Origins  of the Gregorian Calendar

Whether it lives on our wall, in our phone or in our head, calendars allow us to mark the passage of time and important events in our lives. The calendar we use has been in place for almost 500 years, but it has taken several changes over the last 2000 years to ensure its accuracy.

Julian Calendar — The Roman calendar was 355 days long and so was often out of alignment with the solar year, which was approx. 365 days.  The chief priest (or Pontifex Maximus) of Rome would add or subtract days to the calendar every year to ensure that it matched the solar year.  However, most pontifices were also politicians, so there were instances where the calendar would not be updated if it benefited their political career.  For example, they could shorten a rival politician’s term of office by not updating the calendar or lengthen the term of their office by the same method.  This somewhat arbitrary method of keeping the calendar accurate ended with the calendar reforms of Julius Caesar, who officially set the calendar year at 365.25 days (1 leap day every four years).  The problem was that this calendar wasn’t 100% accurate either.  This wouldn’t be corrected for 1500 years, when Pope Gregory XIII sponsored changes that resulted in the calendar we use today.

Gregorian Calendar — The Julian calendar was off by about 11 minutes per year, which adds up over the centuries.  By the time Pope Gregory updated the calendar in 1582, the Julian calendar was off 10 days.  But 10 days as compared to what?  In 325 A.D. the Council of Nicea had officially set down the rule that all Christians must celebrate Easter on the same date.   In order for this to happen, the Julian calendar had to be corrected for the drift that had already occurred and a better calculation developed so that the calendar would be accurate for the future.  Many different re-calculations of the calendar were proposed, eventually the one that Pope Gregory sponsored kept the Leap Year calculation, but modified it slightly to skip 3 leap days every 400 years.  This fixed the calculation error that existed in the Julian calendar; now all that remained was to figure out what to do with the 10 days that the calendars were off.  One idea proposed was to gradually take the 10 days out of the calendar over 40 years by deleting the Leap Day (Feb. 29).  Pope Gregory XIII decided to go with a more direct change and deleted ten days to re-synchronize the calendars; Thursday October 4, 1582 (Julian calendar) was followed by Friday, October 15, 1582 (Gregorian calendar).  The old calculation was off by 1 day every 128 years, the new calculation is off by only 1 day every 7,700 years.

This new calendar, the Gregorian calendar, was adopted by the Catholic church and over the centuries has been adopted by most, but not all, countries around the world.

By Jair Furnas

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