[GoLUG] cal 9 1752 and the Gregorian calendar revision

[Kyle Terrien]

During GoLUG yesterday, I dared everyone to run ‘cal 9 1752’.  No,
this is not a prank or computer cracking trick.  This is the month
that England and her colonies switched from the Julian Calendar to the
Gregorian Calendar.  11 days were skipped to catch up with the
Gregorian Calendar.

   September 1752
 S  M Tu  W Th  F  S
       1  2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30

I did a little bit of digging and went down a historical rabbit hole.

The Gregorian Calendar[1] is a calendar revision promulgated by Pope
Gregory XIII in 1582.  It replaced the Julian calendar, which has leap
years every 4 years.  In the Gregorian system, years divisible by 100
are not leap years unless they are also divisible by 400, for a grand
cycle of 400 years.  This extra rule was invented to align the
calendar better with the summer solstice and make the calculation of
Easter more reliable.

How does one calculate Easter?  The formula is surprisingly
complex[2], despite the formal definition of “Sunday after the Jewish
Passover” being fixed at the Council of Nicaea (AD 325).  If you use
Emacs, take a look at holidays.el for an algorithmic implementation.

Anyway, the purpose of the Gregorian system was to better align the
calendar with the solstice and lunar cycles to make computing the day
of Easter more reliable.

So, now onto what happened after 1582...

Different countries adopted the Gregorian calendar at different
times[3].  There were a couple centuries where the same day had
different designations in different countries.  Sometimes dates were
labeled “Old Style” (O.S.) and “New Style” (N.S.).

But, if you look at the timeline in [3], you will notice a few
patterns.

The first countries that switched to the Gregorian system were the
Catholic countries.  They switched almost immediately.  This shouldn’t
be a surprise because Pope Gregory XIII was Catholic, and the
Catholics have a strong hierarchy where Church state exists above the
civil state.  So, adoption was a no-brainer.

Then came the Protestant countries, Germany and Great Britain (which
was dragged into Protestantism by Henry VIII, who wanted to divorce
his wife---story for another time).  British politics is where we get
the Sep 1752 hard-coded date in cal.

Then, after the Protestant countries, we have the Eastern Orthodox
countries (Russia, Bulgaria, Greece, etc.).  Here is where some
historical context is required.

The Eastern Orthodox religion goes back to the Cerulean Schism of
1054, better known as the East-West Schism or “Great Schism”[4].  In
the Cerulean Schism, the eastern Churches broke unity with the
Catholic Church in the west (or vice versa depending on who you
ask---I’m trying to be diplomatic here).

Importantly, 1054 was 500 years before the Gregorian calendar, so the
Orthodox used the Julian calendar just like all the Catholics of 1054.
The Orthodox were very *very* concerned with tradition, so they kept
using the Julian calendar even after the Gregorian revision.  So, some
of the last countries to switch in the chart in [3] are Orthodox
countries.

But wait, it gets even *better*.  In the 1920s, a Serbian scientist
(Orthodox) created a Revised Julian calendar[5].  This is known as the
“new calendar” within Orthodox circles, and this is the one that the
“Old Calendarists”[6] to which I alluded are adamantly against.
However, the Revised Julian calendar has a rule different from the
Gregorian Calendar:

> years evenly divisible by 100 are not leap years, except that years
> with remainders of 200 or 600 when divided by 900 remain leap
> years[5]

It just so happens this coincides day-for-day with the Gregorian
calendar only from AD 1700 to AD 2800.

And *that* is one of the craziest things I have learned in a while.
The Western countries have one calendar, and the Eastern countries
have another.  They just happen to coincide for several centuries,
partly out of design.  After 28 February 2800, the East and West will
be off by one day, and someone will need to do something about it
again.

--Kyle


[1]: https://en.wikipedia.org/wiki/Gregorian_calendar
[2]: https://en.wikipedia.org/wiki/Date_of_Easter
[3]: https://en.wikipedia.org/wiki/Adoption_of_the_Gregorian_calendar#Timeline
[4]: https://en.wikipedia.org/wiki/East%E2%80%93West_Schism
[5]: https://en.wikipedia.org/wiki/Revised_Julian_calendar
[6]: https://en.wikipedia.org/wiki/Old_Calendarists

-- 
[*] Kyle Terrien
    Terrenus => from the Earth, to the Cloud
    https://terren.us/

Dilexisti justitiam, et odisti iniquitatem.  -- Psalmus 44:8