10. He came. He saw. He concocted a calendar.

Caesar and Cleopatra by Pietro da Cortona (1596-1669) at the Musée de beaux-arts de Leon.  After spending some time in Egypt, Julius Caesar decided to reform the Roman calendar.

Who was born in England on September 3rd, 1752?  No one.  How about on September 4th of that year?  Again, no one.  The 5th, 6th or 7th?  Then too.  How about in New York and Boston?  There too.  No one died either.  

In fact, nothing happened.  Those dates did not even exist.

If you went to sleep on the night of the September 2nd, 1752 anywhere in the British empire, when you woke up, it was September 14th.  September 3rd through September 13th never existed.

How could this have happened?  

In 1752, Britain officially switched from the Julian calendar to the Gregorian calendar.  

The Julian calendar is named after Julius Caesar.  So our story begins in ancient Rome.  

It’s impossible to separate fact from myth when it comes to the original Roman calendar.  After all, it was named after Rome’s founder, Romulus, who was said to have been fathered by the god Mars and suckled by a wolf.  The calendar might have looked like this:

Calendar of Romulus
Martius (31 days)
Aprilis (30 days)
Maius (31 days)
Iunius (30 days)
Quintilis (31 days)
Sextilis (30 days)
September (30 days)
October (31 days)
November (30 days)
December (30 days)

We can see that the ancient Romans had abandoned the Moon as a timekeeper.  Months in lunar calendars are usually either 29 or 30 days long since one cycle of lunar phases takes 29.5 days.  A year consisting of six 29-day months and six 30-day months would be 354 days long, which is shorter than a tropical year by over 11 days. 

It appears that the early Romans wanted the spring equinox to occur during the first month which was then Martius (March).  This explains the choice of 30 and 31 day months.  Seven 30 day months and five 31 day months would produce a 365 day year.  

But notice that the year didn’t have 12 months.  It only had 10.  There were only 304 days in a year.  The days of winter, between December and Martius, were not assigned to any month.

By simply adding another 30 day month and another 31 day month, the ancient Romans could have prduced a 365 day calendar.  They could have done even better by adding a leap day every few years.  It was well known, at least to their neighbors in Greece and Egypt, that a tropical year was a little less than 365.25 days long.

But no.  Things got complicated.

Two more months, Ianuarius and Februarius, were indeed added.  But Februarius would have only 28 days.  Each of the 30 day months would lose a day because even numbers were considered unlucky.  It was okay for Februarius to have an even number of days because it was the month of purification.  The year now had 355 days.  An extra month would be added periodically to bring the year back in line with the seasons.

At some point, the New Year was shifted from the beginning of Martius to the beginning of Ianuarius.  This meant there was no longer any connection between the new year and the spring equinox.  It also meant that the months from Quintilis to December would no longer be the fifth to tenth months, as their names indicate, but the seventh to twelfth months.

This calendar was attributed to Numa Pompilius, Rome’s second king, at least according to legend.

Calendar of Numa
Ianuarius (29 days)
Februarius (28 days)
Martius (31 days)
Aprilis (29 days)
Maius (31 days)
Iunius (29 days)
Quintilis (31 days)
Sextilis (29 days)
September (29 days)
October (31 days)
November (29 days)
December (29 days)

It became clear to Julius Caesar that this calendar had to go.  Not because the month had nothing to do with the Moon.  Or because the names of the last six months did not make sense.  Or because the year did not begin on an equinox or a solstice.  The real problem was that extra months were added not according to how much the year had drifted relative to the seasons but when politicians found it advantageous to do so.  Like the Greeks who had been ruling Egypt, Caesar preferred a 365 day year with a leap day every four years.

This is essentially the calendar we still use today.  It was introduced by Julius Caesar in 46 BCE.  Following his assassination, Quintilis was renamed Iulius in his honor and subsequently Sextilis was renamed Augustus after his successor.

There’s just one little thing.  The tropical year is not exactly 365.25 days long.  The discrepancy is around three days every 400 hundred years.

That was fine for the ancient Romans but not the Christians in charge later.  The problem was Easter.  It was supposed to fall on the Sunday after the full moon following the spring equinox.  This was determined not by observation but by computation.  March 21st was taken to be the date of the spring equinox although the actual spring equinox gradually drifted to earlier and earlier dates.  The ancient Babylonian calculation relating lunar months to tropical years also needed to be revised.  Astronomy again became integral to religion.

During the 16th century, the Council of Trent had astronomers work on the problem for a couple of decades.  The new calendar was instituted in 1582 by Pope Gregory XIII.  The Gregorian calendar would have three fewer leap days every 400 years than the Julian calendar.  Years that are multiples of 100 would not be leap years unless they were also multiples of 400.  So 1600 would still be a leap year but 1700 would not.  

Since the Julian calendar had already drifted relative to the seasons by ten days, a one-time fix was needed to realign the calendar.  In most of the Catholic countries in Europe, October 4th, 1582 was followed by October 15th.

Protestant Europe did not switch to the Gregorian calendar until the middle of the 18th century.  Since they had treated 1700 as a leap year, they now had to drop eleven days from their calendar when they switched.  This was why September 2nd was followed by September 14th in 1752 throughout the British Empire.

The date of the northern summer solstice on the Gregorian calendar.  By BasZoetekouw via Wikimedia Commons.

The Julian calendar has not completely disappeared.  It’s still used by most of the churches in Eastern Christianity to compute the date of Easter.  The heads of major Christian denominations continue to discuss the possibility of a common date for Easter.

When will the Gregorian calendar need to be reformed again?  Not for thousands of years.

A tropical year is 365.2422 solar days while a Gregorian year is 365.2425 solar days.  That would imply an error of one day per 3,300 years but to calculate the actual error, we’d have to take into account all kinds of non-variability in the Earth’s rotation, orbit, and precession.  If we’re still using the Gregorian Calendar thousands of years from now and find out that it’s off by a day, we can simply take out a leap day to bring it back in sync.

This calendar we inherited from the Romans just might endure for another 2000 years or longer.  Even a man as ambitious as Julius Caesar couldn’t have imagined that.



Author: Bhasker Moorthy

I'm a Professor of Astronomy at Harper College in Palatine, IL, USA. My email address is bmoorthy [at] harpercollege.edu.

One thought on “10. He came. He saw. He concocted a calendar.”

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