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Historic Calendar Mystery Solved

July: Peasants cutting grass,
making haystacks.

Or, how the Pope banished 10 days

by Gael Stirler
(April, 2006)

Why is December, which means "tenth", the twelfth month? Why was there a "leap day" in 2000 but there wasn't one in 1900, though they are both divisible by 4? Why do Muslims celebrate all of their holy days on different dates each year? Why did the Pope tell everyone to skip 10 days in 1582 A.D.? Why didn't England and Ireland follow suit for almost one hundred years? How did travelers deal with the discrepancy in calendars when so many places were not using the same method of time calculation?

These mysteries began as soon as people began to keep track of the number days between the most observable celestial events, i.e. the phases of the moon, the vernal and autumnal equinoxes, and the summer and winter solstices. Since the length of the year is not exactly 365 days, ancient astronomers observed that the vernal equinox fell one day later every 4 years. Within a span of only 30 years the calendar was off by more than one week. This could be disastrous for an agrarian society that needed to know the right time to plant each crop. So, from time to time, rulers decreed there would be an extra day or two added to the month of February to compensate. Which begs the question, why February?

September: Sowing the fields


Long before Julius Caesar, around the founding of Rome, the first day of the year was on March 1 which, at that time, roughly coincided with the Vernal Equinox. Originally, the months had numbers for names but later Romans changed the names of some months to honor their gods. March was dedicated to Mars, the god of war. Then came Aprilis for Aphrodite, goddess of love, Maius for Maia, goddess of fertility, Junius for Juno, wife of Jupiter and goddess of marriage, children and home. Even in ancient days June was the most popular month of the year for weddings.

July was called Quintilus, meaning fifth month, until Julius Caesar renamed it after himself to further his assertion of godhood. You know how these things go—Augustus Caesar had to follow his great-uncle's lead and name a month after himself. Thus the sixth month, which was known as Hexilius, became Augustus. A couple of the following emperors tried to make changes to the names of months but people went back to calling the months by the old names as soon as the tyrants were in the ground.

September means "seventh", October means "eight", November means "ninth", and December means "tenth". January, the eleveth month of the old Roman year, was named after Janus, the two-faced god who could look backward and forward at the same time. This cold winter month has always been a good time to take stock and make plans, hence our tradition of making resolutions. One of the Roman emperors though it was better to start the year thoughtfully with Janus rather than violently with Mars and decreed that January 1 would be the first day of the year.

Until then, February was the last month of the year, and the entire month was a celebration of the end of winter. This celebration was like Christmas, New Year's and Mardi Gras all rolled into one big party! Before Julius and Augustus started changing the calendar, the months alternated between 31 and 30 days starting with March. The month of February was shortest because it was made out of whatever numbers of days were left. Before Julius emperors declared leap days whenever they needed to garner the favor of the people with extra party days. This led to calendar chaos. In 46 B.C., Julius Caesar finally put an end to this by making a law establishing regular 4-year intervals for leap years of 366 days.

Pope Gregory XIII
Pope Gregory XIII


The Julian calendar worked well for many centuries until an astronomer priest named Christopher Clavius was trying to create a chart of Easter dates for all the clerics in Christendom to use. The date of Easter, as set forth by the Council of Nicea during the fourth century A.D., moves in relation to the movements of the sun and moon. While Christmas always was celebrated on December 25, no matter what day of the week, Easter had to fall on a Sunday, but not just any Sunday. Each Sunday between Christmas and Easter was dedicated to a different part of the gospel story and there had to be exactly the same number of masses held between Christmas and Easter each year. So the council decreed the "Easter Golden Rule" of celebrating the Resurrection on the first Sunday after the first full moon following the vernal equinox. By the year 1500 A.D. the number of Sundays was no longer the same, and the equinox was occurring on March 12 instead of March 21 as it had been in the fourth century due to a small error in the Julian calendar method.

In 1582, Father Clavius reported to Pope Gregory XIII that the accumulated error was exactly 10 days. It would take dropping 10 days off the calendar to make the correction and a slight change to the way leap year was calculated to keep it on track. Pope Gregory XIII issued a bull (a decree) that the day following October 4, 1582, would be called October 15, 1582, and that from then on leap years would fall on years divisible by 4 unless they ended in double 00 except when they were divisible by 400. Therefore 1600 had a leap day but 1700 didn't. This refinement was called the Gregorian Calendar.


December: Winter activities.
Slaughtering a hog.
Children playing on the hill.


However, at that time, large parts of the world were no longer in allegiance with or in open rebellion to the Roman Church. Protestant kingdoms and states in Western Europe continued to use the Julian calendar for over 100 years. Parts of Germany, France, and the Netherlands adopted it at different times depending on their allegiance. For over one hundred years Europeans found it difficult to coordinate travel and commerce between various locations. But it did have advantages, too. Citizens in Catholic States could celebrate their birthdays twice a year by visiting a Protestant State 10 or 11 days later. England, Ireland, and the British Colonies adopted the Gregorian calendar on September 2, 1752, and even though Scotland used the Gregorian Calendar, no one can really agree when they adopted it. The Scandinavians agreed to make the change over a 40 year period by not adding leap years between 1700 and 1740. But they forgot to make the adjustments in 1704 and 1708; so, in 1712 they went back to the Julian Calendar by having a 30-day February. They finally got on board in 1753. Russia adopted it after the Bolshivik Revolution in 1918 with the final European hold out, Greece, following in 1924. The Eastern Orthodox Church never did switch from the Julian calendar so the Greek Orthodox Easter is usually one week later than Roman Catholic Easter. Soon they will be out of sync by two weeks.

The Julian calendar currently lags the Gregorian calendar by 13 days. Since the Gregorian calendar is close but not perfect, we will still have to make a 1-day correction every 3,323 years.

The Gregorian calendar is the most widely adopted method of dividing the year, but it is not the only method. The Hebrew calendar consists of twelve 29- and 30-day months plus an extra month every year or so. The Muslim calendar is tied to the moon instead of the sun so it consists of twelve 29- or 30-day months with no extra month to make up the difference. This means that, over a period of time, any given date of the Muslim Calendar will fall on any date of the Gregorian calendar. Imagine celebrating you birthday in Winter when you are a baby, in the Spring when you are 9, in the summer when you are 17, and in the fall when you're in your 20's!

So next time you read, "On this day in history," you might take a moment to wonder, "Is that a Julian or Gregorian calendar day?"

See links below for more information on calendars.


Julius Caeser
julius caesar

TimeLine: Ancient Rome a brief history of the origin of the western calendar by David Neelin

Calendars through the Ages, a webexhibit dedicated to the history of Calendars and time keeping methods. Very interesting yet in depth.

The Curious History of the Gregorian Calendar Or, Eleven days that never were by Ben Snowden

Calendars reprinted from the Explanatory Supplement to the Astronomical Almanac, P. Kenneth Seidelmann, editor, with permission from University Science Books, Sausalito, CA On the Units of Time, Part III: The Year by Edward Hahn. A very interesting site with many explanations of the Science an Astronomy involved.

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