Caoimhín's Content

Today, we are going to look a little bit into the past and then a lot into the past. We will start by going back just a couple of days to the 20th of April. This was Easter Sunday this year and some of you may know that Easter Sunday varies a lot between different years, it is a mobile holiday. Easter Sunday is a Christian holiday, the Catholic church, the Protestant church and the Orthodox churches, Eastern Orthodox and Oriental Orthodox, they all celebrate Easter Sunday, just not necessarily on the same dates. It also ties back to another celebration in a different calendar.

In modern times, the date of Easter is calculated in a pretty similar way every year. First off you go to the March equinox, sort of. We don’t really care about the March equinox, just the 21st of March. It was decided that March 21st would be used as the March equinox, even though it can vary a little bit with leap years and similar things. So we’re starting with the 21st, which may or may not be the equinox, and we’re looking for the Moon. We’re looking for the first Full Moon after the equinox, so we’re going to have to go forward quite a bit. The Moon had to go past new and come all the way out the other side this year, so we’re already well into April when the Full Moon occurs. This year, let’s say it was the 12th, already well into April by the time the Moon is fully illuminated. At midday, the Moon was 99.8% full on the 12th and 99.7% on the 13th, so of course, the exact date where the Full Moon occurs can vary as well. The lunar cycle of about 29 and a half days doesn’t line up very neatly with our whole days, and that is a part of the problem. The 14th day of the lunar cycle was selected to be the Full Moon, to reduce this variation. Either way, the first Full Moon after the equinox was the 12th and if we go forward to the first Sunday after that, it’ll bring us to the 20th of April.

This works every year and it works the same every year, and has done since it’s been codified into this repeating process, but that one repeating method leads to different dates every year. Looking ahead to next year, we’re going to go back to the notional equinox, even though it could be the 19th, 20th or 21st, we will go to 21st of March 2026. We’ll then go forward to the first Full Moon. On April 2nd the Moon will be 99.7% full. We’ll say it’s the 2nd, rather than April 1st, though in 2026 it doesn’t matter too much. Regardless, we’re seeing this Full Moon a lot earlier in April next year than this year. The first Sunday after that will be just the the 5th of April, instead of the 20th, so there’s a big difference there. There’s an even greater difference if we were to go back a year from this year. If we go back to 2024 here, and we can do the same thing again. We’ll come all the way back to the equinox, or the 21st, which may or may not be the equinox, and the Moon was at 88.1% full, so we only need to go a few days into the future to get 99.9% on the 25th. That meant that back in 2024, Easter Sunday fell on just the 31st of March.

Even though there is one codified method to figure out when Easter is, starting with the equinox and then the first Full Moon and then the first Sunday after that, the lunar calendar still doesn’t match up with the solar calendar, it doesn’t always produce the same date every year. Another problem on top of this is saying that the 21st is the equinox, even though it can vary with things like leap years and also pinning down the date of the Full Moon to be the 14th day of the lunar cycle, whether or not it’s actually 100% full. The Moon could be 100% crossing midnight, that half day can shift when the Moon appears at its fullest. These kinds of things needed to be sort of simplified. As such, if every church back in the, you know, 1500s tried to figure out what day Easter was themselves, they could end up celebrating it on different dates, and that gets even more complicated when you vary what year you’re using.

In the Stellarium software, whenever I change the date drastically, I bring up this little window. This little window shows what dates we’re looking at, and this date and time is based on the Gregorian calendar. There is also an alternate based on the Julian calendar. The astronomical Julian year or unit, it is shown simply as a count of days, it’s not broken up into months and years the way we normally break up the Gregorian calendar. The actual Julian calendar would have been, and is by those who use it. The astronomical Julian measurement is based on the actual Julian calendar, but it is separate. The Julian calendar gets its name from Julius Caesar. Julius Caesar, way back in the year -46 initiated or ordered a calendar reform. Of course, it wouldn’t have been -46 at the time, it was just another year. There were other calendars before the Julian calendar, but Julius Caesar back in the year -46 by our current reckoning, made the change. The the calendar was going to be reckoned, as 365 days with a leap year every four years. The year -45, this was the first year of the Julian calendar, and of course things got complicated after that. The Julian calendar didn’t account for leap years in the same way as the Gregorian calendar, leading to drift, and of course, it started at a different date. However, the Julian calendar was pretty much the most popular calendar, it was the calendar almost everyone in Europe ended up using.

From the year -45 we’re going all the way through to year 1, or 0, and then out the other side to 33. The celebration of Easter Sunday is to do with the death and rebirth of Christ, the Messiah figure in Christianity and generally a prophet of some sort in the Abrahamic religions. According to the record of the Bible, back in the year 33 is when the death of Christ occurred. That is, if the figure was born in year zero, or one, and then died at the age of 33, it would have been year 33. Also according to the records we are left with, Christ died on the festival of Passover or Pesach, which is a Jewish festival. This is really where things get interesting. The Hebrew calendar that measures out the Jewish festivals, it is a lunar calendar, which is why we have to relate it to the Full Moon, the Moon has a lot to do with when the festival of Pesach or Passover starts. Passover runs for 8 days, measured from sunset to sunset, and it happens around the date of Easter. In fact, the Hebrew calendar could be used to find the date of Easter by Early Christians. However the Christian church, the early Christian church before many of the splits it has today, eventually wanted its own way of figuring out the date. Rather than of tying it to the Jewish celebration of Passover, it got tied to the equinox and the Full Moon directly. This leads to the problem of different churches in different places possibly coming up with different dates for the Full Moon.

As different churches could still potentially end up celebrating Easter on the wrong dates, different dates for different churches in different parts of the world, the decision was made to sort of calculate when all of the Easters were going to occur for years and years in advance. That calculation was the Computus. The Computus Paschalis is latin for the Easter Calculation, so it was called the Computus, mostly in the west, or the Paschalion, mostly in the east. It was an algorithmic way of figuring out for hundreds of years in advance when Easter was going to be. By measuring out the lunar calendar and the solar calendar, you could put a date on when Easter was going to be. The Computus was pretty much the difficult maths puzzle in the Western world for quite a while, for hundreds of years at least. We know it was a problem for hundreds of years because it wasn’t until around the year 525 that the Dionysian Computus or the Dionysian Paschalion calendar, was adopted. Various other computuses were put forward, but many went out of sync with the years over time. It wasn’t until the year 525 that a computus hat worked for a few centuries was established, and even then it only worked for a few hundred years at a time. That Dionysian Computus was also the calendar that selected the start year that we now use, that decided to start the calendar with the birth of Christ and then move on from there. That calendar was eventually established, but there was still difficulties when it came to the Gregorian reform.

The way to figure out the date of Easter had been set, the calculation for it, for all of the churches, had been done, but what calendar was getting used then led to disagreements. The Julian Calendar, with one leap year every four years, went out of drift with the actual solar year over time. This lead to the Gregorian calendar, with a few “gap” years that don’t leap. If you’re using the Julian calendar, you will have your March 21st on a different date to the Gregorian calendar, and the degree of drifting as you go through the year is different as well. This led to schisms in the early church. Some churches wanted to use the Julian calendar and some churches wanted to use the Gregorian calendar, and it’s one of the big splits that still exists today. The Orthodox churches, Eastern Orthodox and Oriental Orthodox, they use the Julian calendar for figuring out when celebrations are. Nearly all of them still use the Gregorian calendar for day to day activities, but for the religious holidays they use the Julian calendar. The other churches, the Roman Catholic Church and the various Protestant sects, they use the Gregorian calendar for holidays instead.

It is, it is a complicated astronomical phenomenon, lining up the solar calendar with the lunar calendar. It is a complicated thing because the lunar calendar doesn’t fit the solar one. The lunar cycle isn’t even really 29 and a half days, it’s 29.53059 on average, it’s not even a round half of a day. It’s similar with our year, you know, it’s 365 days and a quarter, but it’s not even exactly a quarter. The northern tropical year is just about 365.242189 24-hour periods long. Thanks to all this, Easter falls on a different date every year. On a given year, it’s not too hard to calculate, but predicting when it will occur in the future, that became one of the most complicated math puzzles, the Computus or the Paschalion. Of course, we don’t have to worry about it too much anymore, it’s all been sorted out, but it does highlight some interesting things in the history of how we break up the year and how these calendars work. Even though Easter is a religious holiday, it has some secular interest in terms of how it relates to calendars, the lunar calendar, the solar calendar, and how these astronomical motions had a real impact on people here on Earth while they were trying to figure out all of this stuff.

This is a topic that I find particularly interesting, I hope that you found it interesting as well. If you did, please do like this article, you can also subscribe to this website and my YouTube channel if you’d like to see more astronomy like this. Thank you very much for reading, and hopefully I’ll see you back here next time.