One of the most noticeable and immediate differences between Orthodox Christianity and Catholicism is in the calendars they follow, with the Orthodox Church following the Julian calendar, and the Catholic Church adhering to the Gregorian calendar. Using different calendars means that the churches celebrate different days for their holidays and feasts.
Julius Caesar amended the old Roman calendar and introduced the Julian calendar in 46 B.C. The Julian calendar added an extra day to make a year with 365 days. Because Earth takes an extra six hours in addition to the 365 days to orbit around the sun, Caesar added one day every four years, resulting in a more accurate leap year. Although this resulted in a calendar that averaged 365.25 days per year, most Orthodox churches kept the Julian calendar. Orthodox churches in countries such as Russia, Serbia and Macedonia continue to use the Julian calendar.
Pope Gregory XIIII initiated the reform of the Julian calendar in 1582 when he noticed that Easter had drifted 10 days away from its initially agreed upon date in 325 A.D. by the First Council of Nicaea. He proposed resetting the calendar 10 days ahead so that Easter could "catch up" to its original date. The Gregorian calendar took into account solar years -- the amount of time it takes to view the sun in one position starting on the first day of spring in one year and ending on the first day of spring the following year -- 10 minutes and 48 seconds shorter than the Julian calendar. Most Catholic European countries adopted the calendar, and it is the standard calendar used today.
Holidays celebrated on specific days each year are known as "fixed feasts" and include Christmas, New Year's and All Saints Day. In Roman Catholicism, Christmas is on December 25th every year, New Year's Day is always on January 1st, and All Saints Day is always on November 1st. In Orthodox churches, fixed feasts in the Julian calendar occur 13 days later than the Gregorian calendar's fixed feasts. For example, Christmas is celebrated on January 7th.
In the Orthodox church, the calculation of the dates for moveable feasts occur in the Paschal cycle -- feasts are calculated according to days before or after Pascha, the Orthodox Easter. For example, while the liturgical year in a fixed calendar starts on September 1st in the Gregorian calendar, the Orthodox Julian calendar calculates the liturgical year starting on Zaccheus Sunday, which is 11 weeks before Easter. The vernal equinox is used as a starting point to calculate the date of Easter. The first Sunday after the first full moon after the vernal equinox is the date for Easter, and the date varies according to whether the Gregorian or Julian calendar is used.
- Harvard University: The Length of the the Year in the Original Proposal for the Gregorian Calendar
- Harvard University: On a Resolution Concept Concerning the Calendar Reform submitted to the Pan-Orthodox Congress in Constantinople in 1923
- Harvard University: The new calendar of the eastern churches
- Springer Link: Mathematics of the Gregorian Calendar; V. Frederick Rickey
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