The Strange History of the Gregorian Calendar Reforms

For over 1600 years, most European countries used the Julian calendar which was created by Julius Caesar in 46BC as a reform of the old Roman calendar. While the Julian calendar was not perfect, it proved to be fairly reliable. Over the years, however, its inaccuracies were magnified until it was as many as ten days off the solar year. In a time when agriculture was the backbone of all the economies in the world, it was vital to have a reliable way to reckon the time of year (and thus when crops should be planted). To that end, Pope Gregory XIII issued a papal bull in 1582 to reform the old Julian calendar.

One of the problems that one invariably encounters when trying to make a calendar is the inconvenient fact that solar years cannot be divided into a whole number of days. While we say that the year is 365 days long, the solar year is actually about 365.2425 days. Thus, if one were to create a calendar of only 365 days, one would lost one day in relation to the solar year every four years. That difference would grow over time until the solar year and the calendar year were many days or weeks different from each other. Obviously, this is an undesirable effect, so calendars that are not solar based (like Stonehenge) must include some way of dealing with that extra fourth of a day ever year if they want to remain accurate.

That was what the Julian calendar tried to do. Every four years, the Julian calendar added a day to the end of February to account for that extra fourth of a day. While that usually works, it is not entirely accurate either. The solar year is 365.2425 days long, not 365.25 days long. While that extra .0075 of a day may not seem like very much, after a thousand yeas it is enough to make a difference. It losses about three days every 400 years. By 1582, the Julian calendar was about ten days off of the solar year as a result of many small overcorrections in the use of leap years. What the Gregorian calendar did was change the way that leap years were used. Rather than being used every four years, they were used every four years except for those that were divisible by 100 unless that year was also divisible by 400. The result is to call for 97 leap years ever 400 years instead of 100 leap years ever 400. This calendar is quite accurate and loses only one day every three thousand years.