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 the two strongly different lost Metonic 19-year lunar cycles 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 paschalis. 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 respectively 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), .