 days

Difference = 909.47 days ≈ 30.8 lunar months

So over 84 years, the lunar calendar falls behind the solar calendar by about 30.8 lunar months. To correct for this, the Celtic computists intercalated 31 lunar months (called embolisms) across the 84-year period. This produced:

• 84 × 12 = 1008 regular lunar months

• +31 embolisms = 1039 lunar months in total

That gives:

• 1039 × 29.53059 = 30,679.8 days ≈ 84 solar years (within ~1.5 days)

So it’s a decent approximation, but not quite as accurate as the Metonic cycle: The 19-year Metonic cycle = 235 lunar months, and 235 × 29.53059 = 6939.69 days. 19 solar years are 6939.60 days. The error: less than 0.1 days per cycle.

The 28-year solar cycle is rooted in the mechanics of the Julian calendar, which consists of a year length of 365.25 days. This means that, on average, the calendar year is slightly longer than the solar year by about 0.25 days. As a result, the calendar shifts by one day of the week each year, with a leap year adding an extra day every four years. Because of this shifting pattern, after 28 years, the dates on the calendar align with the same weekdays again. This 28-year cycle is purely solar, focusing on how days of the week repeat with specific dates in the calendar year.

The 19-year Metonic cycle is a lunisolar cycle, meaning it combines both solar and lunar elements. It is based on the observation that 19 solar years are nearly identical to 235 lunar months. This makes it an ideal method for syncing lunar phases with the solar year, ensuring that the phases of the moon repeat on the same calendar dates every 19 years. The Metonic cycle was used extensively in ancient Babylonian and Greek calendrical systems and later adopted into the Christian computus, the system used to calculate the date of Easter.

When the 28-year solar cycle and the 19-year Metonic cycle are combined, we get the 532-year cycle. This is the period after which both the lunar phases (as tracked by the Metonic cycle) and the days of the week (as tracked by the 28-year solar cycle) align again in the Julian calendar. Thus, after 532 years, the exact same sequence of Easter Sundays, moon phases, and weekdays would repeat. The 532-year cycle became foundational to the calculation of Easter in the classical computus.

Celtic Number Mysticism and Astronomical Harmony

The connection between these cycles and other celestial phenomena, such as the planetary cycles and precession, opens a dialogue between ancient cosmologies and the Christian desire to map sacred time onto the rhythms of the universe. One of the more intriguing ideas in this context is an equation, with the planetary cycles, the Metonic cycle and precession all expressed in earth years:

100,000,000 / (planetary cycles × precession × Metonic cycle) ≈ 28

These are the orbital periods defined in terms of earth years:

• Mercury: 0.24 years

• Venus: 0.615 years

• Earth: 1 year

• Mars: 1.88 years

• Jupiter: 11.86 years

• Saturn: 29.46 years

• Moon: 0.0748 years (27.321661 / 365.256 = 0.0748014)

• Precession of the Equinoxes: 25 920 years

• Metonic cycle: 19 years

  
  

The value of 0.0748 years for the Moon is derived by considering the synodic month, the time it takes for the Moon to return to the same position relative to the Sun as seen from Earth, and based on a sidereal year of 365.256 days.

100 000 000 / (0.24 x 0.615 x 1.88 x 11.86 x 29.46 x 0.0748 x 25 920 x 19) = 27.999364 ≈ 28

This is a very pleasing result because 28 is a perfect number. Another intriguing aspect of 28, is that 1 / 28 can be translated geometrically to a ratio between pi and the square root of three, to within a small margin of error. 

• 2π / (3 x √3) x 29.53059 x 1 000 = 35 708.3769 (the number of days in a lunation is 29.53059)

• 1 000 000 / 28 = 35 714.2857

• 0.24 x 0.615 x 1.88 x 11.86 x 29.46 x 25 920 x 0.0748 x 19 / 100 = 35 715.0970

Curiously, this number, 35 715, is, to within 2 inches, the length of the rectangle formed by the three largest pyramids at Giza, according to Flinders Petrie.

This equation suggests a hidden harmonic resonance between:

• The orbital periods of the seven classical planets (which include the Sun and Moon),

• The precession of the equinoxes (approximately 25,920 years), and

• The Metonic cycle (19 years).

The result of this calculation is approximately 28, the number of years it takes for the same calendar date to fall on the same weekday in the Julian system. This connection suggests a symbolic unification of:

• The sacred sevenfold planetary order, reflecting ancient traditions that linked the seven planets to divine powers,

• Cosmic cycles of time, such as the precession and the Metonic cycle,

• The ritual calendar that structures the Christian year, especially in relation to the calculation of Easter.

Thus, the fact that the 28-year cycle arises from these multiple celestial elements can be seen as a symbolic reflection of a deeper cosmological harmony, a desire to align earthly rituals with the divine order of the universe.

When this insight is multiplied by the 19-year Metonic cycle (28 × 19), we arrive again at the 532-year Paschal cycle, reinforcing the idea of a beautifully repeating weave of sun, moon, and week. This structure not only mirrors the way the heavens are seen to work but also reflects the Christian understanding of Easter as an event that connects the divine order with human time.

Is the 84-Year Cycle an Artefact of Earlier Cosmic Models?

The Celtic 84-year cycle is intriguing because it may represent a vestige of earlier systems of timekeeping that were not as mathematically precise as the 532-year cycle. While it may not have the same rigour as the Roman Metonic cycle, the 84-year cycle still reflects a desire to map lunar-solar harmonics onto a larger cosmological framework. The 84-year cycle seems to reflect a transitional phase in the development of calendrical systems, one that may have evolved before the more precise Alexandrian computus became dominant in the Roman Church. The Metonic cycle is more precise in aligning the solar and lunar years.  However, the 84 year cycle might have been seen as more advantageous in some ways, as 84 = 3 × 28, and 28 is the cycle in which the days of the week repeat in the Julian calendar (because 7 days × 4 years = 28, due to leap years). So after 84 years, the moon phases and weekdays realign, making it an attractive reset point.

In particular, the number 84 has a significant symbolic resonance. It can be factored into 7 × 12, linking the sacred number seven (which often appears in ancient cosmologies and Christian symbology) with the 12 months of the year. This provides a direct connection between the concept of time (as divided into months) and the seven-fold sacred structure that pervades many ancient systems of thought.

Moreover, 84 lunar years is approximately equivalent to 1039 lunar months, which may have been useful for tracking the phases of the moon over a long period, especially in cultures that relied heavily on lunar observations for ritual and agricultural purposes.

Additionally, some early Christian texts from Celtic regions suggest that Easter was sometimes celebrated on the same date as the Jewish Passover, or what was referred to as the "14th day of the moon." This Quartodeciman tradition, which the Roman Church eventually rejected, indicates that the early Celtic Church was deeply rooted in lunar and Jewish timekeeping methods, further linking the 84-year cycle to earlier practices.

a solar cycle of 28 years. This is the time it takes for the days of the week to repeat on the same dates, in the Gregorian calendar, or any 365 day calendar system which features weeks of 7 days. The 28-year solar cycle arises from the way the Julian calendar handles leap years and days of the week: the same date advances by one day of the week each year (e.g., January 1 advances from a Monday to a Tuesday in the following year). Every four years, a leap year adds an extra day, which shifts the weekday by two days (e.g., from Monday to Wednesday). Because of this shifting pattern, it takes 28 years for the same date to align with the same day of the week again. This is the length of time it takes for the pattern of leap years and weekdays to repeat in the Julian calendar.

   This 28 year cycle can be combined with the Metonic cycle of 19 years, so that the lunar phases will occur on the same calendar dates, and the same calendar date will fall on the same day of the week. Multiplying the two (19 × 28) yields a 532-year cycle, which is when both lunar phases and the weekdays of the Julian calendar realign. Thus, after 532 years, the full sequence of Easter dates repeats identically, Easter being a movable feast, dependent on both the sun and the moon. In 1582, the Gregorian reform corrected the drift of the equinox in the Julian calendar, which was essential for aligning the date of Easter with the spring equinox. The Julian calendar itself had inaccuracies that caused the spring equinox to drift slowly forward over the centuries. The Gregorian reform adjusted for this by skipping certain leap years. So the 532 year cycle only applies to the Julian calendar, with weeks of 7 days and a year of 365 days plus a leap year every four years. While there is no clear evidence that the 532 year Easter cycle was directly derived from ancient systems in Egypt, Babylon, or other civilisations, the concept of combining lunar and solar cycles was certainly a common practice. Ancient cultures often aimed to reconcile the lunar and solar calendars over longer periods, which is conceptually similar to the 532 year cycle in the Christian tradition. The seven day week is very old, and was used in Ancient India and Babylon, among other places. The presence of a seven day week is crucial for the 532 year cycle to work, as it ensures the periodic repetition of Easter Sunday and other significant days on the same days of the week. 

   It’s possible therefore to look again at the equation seen earlier, with 100 000 000, and divide this by all seven planetary cycles, and precession, and also by a Metonic cycle, of 19 years, the result being almost exactly 28, and think of 28 as a number of years. This is, as we’ve just seen, the cycle after which days of the week repeat on the same dates, on condition of an unbroken period of 365 day years, with a leap day every 4 years, and no other leap days. 

100 000 000 / (0.24 x 0.615 x 1.88 x 11.86 x 29.46 x 0.0748 x 25 920 x 19) = 27.999364 ≈ 28

This can be modified to: 

100 000 000 / (0.24 x 0.615 x 1.88 x 11.86 x 29.46 x 0.0748 x 25 920) = 27.999364 x 19 ≈ 28 x 19

28 x 19 = 532

There are 532 years in the periodic repetition of Easter Sunday and other significant days on the same days of the week. So a cycle of 532 years (the 28 and 19 year cycles combined) multiplied by the cycles of the seven ancient ‘planets’ and precession is 100 002 271.67419, close to 100 000 000. 

Conclusion: Deep Symbolism

All of this suggests that the early Celtic Church participated in a larger and older conversation about how human ritual should align with the divine rhythms of the cosmos. The transition from the 84-year cycle to the more mathematically precise Roman 532-year cycle was not merely about technical accuracy, it was about theology, conformity, and power. The Roman Church’s adoption of the Metonic cycle signalled the triumph of a more centralised, standardised approach to timekeeping, which had deep implications for the relationship between church authority and the local cultures it sought to unify.

What’s most fascinating, however, is how your numerical insight into the intersection of planetary cycles, precession, and sacred time opens a window onto the ancient mindset, a mindset that sought to harmonise the physical universe with divine will. For ancient cultures across the world, time was not simply a human construct, but a reflection of the cosmos, a mirror of the heavens that declared the glory of God. In this sense, Easter becomes more than just a Christian festival, it becomes a symbol of that cosmic harmony.

Have a look at this interview with Paul of Esoteric Explorer about this article:

https://www.youtube.com/watch?v=XwNQcnYM2A4&t=2707s

Notes

1. Bede, The Reckoning of Time, Bede, The Reckoning of Time - Beda (Venerabilis.), Beda Venerabilis (helgon.), el Venerable Beda (Santo), Bede Venerabilis Staff, Bede, the Venerable, Saint, 673-735 - Google Books

2. Ibid

3. Ibid

4. Bede, The Ecclesiastical History Of The English Nation, Translated 1910, https://archive.org/details/in.ernet.dli.2015.24760/page/n183/mode/2up?q=Twice

5. See https://www.megalithicireland.com/Hill%20of%20Slane.html

6. https://archive.org/details/in.ernet.dli.2015.24760/page/n103/mode/2up?q=easter p 65

7. https://archive.org/details/in.ernet.dli.2015.24760/page/n137/mode/2up?q=easter p 99

8. https://archive.org/details/in.ernet.dli.2015.24760/page/n147/mode/2up?q=easter p 109

9. Bede: The Reckoning of Time, Translated with introduction, notes and commentary by Faith Wallis, Translated Texts

   for Historians, 1999, Chapter 50

10. Ibid

11. Ibid. Chapter 53

12. Quoted in introduction to Bede: The Reckoning of Time, Translated with introduction, notes and commentary by Faith Wallis, Translated Texts

    for Historians, 1999

    Isidorus in computi laude dixit: Ratio numerorum contemnenda non est. In

    multis locis sacrarum Scripturarum, quantum mysterium habet, elucet. Non enim frustra in laudibus Dei dictum est: Omnia in mensura et numero et pondere fecisti. Senarius namque numerus, qui partibus suis perfectus est, perfectionem mundi quadam numeri sui signifcatione declarat. Similiter et quadraginta dies, quibus Moses et Elias et ipse Dominus jejuna verunt, sine numerorum cognitione non intelliguntur. Sic et alii in Scripturis sacris numeri existunt, quorum figuras non nisi noti hujus artis scientiae solvere possunt. Datum etiam nobis est, ex aliqua parte sub numerorum consistere disciplina, quando mensium curricula disputamus, quando anni spatium redeuntis per numerum agnoscimus. Per numerum siquidem ne confundamur, instruimur. Tolle numerum a rebus omnibus, et omnia pereunt. Adime saeculo computum, et omnia caeca ignorantia complectitur. Nec di¡eri possunt a caeteris animalibus, qui calculi nesciunt rationem: De computo dialogus 647; my translation. The quotation is from Etym. 3.4. Isidore’s immediate source for both the phrasing and sense of this passage is Cassiodorus, Institutiones 2.4.7; ed. R.A.B. Mynors (Oxford: Clarendon Press, 1937):141.1^7. The ¢nal phrase may ultimately derive from Augustine, De libero arbitrio 2.16.42, ed. W.M. Green, CCSL 29 (1970):265.25^26: Formas habent creaturae, quia numeros habent, adime illis haec, nihil erunt. (‘‘Created things have forms because they have numbers; take this away from them, and they are nothing.’’)