You know those deliberately unsatisfying videos?
Where it looks like something’s set up to be absolutely flawless, but then something
misses by inches and the whole thing falls apart?
The lunisolar calendar is like that.
It should be a beautiful, perfect system of wheels within wheels, pleasingly regular orbits
with completely predictable outcomes.
And it kind of is.
But the problem is, the lunar month - aka the time it takes the moon to fully orbit
the earth - is 30 days, and the solar year, the time it takes the earth to fully orbit
the sun, is 365 days, which means twelve lunar months is five days short of a solar year.
That’s already annoying enough.
If you measure your years by the moon, which is extremely convenient due to its phase visibly
changing every day, within six years the sun is a whole lunar cycle off position, which
has a noticeable effect on seasonal weather.
If you measure your years by the sun, which is a little more difficult unless you're really
paying attention to the position of the stars, the seasons might stay consistent but the
lunar cycle is five days offset and you can’t use full moons to demarcate the months anymore.
And it gets worse!
Technically those cyclic lengths are only approximations!
Our day length isn’t some celestial unit inscribed on the heavens that the sun and
moon adhere to, it’s just how fast the earth spins - there’s no reason the moon would
orbit exactly thirty times slower than the earth turns.
So technically a solar year is actually 365 days, five hours, 48 minutes and 46 seconds,
and a lunar month is 29 days, twelve hours, forty-four minutes and 2.9 seconds.
There’s no way to get those perfectly lined up, and any calendar that’s based on whole
days as its smallest unit - which is all of them - will inevitably have to account for
rounding errors within a few short years.
This betrayal of modular arithmetic hurts my soul and single-handedly convinced me that
if there is a creator god responsible for the turning of the celestial spheres, they’re
a real asshole.
Now while almost every culture has its own way of reckoning the calendar, almost all
of them have accounted for this discrepancy in one way or another.
Ancient Babylon favored lunar months for reckoning
time, and made up the time slip with an intercalary
month added every once in a while - roughly seven times every 19 years - to make up the
missing time so the calendar stayed in rough alignment with the observed seasons.
The Hebrew calendar started off very similar to the Babylonian one, but was replaced with
a calculated calendar rather than an observational one over the course of the first millennium
CE, which got pretty solidly locked in by our boy Maimonides in 1178 CE.
The Mayan calendar combined a 365-day solar year with a much more important 260-day cycle
of 13 20-day units called the Tzolk’in that doesn’t have a clear astronomical origin
but does make scholars argue a lot, which is always fun.
And then there’s the ancient egyptians, who created a civil calendar sometime around
2000 BCE by rather ergonomically supplementing their 12 lunar months with a five-day intercalary
period to make sure every year lined up with the 365-day solar cycle.
These five days rounded off the year and were seen as the birthdays of several major gods,
and that’s the kind of concept that definitely has a myth attached, right?
Like, come on.
That’s a golden opportunity.
So today let’s talk about how those oh-so-convenient
five bonus days got tacked onto the calendar
and rounded everything off nicely except for leap years, but they didn’t do those, so
shh So!
This story is recounted by Greek historian, philosopher and priest of apollo Plutarch
in his Moralia, written around 100 AD, in a section that is specifically about Plutarch’s
impression of egyptian culture, their religious practices and their gods.
And while we’re gonna be discussing the egyptian gods he was clearly talking about,
it is important to note that Plutarch went out of his way to localize his story by assigning
corresponding greek gods to several egyptian god in this tale.
It was a popular form of syncretism around this time to basically claim that other cultures
with other gods were really just worshipping the greek gods, but, like… wrong.
So in this story Thoth gets cast as Hermes, Set is regularly referred to as Typhon, primordial
earth and sky gods Geb and Nut are called Cronus and Rhea, Ra gets downgraded to just
“the sun” and Nepthys is briefly called Aphrodite, who is then described as consorting
with Typhon, which I… don’t think is supported in the greek version, but it…… honestly
wouldn’t surprise me?
So our story begins with the sun god Ra pissed off.
This isn’t a strictly uncommon occurrence, but this particular fit of pique is the result
of sky-goddess Nut and earth-god Geb boning down.
It’s not entirely clear why this bugs him so much, but whatever
the reason, Ra lays down an official heavenly
decree that Nut cannot give birth any day of the year.
Yep, all three hundred and sixty of them are strictly off limits.
This is a bummer for a lot of reasons, and Thoth, god of wisdom and assorted smart-guy
things, decides to help her out.
So he gambles with Khonsu the moon god on games of Senet and absolutely smokes him,
winning a small fraction of his total moonlight - about one seventieth.
Now the moon is totally dark only once a lunar cycle, once every thirty days and twelve times
every 360 days, so if you divide up the 348 days when the moon is actually visible, one
seventieth is almost exactly five days.
So Thoth takes his winnings and uses them to expand the calendar, appending his five
new days to the 360 that are already there.
These days are, of course, outside the parameters of Ra’s initial curse, and Nut is free to
have as many babies as she wants!
She proceeds to do just that, popping out Osiris, Horus, Set, Isis and Nepthys in quick
succession, rounding out the Great Ennead in one fell swoop and, as a happy consequence,
perfectly smoothing out the observed solar year.
Except that, you know, they didn’t do leap days, and because this intercalary period
was so deeply religiously significant that the idea of adding a leap day was politically
and religiously controversial and was actively shot down when they tried it in the 200s BCE,
so the whole calendar year very slowly slipped out of alignment with the actual solar year
and the observed seasons - but look, it’s - it's
fine.
Okay?
Sometimes reading the precise mathematical turning of the heavens is - it's a little
more art
than science.
*525,600 Minutes - Rent*