ROME (WHTM) — By the 1500s the Catholic Church was suffering a crisis of bad timing, dating from well back before the beginning of Christianity.
When Rome was founded back on April 21, 753 B.C.E. (Such is the tradition, anyway), Romulus, the co-founder and first ruler of Rome (such is the tradition, anyway) introduced the early Roman calendar. Part of it seems to have been cribbed from the Greeks, who were a bit ahead of the Romans on this sort of thing.
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This first Roman calendar had ten months, Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, and December. (The last six names are based on the words 5,6,7,8,9, and 10) Six of the months had 30 days and four had 31 days, for a total of 304 days. That left a gap of 61 days between December and Martius, which seems initially to have been ignored, possibly because it was winter, and who wants to do anything in winter anyway.
Numa Pompilius, the second king of Rome (such is the tradition, anyway) added the months of January and February, adding 50 days to the calendar to make it 354 days long, then added a day to January to bring the year up to 355 days. This would have been sometime in the 700s or 600s B.C.E.
To deal with the ten-day shortfall in this calendar, the Romans would occasionally stick in a whole additional month called Intercalans or Mercedonius. This would be either 27 or 28 days long. This was repeatedly fine-tuned, with days added here and months shifted there, to come to a reasonably accurate average of 365.25 days per year. The problem is this is an average over a cycle of 24 years. The individual years within this cycle could get, well, sort of messy.
By 46 B.C.E. Julius Ceaser had had enough of the mess, and decided to reform the calendar. He pulled together a group of scholars, most notably a Greek astronomer and mathematician named Sosigenes, to do the heavy mathematical lifting, and in 45 B.C.E. the Julian Calendar began measuring the passing of days in the Roman Republic/Empire.
Perhaps the most important, or at least the most enduring reform was the establishment of regular leap years. Astronomers by then had figured out that a year was 365.25 days long. Instead of a rather haphazard inserting of days, weeks, or even months to make calendar corrections, the Julian Calendar simply called for a leap day to be inserted once every four years. This worked pretty well.
But not perfectly.
It seems the actual length of a solar year is 365.2422 days, just a smidge less than 365.25. Over the centuries, the smidges added up. Every 128 years an error accumulated of about one day. By the 1580s the error had built up to ten days.
For Christians, this meant the Julian calendar was getting out of sync with the liturgical calendar. This was particularly a problem when calculating moveable feasts, days that change dates from year to year. Foremost of these is Easter, whose date needs to be known in advance to calculate things like the number of Sundays that follow Epiphany, the date of Ash Wednesday, and the number of Sundays that follow Pentecost.
The method of calculating the date of Easter was established by the Council of Nicaea in 325 CE. Easter, they decided, would be held on the first Sunday after the first Full Moon occurring on or after the vernal (spring) equinox. At some point, it was decided to lock in the date of March 21st, even though the actual date of the equinox can range from March 19 to March 22. With the Julian calendar out of sync by ten days, it was quite possible to base Easter, and by extension a large chunk of the liturgical calendar, on the wrong full moon.
This is why in the 1580s Pope Gregory XIII decided it was past time to make some changes to the Julian Calendar. After much consultation with astronomers and mathematicians, the new improved calendar was built around one designed by Italian doctor/astronomer/philosopher Luigi Lilio, aka Aloysius Lilius. (Lilius didn’t live to see his calendar adopted; he died in 1576.)
The most important change in the Lilius/Gregorian calendar is the method of calculating leap years. Instead of a day added every four years, they were added by a formula that reduced the total number of leap years. As explained by the United States Naval Observatory:
“Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.”
How much more accurate is this calendar? The Julian Calendar gained 10 days in a little over 1500 years. The Gregorian Calendar, it is calculated, will gain a single day every 3236 years.
But what to do about those extra 10 days? Gregory decided that rather than finding ways to get rid of them one at a time, here and there, over the course of a few years, it would be easier on everyone to just drop them all at once.
Pope Gregory instituted this reform by the papal bull “Inter Gravissimas” on Feb. 24, 1582. The change went into effect in October, when Thursday, Oct. 4 was followed by Friday, Oct. 15. Predominately Catholic countries adopted the new calendar quickly; Protestant countries took longer. When the British (including the American colonies) adopted the reform in 1752, the two calendars were out of sync by 11 days.
Today most countries have moved to the Gregorian Calendar. Meanwhile, the error in the Julian Calendar has continued to pile up. If you were to make the switch from Julian to Gregorian today, you would have to delete 14 days-almost half a month.