This book explains, by following the mainstream of the history of the computus paschalis, i.e. the science developed from the beginning of the third century on behalf of the determination of the date of Paschal Sunday, which rised around AD 250 in Alexandria (Egypt), how of old the date of Easter depends on the phases of the moon, and provides, on the basis of NASA’s Six Millennium Catalog of Phases of the Moon, the reconstruction of both the strongly different lost Metonic 19-year lunar cycles which must have been constructed before the first council of Nicaea (AD 325), turning point in the history of Christianity. The author of this book was born in 1938, studied mathematics, physics, and astronomy at the university of Utrecht from 1960 to 1969, and was a teacher of mathematics from 1970 to 2001 at the Gymnasium Celeanum in Zwolle. After having steeped himself in the fields of history of mathematics, history of early Christianity, and chronology, he became fascinated by the computus. In 2009 he succeeded in determining the initial year (AD 271) of De ratione paschali, i.e. the medieval Latin text containing the Paschal tract of the famous third century Alexandrian computist Anatolius, founder of the modern way of determining the date of Easter. This was done by reconstructing, on the basis of the Six Millennium Catalog, the proto-Alexandrian 19-year lunar cycle defined to be the Metonic 19-year lunar cycle Anatolius must have used to construct his legendary 19-year Paschal cycle. The presentations the author gave at the conferences on the science of computus at the university of Galway in 2010 and 2018 resulted in an article entitled “The initial year of De ratione paschali and the relevance of its paschal dates” (in 2017) and the present study (in 2019), respectively.
It is the development of the Metonic 19-year lunar cycle which formed the mainstream of the history of the computus paschalis that had risen in third century Alexandria (Egypt) and would in 1582 flow out 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 first version of this lunar cycle by Anatolius (somewhere between AD 250 and 270) and the replacement of the Julian calendar with the Gregorian one (in 1582) it happened only one time, namely somewhere between AD 300 and 325, hence still before the first council of Nicaea, that a new version of it was actively constructed. After having reconstructed (on the basis of NASA’s Six Millennium Catalog) the two lost ante-Nicene versions of the Metonic 19-year lunar cycle in question, we establish that: 1) the first of them (referred to as ‘Anatolius’ 19-year lunar cycle’) is nothing but the proto-Alexandrian 19-year lunar cycle (reconstructed ten years ago); 2) the second (referred to as ‘the archetypal 19-year lunar cycle’) must considered to be the archetype from which after AD 325 one after another each of the three well-known post-Nicene Metonic 19-year lunar cycles was obtained simply by moving only 1 of the 19 different dates of its predecessor one day forward or back; 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 one 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. ×