This book explains, by following the mainstream of the history of the computus (i.e. Paschal reckoning, developed from early in the third century for the purpose of determining Julian or Alexandrian calendar dates of Paschal Sunday) which rose shortly after AD 250 in Alexandria (Egypt) to ultimately in AD 1582 (turning point in the history of chronology) flow into a more realistic method for determining Gregorian calendar dates of Easter, how at the time calendar dates of Paschal Sunday depended on phases of the moon and how recently both lost Metonic (19 year) lunar cycles constructed in Alexandria before the first council of Nicaea in AD 325 (turning point in the history of Christianity) were reconstructed on the basis of Espenak’s Six Millennium Catalog of Phases of the Moon.
The author of this groundbreaking 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 gone deeply into the history of mathematics, chronology, and early Christianity, ultimately resulting in his lucid webpage “Christian Era and Universal Time”, he became fascinated by the Alexandrian computus. In 2009 he succeeded, by using the Six Millennium Catalog, in determining the Metonic lunar cycle underlying the legendary 19 year Paschal cycle of the great third century Alexandrian computist Anatolius which is part of the fourth century Latin text De ratione paschali. The two presentations he gave at international conferences on the science of computus at the university of Galway resulted in 2017 in a pioneering article entitled “The initial year of De ratione paschali and the relevance of its paschal dates” and in 2019 in the first edition of this book reducing that article to a preparatory study.