In the 16^th Century, Pope Gregory XIII had a problem. It had been known for a long time, and no one really wanted to deal with it, but the problem wouldn’t go away. It couldn’t go away. It only got worse. It wasn’t just his problem, either. Christendom, long ago fractured into Roman Catholic and Eastern Orthodox, had fractured again with Protestantism. And all faced the same problem: Easter.

When to celebrate Easter had been an issue with Christians for centuries. Christianity was first a Jewish sect, and the very first Christians were Jews. The day of Jesus’ resurrection was known, but as a date in the Hebrew Calender. Unlike the Julian Calendar used by Rome, which is tied to the Sun with automatic adjustment, the Hebrew Calender is tied to both the Sun and the Moon with varying adjustments. It even may not have been mathematically based: the modern Hebrew Calender is, and like the Julian Calender has automatic adjustments, but development of that didn’t start until the 4^th Century. This meant that while the date of the resurrection was a set point on the Hebrew Calender, it bounced around the Julian calender.

To the increasing gentile eyes used to the Julian Calender, it was obvious that Jesus’ resurrection was tied to the Jewish Passover. Jesus was crucified at the Passover, and was resurrected the first day of the week. So if you wanted to observe the actual day as close as you could, you followed Passover. Except this varied, and by the 4^th Century there were suspicions that the rabbis weren’t doing such a hot job keeping their calendar, as a few times Passover occurred prior to the Spring Equinox. Some Christians didn’t even follow the Passover observance at all, and came up with their own method. Different churches were observing Easter at different times.

The problem got so bad that Constantine called a conference primarily to discuss when to observe Easter. What shook out of that was a unified method of calculating the date of Easter, one that didn’t rely on the Jewish Calendar, but was a close approximation. The idea was that Easter would fall on the 1^st Sunday after the 1^st Full Moon after the Spring Equinox, which was close to the Jewish Passover. If the Pascal Full Moon fell on Sunday, Easter fell on the next Sunday.

This worked pretty well, especially since lunar phases repeat ever 19 years, and from this came Dominical Letters and Golden Numbers, which were used in determining Easter by previously computed tables. With the date of Easter determined, other movable church observances fell into place as time went on.

Except there was one tiny problem: The Julian Calendar itself.

Time keeping is as much a matter of what we choose to measure as how we do it. In the case of a solar calendar, like the Julian, it measures the time it takes for the return of the seasons. Before the Julian Calendar, the Romans did this manually, or were supposed to have, but they stretched or shortening the years for political convenience. But in addition to discovering Cleopatra, Julius Caesar found an Egyptian calendar of three regular years of 365 days with a fourth one of 366 days to keep everything automatically on track. Julius saw it as a solution to the Roman calendar problem, and adopted their own version with three 365 day years followed by one 366 day year, and, after a few blips, everything continued right on track.

Except the earth doesn’t take 365 days, 6 hours to return from Spring Equinox to Spring Equinox. No, it takes, on the average, 365 days, 5 hours, 48 minutes, and 45 seconds. The Julian Calendar runs 11 minutes, 15 seconds too fast. That doesn’t seem like much, but that’s 45 minutes every four years, which comes to one day every 128 years.

The Romans adopted the Julian Calendar in 45 BC. The 1^st Council of Nicea, that set calculation of Easter, met in 325 AD. By then the West had been using the Julian Calendar for 369 years, and those extra 11 minutes, 15 seconds now tallied up to 2 days, 21 hours, 11 minutes, 15 seconds. In 325 AD the Spring Equinox fell around March 21, but in 45 BC it fell around March 24. The seasons were edging backward in relation to the calendar.

So as the Julian Calendar slowly creeped forward, the seasons returned earlier and earlier in the calendar year, until, by the time of Pope Gregory XIII, the Spring Equinox fell around March 11. In effect, Easter was occurring later and later in respect to the Spring Equinox. If the whole point was to keep the observance of Jesus’ resurrection as close to the actual date as possible, then something must be done.

That something was the Gregorian Calendar. The Gregorian Calendar is just the Julian Calendar with a slight tweak. In a Julian Calendar, every four years is a leap year. But in a Gregorian Calendar, every four years is a leap year except for years divisible by 100. Years divisible by 100 were regular years unless they are also divisible by 400. So while 1600 would be a leap year, 1700, 1800, and 1900 would not. That shortens the Gregorian Calendar by three days every 400 years as compared to the Julian. Since the Julian Calendar gains a little over 3 days in 400 years, removing 3 leap days makes the calender pretty accurate in regard to seasons. Since the calendar is still gaining 3 hours every 400 years, it won’t be off by a full day until around 4,782 AD.

That solved the calendar problem. But there was another detail: Removing the extra days. Pope Gregory XIII could have restored the calendar to where it was in relation to the seasons in 45 BC. But remember that when the 1^st Council of Nicea met, the calendar had crept forward by nearly 3 days. Since 325 AD was when the Roman Catholic Church adopted its method of calculating Easter, Pope Gregory XIII corrected the calendar back to that point. This required removing 10 extra days. So it was that October 4, 1582, was followed by October 15, 1582.

The change in the calendar also required a change in the method of calculating Easter. Instead of the old system, the new method was based on epacts. There’s a difference between a year of 12 months based on the Moon and a year based on the Sun, with the lunar year shorter than the solar year. An epact is this difference in days, expressed as how many days the Moon is into its cycle of phases on January 1. Now everything was back on track.

Of course, in 1582 the pope’s words carried no weight outside of the Roman Catholic nations. Eastern Orthodox didn’t care, and Protestant countries sure didn’t. In 1583, whether a nation was on the Gregorian Calender depended on whether they were Roman Catholic.

Here things got messy. Not only were different countries on different calendars, but they were also celebrating New Year’s Day at different times. Early 18^th Century England celebrated New Years’ Day on March 25, but across the channel, France celebrated it on January 1. Countries gradually went to the Gregorian Calendar because it was an improvement to the Julian and due to commerce. England held out until 1752, moving it’s New Year to January 1, just like everyone else. Some nations didn’t abandon the Julian and other calendars until the 20^th Century. Now the entire world uses it.

However, this doesn’t mean Easter falls on the same day across all of Christendom. The Greek Orthodox Church uses another method to calculate Easter.

Meanwhile, there’s still that slow accumulation of 3 hours every 400 years. At some point there will have to be another adjustment. Some, like astronomer John Hershel, have proposed tweaking the Gregorian Calendar so that it drops one leap day every 4,000 years. Under this plan, the year 4000 AD, which is divisible by both 100 and 400, and which would normally be a leap year, would be a regular year. This would put the calendar back on track. If the nations adopt this plan, the calendar will be accurate until around 16000 AD, when another leap day would have to be dropped. So far, the nations haven’t agreed to this proposal.

I think we can wait.