**This**** webpage belongs to www.alexandriancomputus.net,
which is a website promoting [Jan Zuidhoek
(2019) Reconstructing Metonic
19-year Lunar Cycles (on the basis of NASA’s Six Millennium Catalog of Phases of the Moon): Zwolle], and
shows section Summary of this pioneering book, which is available via this website.**

**Summary**

**It**** is the development the Alexandrian computus underwent which formed the mainstream of the history of the computus paschalis that had risen in third century Alexandria
(Egypt) to ultimately flow into the modern method of determining Gregorian calendar dates of Easter Sunday. Between the construction of the very first Metonic 19‑year lunar cycle by
the Alexandrian computist Anatolius (between AD 250
and 270) and the replacement of the Julian calendar with the Gregorian calendar (in AD 1582) it was
only once, namely shortly before the council of Nicaea in AD 325, turning
point in the history of Christianity, ****that**** in Alexandria a completely new Metonic 19‑year lunar cycle was constructed, like its predecessor
directly or indirectly on the basis of at the
time recent lunar tables, but
according to a different
set of computistical principles.
After having reconstructed both of these lost ante‑Nicene Metonic 19‑year
lunar cycles, we establish that:**

**1) the first of them
(referred to as ‘Anatolius’ 19-year lunar cycle’) is equal to the proto-Alexandrian 19-year lunar cycle reconstructed
in 2009;**

**2) the second of them (referred to as ‘the archetypal Alexandrian 19‑year lunar cycle’) is nothing but the archetype from which after AD 325 one after another
each of the three well‑known post-Nicene Alexandrian Metonic 19‑year
lunar cycles was obtained either by adopting the very same Metonic
structure of the archetypal
Alexandrian cycle or by moving
the saltus of its predecessor 1 or 2 years forward or
afterward (see Table 8)****;**

**3) the cause of the 2-day gap between them (referred
to as ‘the ante-Nicene Alexandrian 2-day gap’) must be sought in the transition from the more Jewish Christian world of the third century to
the more Gentile Christian world of the fourth (as a result of which Alexandrian computists began to use
the Egyptian lunar calendar more familiar to them instead
of the Alexandrian version
of the Jewish lunar calendar);**

**4) both Anatolius’
19-year lunar cycle and the
sequence of Paschal dates generated by it
according to the old Alexandrian Paschal rule have de facto lower limit date 23 March;**

**5) the archetypal Alexandrian 19-year lunar cycle has de facto
lower limit date 21 March
but the sequence of Paschal dates generated by it according
to the new Alexandrian Paschal rule has de facto
lower limit date 22 March
(the same applies to the well-known three post‑Nicene Alexandrian Metonic 19-year lunar cycles).**

**We conclude that
Anatolius can be considered to
be the founder of the efficient Alexandrian Metonic 19-year lunar cycle method of determining the Julian calendar date of Paschal Sunday from which
after the Gregorian
reform in AD 1582, after preparatory
work by the Italian astronomer Luigi Lilio, the German mathematician Christoph Clavius would develop a modern system for determining
(Gregorian calendar) dates
of Easter Sunday.**

**© Jan Zuidhoek 2019-2021**