This book explains, by following the mainstream of the history of computus (i.e. Paschal reckoning developed from early third century for determining Julian or Alexandrian calendar dates of Paschal Sunday) which shortly after AD 250 rose 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 ...