What fun. I've liked a good calendar deep dive, ever since finally finding out that leap year isn't just "every 4 years", back when working on the Year 2000 problem. This also feels like exactly the sort of entertaining rabbit hole that comes up whenever I look into Passover things, like the first time I participated in the Seder and was deeply confused why Maxwell House Coffee, of all people, published the Haggadah. That's right up there with Michelin Stars and Guinness Book of World Records for advertising pitches that became so culturally fundamental that they transcend sales and actually become somehow "good", which is a pretty wild transformation.
Huh, and I guess actually fits the Easter theme in a weird way.
I … had never heard of the Maxwell House Haggadah, Brian—thanks for bringing that to my attention! That is indeed wild, as you say. Glad you enjoyed my calendar deep dive!
There was even a cheerful KJV-vs-NIV style debate at dinner this year, over which Maxwell House translation is the "good one". The older generation was swearing by the old blue edition, with its archaic "thees and thous" English and pencil art, while the kids preferred the new edition, with modernized wording and color photos. Of course, the Hebrew wording and ceremonial structure is the same either way.
An interesting wrinkle re. the problems in your footnote 11: now that we have atomic clocks, the official clock time is actually more "accurate" (i.e. more mathematically regular) than the solar phenomena themselves, since the earth's rotation is affected stochastically by geological events. Hence "leap seconds" are not added on a fixed schedule, but scheduled as needed by some person in authority (very likely the Astronomer Royal).
(Or were. Apparently leap seconds are difficult for programmers to deal with, and the current plan is to let the error accumulate up to a minute or more and let posterity deal with it. I find this vaguely upsetting.)
However, IIUC, leap seconds are only designed to correct the length of the calendar *day*, not the year. If the Gregorian calendar continues to be used for centuries into the future, some yet-to-be-determined correction will still have to be applied. Per Wikipedia, the 4000-year idea has been proposed before,[1] though I suspect it would make more sense to apply one-off corrections at somewhat shorter intervals, since the 4000-year cycle *still* wouldn't reduce the error to zero, and the length of the cycle is getting a bit ridiculous at that point.
For ecclesiastical purposes, of course, the kind of precision you’re talking about is much more than we need, P! For example, the Easter Vigil has to start on Holy Saturday, “after nightfall,” i.e., with the sky completely dark. (I originally wrote “45 minutes after sunset” but after rechecking I see that’s an application of the law, not the law itself. Still, it represents a reasonable application of how the law should work.)
For most other purposes, like days of fast and abstinence, the rule is midnight to midnight, but very few of us are chowing down in the seconds before or after midnight.
How could we make smaller, more frequent corrections to the Gregorian calendar? A single calendar date is the indivisible base unit of the Gregorian calendar.
Yes, certainly this all is not very relevant for ecclesiastical purposes. Even if you're given to midnight snacks, watching the clock for precision down to the second would seem somewhat legalistic, akin to measuring out the exact maximum amount of food allowed on fast days. I realize there are pastoral reasons to discourage this kind of thinking.
The day is far from indivisible! For example, if we would need an extra leap day every 4000 years, we could instead set UTC time back by one hour every 166.67 years. This would be no more disruptive than the current semi-annual Daylight Saving Time adjustment. (Yes, doing the transition twice a year is annoying, but if daylight saving time is abolished in the enlightened future times, a time adjustment every century or two wouldn't be so bad!)
“if we would need an extra leap day every 4000 years, we could instead set UTC time back by one hour every 166.67 years. This would be no more disruptive than the current semi-annual Daylight Saving Time adjustment”
Maybe I’m not thinking straight, but, I … don’t think this would work? Wouldn’t this mean that after, for example, 2,000 years we’d be marking noon at midnight and midnight at noon?
Wait, no, you're right, that wouldn't work. An error in the solar year does *not* manifest as a gradually increasing error in the clock. That means this tangent is even less relevant than I thought! (All I can say in my defense is that "These are not problems we need to solve today on Substack" was a provocative line.)
Thank you, B! It’s a significantly better piece thanks to you underscoring for me that the Gregorian calendar was not embraced everywhere in the 16th century (or even in the 19th). I caught my breath when I realized that where I live 1776 followed the adoption of the Gregorian calendar by less than a quarter century!
Duh, I probably should have applied the principle to 2025, shouldn’t I! No, Melanie, the reason is the the full moon that matters is not the actual, observed full moon, but the full moon predicted by tables used by the Church based on the Metonic cycle—which I believe happens to be off this year by one day. So the Church reckons the full moon as today, not yesterday, while the Orthodox full moon is probably still a couple of days away—but we agree that next Sunday is Easter! I’ll try to verify this.
Good question, Melanie! I expect the reason we still use a version of the Metonic cycle is … that’s what was approved at the Council of Nicea! Pope Gregory XIII allowed slight corrections to the math, but still the Church wants a method written down in a book in continuity with the historical formula.
Fascinating essay. A couple of weeks ago for work I had to modernize the dates of a book auction that 1) took place in London after the Gregorian calendar was established but before England adopted it, 2) happened in late February to early March so before Lady Day, and 3) took place in a leap year. There may have been some swearing as I created the record.
What fun. I've liked a good calendar deep dive, ever since finally finding out that leap year isn't just "every 4 years", back when working on the Year 2000 problem. This also feels like exactly the sort of entertaining rabbit hole that comes up whenever I look into Passover things, like the first time I participated in the Seder and was deeply confused why Maxwell House Coffee, of all people, published the Haggadah. That's right up there with Michelin Stars and Guinness Book of World Records for advertising pitches that became so culturally fundamental that they transcend sales and actually become somehow "good", which is a pretty wild transformation.
Huh, and I guess actually fits the Easter theme in a weird way.
I … had never heard of the Maxwell House Haggadah, Brian—thanks for bringing that to my attention! That is indeed wild, as you say. Glad you enjoyed my calendar deep dive!
There was even a cheerful KJV-vs-NIV style debate at dinner this year, over which Maxwell House translation is the "good one". The older generation was swearing by the old blue edition, with its archaic "thees and thous" English and pencil art, while the kids preferred the new edition, with modernized wording and color photos. Of course, the Hebrew wording and ceremonial structure is the same either way.
An interesting wrinkle re. the problems in your footnote 11: now that we have atomic clocks, the official clock time is actually more "accurate" (i.e. more mathematically regular) than the solar phenomena themselves, since the earth's rotation is affected stochastically by geological events. Hence "leap seconds" are not added on a fixed schedule, but scheduled as needed by some person in authority (very likely the Astronomer Royal).
(Or were. Apparently leap seconds are difficult for programmers to deal with, and the current plan is to let the error accumulate up to a minute or more and let posterity deal with it. I find this vaguely upsetting.)
However, IIUC, leap seconds are only designed to correct the length of the calendar *day*, not the year. If the Gregorian calendar continues to be used for centuries into the future, some yet-to-be-determined correction will still have to be applied. Per Wikipedia, the 4000-year idea has been proposed before,[1] though I suspect it would make more sense to apply one-off corrections at somewhat shorter intervals, since the 4000-year cycle *still* wouldn't reduce the error to zero, and the length of the cycle is getting a bit ridiculous at that point.
[1] https://en.wikipedia.org/wiki/Gregorian_calendar#Accuracy
For ecclesiastical purposes, of course, the kind of precision you’re talking about is much more than we need, P! For example, the Easter Vigil has to start on Holy Saturday, “after nightfall,” i.e., with the sky completely dark. (I originally wrote “45 minutes after sunset” but after rechecking I see that’s an application of the law, not the law itself. Still, it represents a reasonable application of how the law should work.)
For most other purposes, like days of fast and abstinence, the rule is midnight to midnight, but very few of us are chowing down in the seconds before or after midnight.
How could we make smaller, more frequent corrections to the Gregorian calendar? A single calendar date is the indivisible base unit of the Gregorian calendar.
Yes, certainly this all is not very relevant for ecclesiastical purposes. Even if you're given to midnight snacks, watching the clock for precision down to the second would seem somewhat legalistic, akin to measuring out the exact maximum amount of food allowed on fast days. I realize there are pastoral reasons to discourage this kind of thinking.
The day is far from indivisible! For example, if we would need an extra leap day every 4000 years, we could instead set UTC time back by one hour every 166.67 years. This would be no more disruptive than the current semi-annual Daylight Saving Time adjustment. (Yes, doing the transition twice a year is annoying, but if daylight saving time is abolished in the enlightened future times, a time adjustment every century or two wouldn't be so bad!)
“if we would need an extra leap day every 4000 years, we could instead set UTC time back by one hour every 166.67 years. This would be no more disruptive than the current semi-annual Daylight Saving Time adjustment”
Maybe I’m not thinking straight, but, I … don’t think this would work? Wouldn’t this mean that after, for example, 2,000 years we’d be marking noon at midnight and midnight at noon?
Wait, no, you're right, that wouldn't work. An error in the solar year does *not* manifest as a gradually increasing error in the clock. That means this tangent is even less relevant than I thought! (All I can say in my defense is that "These are not problems we need to solve today on Substack" was a provocative line.)
What an amazing and informative piece (and thanks for the shoutout, of course)! Calendar stuff is endlessly fascinating to me!
Thank you, B! It’s a significantly better piece thanks to you underscoring for me that the Gregorian calendar was not embraced everywhere in the 16th century (or even in the 19th). I caught my breath when I realized that where I live 1776 followed the adoption of the Gregorian calendar by less than a quarter century!
I think you’ll enjoy this video! https://youtu.be/awhGbKH3mGk?si=iWCPM4bYbqJIXJR5
This video is good.
Hmmm. So even though the full moon was yesterday, Easter is next Sunday and not today. Is that because the full moon is observed after sunset?
Duh, I probably should have applied the principle to 2025, shouldn’t I! No, Melanie, the reason is the the full moon that matters is not the actual, observed full moon, but the full moon predicted by tables used by the Church based on the Metonic cycle—which I believe happens to be off this year by one day. So the Church reckons the full moon as today, not yesterday, while the Orthodox full moon is probably still a couple of days away—but we agree that next Sunday is Easter! I’ll try to verify this.
I’m still confused as to why the Metonic cycle is preferred to the actual astronomically observed full moon.
Good question, Melanie! I expect the reason we still use a version of the Metonic cycle is … that’s what was approved at the Council of Nicea! Pope Gregory XIII allowed slight corrections to the math, but still the Church wants a method written down in a book in continuity with the historical formula.
Terrific explanation. A few days ago, I offered my own attempt to simply all this. Yours is much more thorough. Thank you!
https://open.substack.com/pub/rosselli/p/different-dates-for-easter?r=mtvnz&utm_medium=ios
Fascinating essay. A couple of weeks ago for work I had to modernize the dates of a book auction that 1) took place in London after the Gregorian calendar was established but before England adopted it, 2) happened in late February to early March so before Lady Day, and 3) took place in a leap year. There may have been some swearing as I created the record.