document the state of the calendars in one place

docs/calendars.md: the three independent questions a reckoning answers, how
the 1923 split came about and the three changes since that catch out a printed
reference, which churches keep what, and where the line falls between the
Byzantine rite and the nine Eastern Catholic churches that do not use it.

The README and --list lose the prose that had grown around the same subject.

README.md

@@ -174,28 +174,9 @@ See [the law and the fast](#the-law-and-the-fast-are-not-the-same-question)
 for what `--typikon` does with this, which is the flag most Byzantine Catholics
 will want.
 
-### The other ten
-
-There are fourteen Byzantine-rite Eastern Catholic churches. The Slovak,
-Hungarian, Krizevci, Italo-Albanian, Greek, Bulgarian, Macedonian, Belarusian,
-Russian and Albanian ones are **not** here, and the reason is worth stating:
-their calendars could not be verified against anything published. General
-references do not record them, and the churches' own sites render their
-liturgical calendars in JavaScript. Guessing from geography would put an
-unsourced claim in the one place this program is actually for.
-
-You do not need an entry to get a correct calendar. Ask your parish one
-question, and the answer picks a reckoning:
-
-| is Pascha kept with… | and the fixed feasts… | |
-|---|---|---|
-| Rome | on the civil calendar | `--calendar western` |
-| the local Orthodox | on the civil calendar | `--calendar new` |
-| the local Orthodox | thirteen days behind | `--calendar old` |
-
-Add `--tradition slavic` or `--tradition greek` for the usage your books
-follow. A patch adding your church, with a source for its calendar, is
-welcome.
+Ten of the fourteen Byzantine-rite Eastern Catholic churches have no entry of
+their own. Each keeps one of the three reckonings, and
+[docs/calendars.md](docs/calendars.md) sets out which questions decide it.
 
 ## The fast
 
@@ -442,6 +423,11 @@ $ typikon --update-menaion ruthenian
 
 It lands in `~/.local/share/typikon/` and is overlaid automatically.
 
+## Further reading
+
+[docs/calendars.md](docs/calendars.md) sets out which churches keep which
+reckoning, how the split came about, what is modelled here and what is not.
+
 ## Accuracy
 
 `typikon` is pinned against published calendars: the paschalion for six years

docs/calendars.md

@@ -0,0 +1,141 @@
+# The state of the calendars
+
+Which churches keep which reckoning, and what this program models of each.
+
+## Three independent questions
+
+Almost every disagreement about the date of a feast comes from conflating
+these. They vary separately.
+
+**The paschalion** decides Pascha and everything counted from it: the Triodion,
+Great Week, the Pentecostarion, the Apostles' Fast. Two tables are in use. The
+**Julian Paschalion** is the old computus, worked in the Julian calendar and
+read onto a civil date. The **Gregorian Paschalion** is the Western one. They
+agree in some years and can diverge by five weeks.
+
+**The menologion** decides the fixed feasts: the Nativity, Theophany, the
+Dormition, and every saint's day. Two calendars are in use. The **Julian**
+menologion runs thirteen days behind the civil calendar until 2100 and fourteen
+after it. The **Revised Julian** menologion agrees with the Gregorian for every
+date until 2800, so in practice it is the civil calendar.
+
+**The usage** decides rank and some of the fast: Greek books and Slavic books
+read the same Typikon a little differently.
+
+The three combinations in use have short names:
+
+| | paschalion | menologion | kept by |
+|---|---|---|---|
+| **old** | Julian | Julian | Moscow, ROCOR, Serbia, Georgia, Jerusalem, the Holy Mountain, Poland, Macedonia, ACROD |
+| **new** | Julian | Revised Julian | Constantinople, Greece, Antioch, the OCA, Romania, Bulgaria, Cyprus, Albania, Ukraine |
+| **western** | Gregorian | Revised Julian | the Finnish Orthodox Church; most Byzantine Catholic churches |
+
+A new-calendar church therefore keeps the Nativity with the West and Pascha
+with the East. That is not a compromise but what the 1923 congress at
+Constantinople decided: adopt the Revised Julian menologion, leave the
+paschalion alone.
+
+## How it came about
+
+The Pan-Orthodox Congress of May 1923 proposed the Revised Julian calendar.
+Constantinople, Greece, Romania, Poland and others adopted it over the
+following years; Alexandria, Antioch, Bulgaria and Cyprus followed later.
+Jerusalem, Russia, Serbia, Georgia and the Holy Mountain did not, and have
+not.
+
+Three changes since are worth knowing because they are recent enough to catch
+out a printed reference:
+
+- **1923.** The Council of Bishops of the Russian Church Abroad rejected the
+  new calendar outright, which is why ROCOR is Julian throughout and why that
+  became part of its identity rather than a regional custom.
+- **2014.** The Polish Orthodox Church returned to the Julian menologion after
+  decades on the Revised Julian. Some parishes kept the new calendar.
+- **1 September 2023.** The Ukrainian Greek Catholic Church and the Orthodox
+  Church of Ukraine moved their fixed feasts to the Revised Julian calendar,
+  keeping the Julian Paschalion.
+
+One Orthodox church keeps the Gregorian Paschalion: the Finnish Orthodox
+Church, which has done so since the 1920s.
+
+## The Orthodox churches
+
+All twenty are modelled, with the Typikon's own discipline. The Orthodox Church
+legislates fasting as tradition rather than as precept, so nothing is reported
+as binding for any of them.
+
+| jurisdiction | reckoning | usage |
+|---|---|---|
+| `moscow` `rocor` `serbian` `georgian` `jerusalem` `athos` `polish` `macedonian` `acrod` | old | Slavic, except `georgian` `jerusalem` `athos` |
+| `constantinople` `goarch` `greece` `antiochian` `cyprus` `albanian` `romanian` | new | Greek |
+| `oca` `bulgarian` `ukrainian` | new | Slavic |
+| `finnish` | western | Greek |
+
+Where a church is not uniform, the jurisdiction's note says so: most OCA
+parishes keep the new calendar and a minority of Carpatho-Russian and Alaskan
+background keep the old; ACROD publishes both cycles and its parishes keep one
+or the other; Polish parishes vary since 2014.
+
+## The Byzantine Catholic churches
+
+Fourteen Eastern Catholic churches use the Byzantine rite. Four have entries.
+
+| church | reckoning | particular law on fasting |
+|---|---|---|
+| `ruthenian`, the Metropolia of Pittsburgh | western | modelled, from the Metropolia's own published rule |
+| `ugcc`, Ukrainian Greek Catholic | new since 2023 | its own; not modelled |
+| `melkite`, Melkite Greek Catholic | western | its own; not modelled |
+| `romanian-catholic`, Romanian Greek Catholic | western | its own; not modelled |
+
+The other ten are the Slovak, Hungarian, Križevci (Croatia and Serbia),
+Italo-Albanian, Greek, Bulgarian, Macedonian, Belarusian, Russian and Albanian
+Greek Catholic churches. They have no entry because their reckonings are not
+uniform and are not recorded in general reference: several vary by eparchy, and
+the churches that sit beside a local Orthodox majority often keep that
+majority's paschalion for the sake of a common Pascha, as Melkite parishes in
+the Middle East do.
+
+Any of them keeps one of the three reckonings above, and two questions decide
+which:
+
+1. Is Pascha kept with Rome, or with the local Orthodox?
+2. Are the fixed feasts on the civil calendar, or thirteen days behind it?
+
+| | | |
+|---|---|---|
+| with Rome | civil calendar | `--calendar western` |
+| with the local Orthodox | civil calendar | `--calendar new` |
+| with the local Orthodox | thirteen days behind | `--calendar old` |
+
+Add `--tradition slavic` or `--tradition greek` for the usage the books follow.
+A patch adding a church, with a source for its calendar, is welcome.
+
+### Why the law is never borrowed
+
+Every Eastern Catholic church legislates its own fasting discipline, and they
+differ. Ruthenian particular law binds on four kinds of day in the year:
+strict abstinence on the first day of the Great Fast and on Great and Holy
+Friday, and abstinence from meat on the Wednesdays and Fridays of the Great
+Fast. That is the Metropolia of Pittsburgh's law and is reported for that
+church alone.
+
+The others are shown at the Typikon's discipline with nothing reported as
+binding. Telling a Melkite that fourteen days a year oblige him under a law
+written for Pittsburgh would be a false statement about his obligations, which
+is a worse failure than saying nothing. When a church's law is sourced, its
+entry will carry it.
+
+`--typikon` shows any of these churches at the full Byzantine discipline while
+still reporting whatever its own law obliges. For a Byzantine Catholic that is
+usually the flag you want: the law is the floor, not the fast.
+
+## Not in scope
+
+Nine Eastern Catholic churches do not use the Byzantine rite: the Maronite,
+Chaldean, Syro-Malabar, Syro-Malankara, Syriac, Armenian, Coptic, Ethiopian and
+Eritrean churches. They have their own rites, their own sanctorals and their own
+fasting structures, and none of the machinery here applies to them: there is no
+Triodion, no Pentecostarion, no Byzantine menologion. The Latin Church likewise.
+
+Supporting any of them would be a different program sharing nothing but the
+word "calendar".

src/main.rs

@@ -149,17 +149,10 @@ fn list_jurisdictions() {
         }
         println!();
     }
-    println!("  Not listed? There are fourteen Byzantine-rite Eastern Catholic");
-    println!("  churches and ten of them are not here, because their calendars");
-    println!("  could not be verified against anything published. Ask your parish");
-    println!("  one question: is Pascha kept with Rome or with the local");
-    println!("  Orthodox? The answer picks a reckoning above:\n");
-    println!("    with Rome, fixed feasts on the civil calendar   --calendar western");
-    println!("    with the Orthodox, fixed feasts on the civil     --calendar new");
-    println!("    with the Orthodox, fixed feasts thirteen days    --calendar old");
-    println!("                                                     behind\n");
-    println!("  A patch adding your church, with a source for its calendar, is");
-    println!("  welcome.\n");
+    println!("  A church not listed here keeps one of the three reckonings above.");
+    println!("  Which one follows from two questions: is Pascha kept with Rome or");
+    println!("  with the local Orthodox, and are the fixed feasts on the civil");
+    println!("  calendar or thirteen days behind it?\n");
 }
 
 fn elide(text: &str, width: usize) -> String {