@johncarlosbaez I keep thinking about how one might /guess/ this missing digit to be the at first implausible sounding 7. I imagine that if it was an 8 (or later), it would be too late for some further context of the stone?
@johncarlosbaez
This made think, and what I'm going to say is probably going sound schizo lol.
That said, I was thinking maybe both calendars even though different, they are both completely accurate. For example gravity is 9.8 m/s^2 but that's an average. If you go to a different location, the gravity has a slight difference based on your location. gravity was working before Newton but Newton was the first to label the phenomenon in literature.
People used to think the Olmecs, who made those colossal stone heads, were not very old.
But in 1939, an archaeologist couple, Marion and Matthew Stirling, found the bottom half of this Olmec rock, which had part of a date carved on it.
They guessed the date was 7.16.6.16.18. In the Meso-American Long Count calendar this corresponds to September 3, 32 BC. That meant the Olmecs were extremely old!
But the first digit was missing - Marion just *guessed* it was a 7 - so few believed them.
(2/n)
@johncarlosbaez I keep thinking about how one might /guess/ this missing digit to be the at first implausible sounding 7. I imagine that if it was an 8 (or later), it would be too late for some further context of the stone?
@Ardubal - that's a great question, and there should be a paper that answers. I doubt Marion Stirling simply guessed that initial 7 based on no evidence at all! She (or her husband) would have given some explanation.
Time for some research!
30 years later a farmer found the other half of the rock and confirmed Marion Stirlling's guess: yes, the date was September 3, 32 BC!
That's a wonderful story of delayed vindication. But here's the absolutely chilling part: the Mesoamerican Long Count calendar was so damn good that we can look at that date and know it meant September 3, 32 BC... to within a few days!
I'll explain how, quoting my friend Gro-Tsen (who alas chose Bluesky rather than Mastodon because it's easier to move your posts somewhere else).
https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calendar
(3/n)
Gro-Tsen writes:
I did the math. 🙋
👉 It's Sept. 3, 32BCE (reminder: “32BCE” actually means “−31” 😒) in the proleptic Julian calendar = Sept. 1 prol. Gregorian.
The Western equivalent of the Mesoamerican Long Count is the “Julian Date” (NB: “Julian” here refers not to Julius Cæsar as in “Julian Calendar” but to the 16th century scholar Julius Scaliger). The Julian Date simply counts the number of days from an arbitrary remote reference point (Nov. 24, 4714BCE proleptic Gregorian). More practically, on 2000-01-01 it equaled 2 451 545 (at 12:00 UTC if we want to use fractional Julian dates).
For example, today as I write is Julian Date 2 461 082 (well, 2 461 081.9 because it's not yet noon UTC). And the date of Sept. 1, 32BCE [prol. Greg.] we're talking about corresponds to Julian Date 1 709 981. More convenient than all this dealing with complicated calendar conventions.
So to convert a Long Count date to the Western calendar, we first convert the Long Count to an integer (trivial: it's already just an integer written in base 20-except-18-in-the-penultimate-digit), we add a constant (C) to get a Julian Date, and we convert to our messy calendars.
BUT! What is this constant C? This is known as the “Mayan correlation”. For a long time in the 20th century there was a debate about its value: scholars could relate any two Mayan dates, but not situate them exactly w.r.t. our own calendar. Various values were proposed, ranging from the (frankly rather ludicrous) 394 483 to 774 078, an interval of about 1000 years! (😅)
https://bsky.app/profile/gro-tsen.bsky.social/post/3meiqswj7b22a
(4/n)
@johncarlosbaez Are you sure about your Juliuses and your Scaligers?
It looks to me as if:
Julius Caesar (the emperor) introduced the Julian calendar.
Julius Caesar Scaliger was an erudite chap but had nothing much to do with calendars.
Joseph Justus Scaliger, Julius Scaliger's son, introduced the "Julian period" and (kinda) the "Julian day number".
The "Julian" there is a reference to the Julian calendar, _not_ to his dad.
... But I am not an expert, I'm just reporting what I find in Wikipedia, and maybe the above is all wrong?
Gro-Tsen continues:
This C = 584 283 or “GMT” correlation value places the “Long Count epoch” 0.0.0.0.0 on August 11, 3114BCE in the proleptic Gregorian calendar (the day with Julian Date 584 283), although IIUC it's not clear if this precise date held any particular importance to the Olmecs (or later Mayans).
Maybe it was just arbitrary like the start of our own Julian Date (because, no, Julius Scalier didn't think the world started on November 24, 4714BCE proleptic Gregorian).
One Mayan inscription suggest that the Long Count was the truncation to the last 5 “digits” of an even longer count, and that a Long Count value such as 9.15.13.6.9 was in fact 13.13.13.13.13.13.13.13.9.15.13.6.9 in this Even Longer Count (why 13 everywhere? I don't know!). But this may be one particular astronomer's weird ideas, I guess we'll never know.
But back to the Mayan correlation constant C.
Wikipedia suggests that this “GMT” value C = 584 283 for the Mayan correlation is now settled and firmly established. But between 1905 and now there was some going back and forth with various authors (including the three Goodman, Martínez and Thompson after which it is named) adding or removing a day or two (I think Goodman first proposed 584 283, then changed his mind to 584 280, but nobody really cared, Hernández resurrected the proposal in 1926 but altered it to 584 284, then Thompson to 584 285 in 1927, and then Thompson later said Goodman's initial value of 584 283 had been right all long, and while this is now accepted, the confusion of ±3 days might still linger).
(5/n)
@johncarlosbaez On Gro-Tsen's question "Why 13?": I vaguely remember the number 13 to be symbolic in meso-American culture, a bit like 7 in Europe. At least, there is a 13 day cycle, the trecena https://en.wikipedia.org/wiki/Trecena?wprov=sfla1, used in many meso-American calendars. It was used by Mayas in the Tzol'kin, alongside the long count
Gro-Tsen continues:
The Emacs program's calendar (M-x calendar) can give you the Long Count date (type ‘p m’ for “Print Mayan date”) and uses the GMT value C = 584 283. Today is 13.0.13.5.19. (You can also go to a particular Long Count date using ‘g m l’ but Emacs won't let you go to 7.16.6.16.18 because its calendar starts on January 1, 1 prol. Gregorian = Julian Date 1 721 426 = Long Count 7.17.18.13.3. So close! This caused me some annoyance in checking the dates.)
So anyway, 7.16.6.16.18 is (((7×20+16)×20+6)×18+16)×20+18 = 1 125 698 days after the Long Count epoch, so Julian Date 1 125 698 + 584 283 = 1 709 981 if we accept the GMT value of C = 584 283 for the Mayan correlation, and this is September 1, 32BCE in the proleptic Gregorian calendar, or September 3, 32BCE in the proleptic Julian calendar. (I write “proleptic” here, even though the Julian calendar did exist in 32BCE, because it was incorrectly applied between 45BCE and 9BCE, with the Pontiffs inserting a leap year every 3 years, not 4, and Augustus had this mess fixed.)
Also, confusingly, if we use Thompson's modified (and later disavowed) correlation of 584 285, then we get September 3, 32BCE in the proleptic Gregorian calendar, so maybe this could also be what was meant. Yeah, Julian Dates are a great way of avoiding this sort of confusion!
PS: I wrote the pages
http://www.madore.org/~david/misc/calendar.html
(and also http://www.madore.org/~david/misc/time.html) many years ago (starting on Long Cont 12.19.10.13.1), which I just used to refresh my memory on the subject.
(6/n, n = 6)
@johncarlosbaez
That's all well and good, but how does one convert from Blue Sky dates to Mastodon dates? 😆
@dougmerritt @johncarlosbaez why would you date anyone from Bluesky
