This book by the mathematician and astronomer Johannes Mueller, known as “Regiomontanus,” began the transition to the new, reformed Gregorian calendar.  Because Easter was based on the flawed Julian calendar introduced by Julius Caesar (100–44 BC), the day had gradually drifted away from its spring observance tied to the Jewish Passover. Regiomontanus died soon after being summoned to Rome by Pope Sixtus IV (reigned 1471 to 1484) to begin the reform. The calendar change was not achieved until 1582 under Pope Gregory XIII (reigned 1572 to 1585).