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How Easter made the modern calendar
Every spring, I have the vague sense that Easter is coming, but my only way to figure out its date is to look at a calendar. This year, Easter is on April 9. What makes Easter so special?
In secular American life, the dates of most holidays are easy to figure out. Some holidays are “fixed” and always fall on the same date, like Christmas on December 25 or Independence Day on July 4. (If a fixed holiday falls on a weekend, we “observe” the holiday on the nearest weekday, presumably to keep the number of days off of work the same across years.)
Other holidays are “floating” and fall on some regular day, week, and month, like Thanksgiving on the fourth Thursday in November, or Memorial Day on the last Monday in May. Many of these floating holidays were originally fixed on some meaningful date but were then changed to create more three-day weekends. For example, Americans used to celebrate George Washington’s birthday on February 22, which is his actual birthday. But the travel industry, hoping to get federal employees to take more vacations, drafted the text of the 1971 Uniform Monday Holiday Act, which moved Washington’s Birthday, Memorial Day, Columbus Day, and Veterans day from their fixed dates to floating Mondays. (Veterans Day was later re-fixed at November 11.)
Other holidays move around even more than this. Growing up in northern New Jersey, I got many of the Jewish holidays, like Rosh Hashanah and Yom Kippur, off from school. I noticed how these holidays weren’t always even in the same month every year. Same story for Chinese New Year: it is sometime in the winter, but it moves around.
These other holidays “move around” more because they are based on lunar calendars. Now, the Sun is a really good tool for measuring the passage of individual days. Even very near the poles, the light-dark transition is hard to miss. But it’s tough to measure longer lengths of time with the Sun. Sure, days are longer in winter than in summer, but it’s pretty tricky to tell what day is the solstice or the equinox without some careful observation or day-counting.
The Moon, on the other hand, is great for measuring month-like lengths of time. Every 29 or 30 days, the Moon becomes basically invisible, and then there is a new moon, which traditionally meant the first glimpse of the waxing crescent moon, the first thing you could see after the “old” moon disappeared. So long as the sky is clear from time to time, everyone can tell the lunar phase and count lunar months. Many, if not most, civilizations have used lunar calendars, including the Hebrews and the Chinese.
Lunar calendars face an important limitation: the lunar months and the solar year don’t line up. A solar year is 365.24 days long, and a lunar month is 29.53 days long, and so 12 lunar months comes to about 11 days short of a solar year. This means that the same lunar month isn’t always at the same time of the year, and without correction, the same lunar month will fall during winter and then during summer. For example, say you started your lunar year right in the dead of winter, on the winter solstice. Then 16 lunar “years” later, the start of the lunar year would be in the middle of summer!
The solution is “intercalation:” from time to time, you add in some extra days into the lunar calendar, to make it align with the solar calendar. Intercalation is hard to do right, and there are often motivations to do it wrong. The Romans adopted the Julian calendar, the forerunner of our modern calendar, because Julius Caesar was irritated that the priests who controlled the old Roman calendar were making intercalary periods longer during years that their friends were in power but shorter during years that their enemies were in power. He mostly fixed the problem by setting the year at 365 days, with a leap day every fourth year. (Conveniently, this also entailed a one-time, extra-special intercalary period that made one of his years in power last for 446 days.)
Easter is the celebration of Jesus’s resurrection. After the Last Supper, Judas betrayed Jesus, who was imprisoned and crucified. He died, rose from the dead, and disappeared from the Earth. Resurrection is a central theological concept in Christianity, and Easter is considered as important as Christmas, or more important, in Christian theology and worship.
So getting the date for the celebration right seems important! And in fact the date of Easter led to controversies as bitter and incendiary as the theological debates that split the Western and Eastern churches.
Even from the beginning, early Christians didn’t all agree on when to celebrate Easter. From the Bible and other sources, it is fairly clear that the Last Supper was a Passover dinner, so some Christians celebrated Easter on the same day that the Jews celebrated Passover. Other Christians had taken to celebrating Easter on a Sunday, probably because Sunday was their main day of worship. This tradition is compatible with the Bible, which makes it seem like the day of Jesus’s resurrection was a Sunday, plus or minus a day or two. But the relevant parts of the Bible weren’t written for at least 50 years after Jesus’s death, so people had to have found ways to figure out the relevant day for worship without it!
By sometime around 100 or 200 AD, the Christian community mostly settled on celebrating Easter on the Sunday following Passover, mimicking the original story of Jesus’s betrayal and death.
By the late 200s AD, a new problem arose, and Christians became concerned. Celebrating the holiest day of the year on the Sunday after Passover meant that the Christians had to rely on the Jews to correctly identify the date of Passover. Passover is a Jewish holiday commemorating the Exodus of the Jews from Egypt, a mythical story based on historical events that likely took place 1,000 years before Jesus lived. This meant that the Jewish community had a lot of experience deciding on the date of Passover!
However, in the 200s AD, the Jewish community did not have a highly accurate system of intercalation. Religiously resonant but highly qualitative measures, such as whether the barley crop was ripe, were used as signals for when to intercalate. The Christians noticed that, under this system, the date of Passover was sometimes before the spring equinox. The Christians knew that Passover was a spring celebration, and they were alarmed that the Jews were not managing their calendar correctly, if the reported date of Passover was coming so early, before the equinox.
Logistical constraints added another problem. “Ask the Jews” was a practical system when the Christians were mostly confined to the Middle East, but as Christianity spread to parts of the world with no Jewish communities, a new system was needed.
Anti-Semitism played a substantial role too. Insofar as the early Christians held contemporaneous Jewish communities responsible for the death of their prophet, it was not a good feeling to have to ask the Jewish people when to commemorate the death of their prophet.
A solution of sorts came during the first great council of Christian bishops. There were 7 of these councils recognized as “ecumenical,” or embracing the whole of the Christian world, with the first in 325 AD and the last in 787 AD. These councils charted the course of the early church and had a pivotal role in shaping Christianity as it exists today.
At the first great council, the first agenda item was to come to agreement about key theological underpinnings of the nature of Christ, what it meant for Jesus to be both humanity and God, whether the humanity and godhood were the same thing, or whether they were separate things in one body, and so forth. The second agenda item was to decide on when to celebrate Easter. This is how important the Easter controversy was.
The council decided two things: first, all Christians should celebrate Easter on the same day, and second, they shouldn’t rely on the Jewish calendar. How to actually compute the date of Easter was not decided, and there were multiple competing methods. Christians in Rome had one approach; people in Alexandria in northern Egypt had another; the Christians in Britain had a third.
Over the next few hundred years, a system emerged: Easter would fall on the first Sunday after the first full moon after the spring equinox. This was an approximation of the date of Passover but did not rely on the Jewish calendar.
This system sounds tidy to me, living today: a quick Google shows me that the 2023 spring equinox will occur on March 20 at 5:24pm Eastern Time, and the full moon after that will occur on Thursday, April 6 at 12:34am, so I can easily conclude that Easter comes on the following Sunday, April 9.
But people had to figure out the date of Easter before the Scientific Revolution, before astronomical observations were so precise they could know the equinox down to the second, and before communication was so rapid. Today, I can look up the astronomical equinox, the instant the plane defined by the Earth’s equator passes through the geometric center of the Sun, and the astronomical full moon, the instant the moon passes through the plane defined by the Sun and the Earth’s poles. Because these kinds of precise observations were initially infeasible, the date of Easter was computed using an ecclesiastical equinox and ecclesiastical full moon. These are calendar dates that approximate the astronomical phenomena. The ecclesiastical equinox is fixed at March 21, and the ecclesiastical new moon is based on an approximation that 19 solar years is almost exactly equal to 235 lunar months, minus a few hours. This approximation essentially allows for predictable and precise intercalation.
Confusingly, because the ecclesiastical equinox and full moon are approximations, there’s no guarantee that the astronomical procedure I outlined will actually produce the recognized date of Easter. The next “paradoxical” Easter date, when the ecclesiastical approximations will disagree with the astronomical reality, will be 2049, assuming that various efforts to unify the Western and Eastern churches’ celebrations continue to fail.
As tidy as this system sounds, it is not easy to understand or to compute by hand. It can now be done pretty efficiently in a few dozen lines of code, but the computation is tricky enough to have confused Gauss, who was probably one of the smartest people to have ever lived. In fact, the Latin word computus, literally meaning “computation,” has come to refer specifically to the algorithms used to compute the date of Easter. (Star Trek fans know what joke I would make about computus.)
Computus eventually led to our modern calendar. The earlier Julian calendar, established by Julius Caesar in 45 BC, had made important innovations, by fixing the length of the year at 365 days and allowing for a leap day every 4 years. Unfortunately, this is just slightly too many leap days. By the 1500s, it was becoming clear that the ecclesiastical equinox was no longer aligned with the astronomical equinox. The calendar and the seasons were starting to get out of whack, and the most troubling consequence was the Easter might fall ahead of the spring equinox, just as it had 1,200 years before, during the time of the imprecise intercalation in the Jewish calendar.
In 1582, Pope Gregory XIII instituted two changes to the church calendar. First, every year evenly divisible by both 4 and 100 would not be a leap year. (This is why the year 2000 was not a leap year, despite being divisible by 4.) This improvement has made the Gregorian calendar so accurate that the limiting factor is now the irregularity of astronomical phenomena, like the precession of the equinoxes or the tidal deceleration of Earth’s rotation, rather than because of imperfect calendars.
Second, Pope Gregory deleted 10 days from the calendar: the day after Thursday, October 4, 1582 would be Friday, October 15, 1582. It was important that Friday follow Thursday, without changing the days of the week, so that Sundays would continue to fall every 7 days as they had, presumably, since Jesus’s resurrection.
The Gregorian calendar was used in the Catholic church, and it slowly made its way into the secular world. For a time, the two calendars co-existed. The United Kingdom and its colonies switched in 1752, so that George Washington was born on February 11 in the “Old Style” but February 22 in the New Style. The last country to adopt the Gregorian calendar for civil purposes was Greece, which took until 1923.
The Greeks and the rest of the Eastern Orthodox Christians still use the Julian calendar to compute the date of Easter. Because the same date occurs first in the Gregorian calendar, which skipped days relative to the Julian calendar, Orthodox Easter is most often behind Western Eastern by a week or two. However, because of the complexities of how the ecclesiastical equinox and full moon fall in the two calendars, the two Easters are sometimes on the same date, and sometimes Orthodox Easter is many weeks later.
Religion has shaped so much of our world and society, but it surprised me that even the Gregorian calendar reform, something I took for granted as an element of “progress” and scientific advancement, was mostly motivated by making sure that Easter would land in a way that would have made sense to a council of bishops some 1,700 years ago.