An Easter Puzzle & What ChatGBT Is Good For

Easter is a “movable feast,” meaning it jumps around the calendar every year. Calculating what that date will be in any given year gets complicated. Easter has to be on a Sunday. It also needs to correlate somehow with the Jewish Passsover, which is determined by a lunar calendar (based on the phases of the moon), whereas we use a solar calendar (based on our planet’s position in relation to the sun).

As the Wikipedia article on the subject explains, “Easter is celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or after 21 March (a fixed approximation of the March equinox). Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the Julian or Gregorian calendar.” The article goes on to discuss “the complexity of the algorithm,” giving different formulas and ways of calculating Easter math.

I bring this up because an item from Christianity Today‘s Daily Briefing e-mail newsletter, 4/4/2023 (I can’t find the issue online), had this from the magazine’s news editor Daniel Silliman:

As I prepare for Easter this week, a fragment of a 14th- or 15th-century Latin poem has been rattling around my brain: “Quaerit amor Christi multos dum regnat iniquus.” In English: “The love of Christ seeks many while injustice reigns.”. . .

The history of the Easter equations is fascinating—and if you want to buy pages of a medieval prayer book that include this poem/cipher, they are for sale—but I’m thankful for the not-cryptic part of this poem that reminds me that something is happening behind the scenes “while injustice reigns.”

That’s a great quotation, encouraging us that in evil times (like ours), Jesus is especially active in bringing people to Himself. But Silliman doesn’t explain how it helps us calculate the date of Easter. While I found the quotation online, nothing told me anything about the embedded math.

Now I was going to use this as a brief item of interest in my Monday Miscellany. But then I had an idea: Maybe ChatGPT, the internet-scouring, artificial intelligence-driven chatbot that we’ve been playing around with could help me.

So I went to the site and asked it this: “Quaerit amor Christi multos dum regnat iniquus” contains a cipher used to calculate the date of Easter. Could you explain what the cipher is?

Here is the answer:

“Quaerit amor Christi multos dum regnat iniquus” is a Latin sentence that means “The love of Christ seeks many while injustice reigns.” This sentence contains a cipher that was traditionally used to calculate the date of Easter in the Western Christian tradition.

The cipher is based on the number of letters between the first letter “Q” and the last letter “S” in the sentence, which is 23. This number is then added to the number of days in March, which is the month in which the Spring Equinox occurs. In the Western Christian tradition, Easter is celebrated on the first Sunday after the first full moon that occurs on or after the Spring Equinox.

Therefore, to calculate the date of Easter using this cipher, you would add 23 to the number of days in March and then find the first Sunday after the first full moon that occurs on or after that date. This is known as the “ecclesiastical full moon,” which is slightly different from the astronomical full moon.

While this cipher was used in the past to calculate the date of Easter, it is important to note that it is no longer used in modern times. Today, the date of Easter is determined by a set of complex rules and calculations that are based on the Gregorian calendar.

OK! The answer is a little mixed up: March has 31 days, so if you add 23 to that, you’d get 54. So “the first Sunday after the first full moon that occurs on or after” March 54 doesn’t make much sense, there being no such date as March 54. I think the chatbot means that we should go to that number of days in March–that is, March 23–and start counting from that.

And it isn’t clear why folks in the Middle Ages needed a Latin quotation if the answer is always 23. Isn’t it easier to remember a single number than to remember a seven-word line of Latin poetry. I suspect there may be other aspects to the cipher, such as clues to finding the “ecclesiastical full moon.”

Still, ChatGPT did find the basic explanation. I couldn’t find anything online about it, but something obviously exists somewhere. ChatGPT was able to sort through the whole internet very quickly, retrieve the information, and present it to me in plain English.

Which shows the true use of this technology: Not to replace human thinking as alarmists about Artificial Intelligence are worried about, but to serve as a really good search engine. ChatGPT won’t replace human beings, but it might replace Google.

Photo: A calendar of the dates of Easter, for the years 532–632, marble, in the Museum of Ravenna Cathedral, Italy.Photo by Apatak – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55916988