This webpage is part of www.janzuidhoek.com, which is a website promoting [Jan Zuidhoek (2021) 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 groundbreaking book, which is available via this website.
It is the development the Alexandrian computus underwent which forms the mainstream of the history of the computus paschalis that had risen in third century Alexandria (Egypt) to ultimately (in the sixteenth century) flow into a more realistic method for determining Gregorian calendar dates of Easter. In this mainstream there were only two real rapids:
1) the construction of the proto‑Alexandrian cycle being the lost very first Metonic 19‑year lunar cycle from which the Alexandrian computist Anatolius originally (around AD 260) started to construct his 19‑year Paschal cycle;
2) the construction of the archetypal Alexandrian cycle being the also lost ante‑Nicene common archetype of the three well‑known post‑Nicene Alexandrian Metonic 19‑year lunar cycles.
The proto‑Alexandrian cycle has appeared to be equal to Anatolius’ 19‑year lunar cycle being the also lost Metonic 19‑year lunar cycle from which Anatolius ultimately (around AD 270) started to construct his 19‑year Paschal cycle. The completely different archetypal Alexandrian cycle was constructed by Alexandrian computists shortly before the council of Nicaea in AD 325, turning point in the history of Christianity.
Of course the archetypal Alexandrian cycle as well as the proto‑Alexandrian cycle was constructed essentially on the basis of at the time recent lunar tables, but the sets of computistical principles according to which they were constructed were rather different. After having reconstructed them, we establish (see Table 8) that:
1) there exists an almost perfect 2-day gap (referred to as ‘the ante‑Nicene Alexandrian 2-day gap’) between Anatolius’ 19-year lunar cycle and the archetypal Alexandrian cycle, the cause of which 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);
2) 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;
3) the archetypal Alexandrian cycle is the archetype from which after Athanasius’ death in AD 373 one after another each of the three well‑known post-Nicene Alexandrian Metonic 19‑year lunar cycles would be obtained either by adopting the archetypal Alexandrian cycle or by simply adapting it by only moving its saltus 1 year afterward or 2 years forward;
4) the archetypal Alexandrian 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 for determining Julian calendar dates of Paschal Sunday from which around the great calendar reform in AD 1582, turning point in the history of chronology, a more realistic (but necessarily also much more complicated) method for determining Gregorian calendar dates of Easter would be developed.
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