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.
It is the development the Alexandrian Metonic 19-year lunar cycle method underwent which formed the mainstream of the history of the computus paschalis that had risen in third century Alexandria (Egypt) to eventually flow into the modern method of determining the Gregorian calendar date 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 the year 1582) it was only once, namely shortly before the first council of Nicaea (AD 325), that in Alexandria a completely new Metonic 19‑year lunar cycle was created, if not by only simply advancing the dates of Anatolius’ 19‑year lunar cycle by 2 days, then by constructing it directly or indirectly on the basis of contemporary lunar tables. After having reconstructed both of these lost ante‑Nicene Metonic 19-year lunar cycles, we establish that:
1) the first of them (indeed referred to as ‘Anatolius’ 19-year lunar cycle’) is nothing but 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 simply by moving only 1 of the 19 different dates of its immediate predecessor one day forward or back (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 may 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 thirteen centuries later the Italian astronomer Luigi Lilio and subsequently the German mathematician Christoph Clavius could develop a modern, astronomically more correct, system for determining the Gregorian calendar date of Easter Sunday.
© Jan Zuidhoek 2019-2021