The answer
In 46 B. C., Julius Caesar adopted the ancient Egyptian solar calendar, with the help of the Alexandrian astronomer Sosigenes. The original Roman calendar was lunar, and its year had 355 days. The Romans had reckoned thirty-one days in the months of March, May, Quintilis (July634), and October and twenty-eight days in February, while the rest of their months were twenty-nine days. In their new calendar, they added one day to each of April, September, and November, and two days to the months of January, Sextilis (August635), September, and December, bringing the Julian year to a total of 365 days. In the first thirty- six years from the inception of the Julian calendar, the extra day required in a leap year was added every three years instead of every four years, as per the ancient Egyptian calendar.
The Julian calendar was adopted throughout the Western world. But around the year 325 A. D., its inaccuracy became apparent as it was realized that the Julian year of exactly 365.25 days was slightly longer than the actual solar year, which is precisely 365.24217 days long. This difference amounts to eleven minutes and fourteen seconds each year, which adds up to approximately a day and a half every two centuries or seven days every thousand years.
In 1582 A. D., Pope Gregory XIII of Rome omitted ten days from that year to correct for this accumulated error in the calendar. Thus, for that year only, the day after 4th October 1582 became 15th October. He further adjusted the calendar such that three times in every four hundred years (century years that are not divisible by 400), a leap year is omitted. The Gregorian calendar gradually replaced the Julian calendar in different parts of the Western world over the period from 1582 to 1924. For example, England adopted the Gregorian calendar in 1752. The Julian calendar is still used in the calculations of the Feast of the Resurrection and the moveable feasts that depend on it in the Orthodox Churches, which has resulted in the liturgical year of the Church today diverging from the Gregorian calendar by thirteen days.
Fr. Tadros Yacoub Malaty
Notes
633 See entries on “Calendar, Gregorian” and “Calendar, Julian” in The Coptic Encyclopedia, vol. 2, pp. 436– 437; and Elisabeth Achelis, Of Time and Calendar (New York: Hermitage House, 1955).
634 Later named after the Emperor Julius Caesar.
635 Later named after the Emperor Augustus Caesar who changed its number of days from thirty to thirty-one.