This webpage belongs to, 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 underwent that formed the mainstream of the history of the computus paschalis which had risen in third century Alexandria (Egypt) and would in the year 1582 flow into the modern method which since then is used in order to determine the Gregorian calendar date of Easter Sunday. Between the active construction of the very first Metonic 19-year lunar cycle by the Alexandrian computist Anatolius, somewhere between AD 250 and 270, and the replacement of the Julian calendar with the Gregorian calendar (in the year 1582) it happened only one time, namely somewhere between AD 300 and 324, in any case already before the first council of Nicaea, that (under the auspices of the church of Alexandria) a radically new Alexandrian Metonic 19-year lunar cycle  was actively constructed. After having reconstructed (on the basis of NASA’s Six Millennium Catalog) both of these ante-Nicene Metonic 19-year lunar cycles, we establish that:

1) the first of them (referred to as ‘Anatolius’ 19-year lunar cycle’) is nothing but the lost 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 lost ante-Nicene 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 dates of Paschal Sunday 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 dates of Paschal Sunday 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 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.



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