Thread by @BretDevereaux

Ok so I enjoy this joke a lot too, but the actual history behind this is a bit more complicated and honestly illuminating in its wackiness.

Let's talk about Roman calendars!

So why do the 'number months' (Sept-Dec) not correspond to their numbers (7-10)? 1/

https://twitter.com/TVAyyyy/status/1598352723833131009

As many people know, the back four months of our calendar are effectively numbered:
September - Seventh Month
October - Eight Month
November - Ninth Month
December - Tenth Month

Except none of them are in their right numerical position. Why?

The Romans. 2/

The normal version of this story you'll hear is that this is all Julius Caesar's fault for 'adding' July, but that's not quite right, a bit of folk history.

July and August existed long before, they were called Quintilis (fifth) and Sextilis (sixth).
3/

So what happened? At some early point, the Romans do seem to have had a ten-month calendar. Later sources (Macrobius, Plutarch) place this in the period of the legendary kings. 4/

By the point where we can observe them (centuries before Caesar) the Romans are, however, operating a twelve month calendar. Our sources (Plut. Numa 18; Liv. 1.19.6-7) credit the legendary king Numa (r. LOL, not a real person or 715-672) with the 12 month calendar. 5/

Numa isn't really a historical figure, so the best we can say from this is that the shift to a twelve month calendar happened so far in the past that by the historical period the Romans only dimly remembered it. 6/

That newer 12-month calendar has some wackiness though, because it's trying to harmonize the lunar and solar calendars and they don't actually fit, so the standard year on the calendar is 355 days, which is notably not 365 days. 7/

The patchwork fixes to this was to add an 'intercalary month' - basically a bonus month - every other year at the end of the year to 'reset' the system. Put a pin in that because we'll come back to it.

But what about the names of the months? 8/

The names of months are, except Quintilis and Sextilis, the same, but for a slight difference in the order, see if you can spot it:
Martius -> Aprilis -> Maius -> Iunius -> Quintilis -> Sextilis -> September -> October -> November -> December -> Ianuarius -> Februarius. 9/

That's right!

The year starts in March! That intercalary month, placed at the end of the year, happens at the end of *February*! 10/

And this makes a lot of sense. March - the month of Mars - marked the beginning of the Roman religious calendar and the start of the campaigning season, with lots of rituals preparing for an army to be raised. It makes sense to start the year there. 11/

And with March coming first, all of the 'number months' fall neatly on their numbers before January and February (likely the two months added to get to 12 way back when) come last as months 11 and 12. 12/

So why do we start the year with January? The Romans again, but *still* not Julius Caesar.

In this case our sources tell us. Livy reports that in 153 the consuls first began to enter office (and thus start the year) in January. 13/

Or more correctly, the Periochae, a summary of Livy's history which gives us some insight into books that are lost (such as Liv. 47, which covered 153) reports that this happened "because of a rebellion in Spain." (Liv. Per. 47.13 🤷‍♂️ 14/

Still, that's pretty reliable: Livy will have been able to confirm this detail and get it right. But why?

Rome's consuls, the two highest officials, had the job of raising and commanding Rome's main armies. So coming into office in March...15/

...and then immediately enrolling those armies initially made a lot of sense. But the tight timing of all of that may have become a problem as Rome's empire grew and the Romans found themselves embroiled in major wars far afield. 16/

Bringing the consuls in two months early would give them time to handle domestic affairs before the campaigning season before the draft in March and still have time to get to armies already in the field in places like Spain before the fighting season began in earnest. 17/

But the Roman calendar was religious, as was Roman politics. The calendar was kept by the Pontifex Maximus, while consuls were ritually inaugerated ('augered in' where augury is 'divining the will of the gods from the flights of birds) for the calendar year. 18/

So moving the date the consuls entered office meant shifting the entire calendar, taking the two last months and making them the two first months, throwing off the position of all of the number-months.

Alas, the Periochae does not tell us who specifically was responsible. 19/

So what did Caesar do with the calendar? Well, the thing is, the Roman calendar here is still a mess - it requires the Pontifex Maximus be very active in setting intercalary months of the right length or the whole thing drifts off of the correct seasons. 20/

And that matters a lot - these are agricultural societies where seasonal cycles are very important.

In the chaos of the first century, those adjustments had been repeatedly not happening. It didn't help that the Pont. Max spent the 50s in Gaul, not in Rome. 21/

That Pont. Max' name was Julius Caesar, of course.

By the mid-40s BC, the calendar was almost a hundred days off. Finally in 46, Julius Caesar takes some time out of his busy schedule of Doing Wars to actually be in Rome to fix the calendar. You know, his job as Pont. Max. 22/

Caesar brings in the best astronomers and mathematicians - from Egypt, where the study of those things was more advanced (Plin. NH. 18) - tweaking the lengths of all of the months to get the year to the 365 days(ish) of the solar calendar. 23/

Then, to reset the start of the year to the right spot, he extended 46 BC to 445 days. Since we still use this calendar system (in many but not all countries), 46BC remains the longest year.

Wikipedia's note that this did not change planetary orbits is hilarious. 24/

Quintilis was only renamed 'July' in 44 after Caesar was dead (in March, you may recall), in his honor. Sextilis was renamed 'August' in 8BC, giving us the months as we have them now.

/end