The Century Numbering Mystery

Arch of Titus, Via Sacra, Rome

Arch of Titus, Via Sacra, Rome

The photo above shows the Arch of Titus, spanning the Via Sacra near the forum in Rome, taken by me while on holiday there in 1978. Although the arch displays several Latin inscriptions commemorating the achievements of the Emperor Titus, nowhere in those inscriptions are any numeric year dates provided.

Historians believe that the arch was built in 82 AD, where AD (Anno Domini) is of course a reference to the supposed year of the birth of Jesus of Nazareth, but at that time Rome was not Christian, so obviously they would not have been numbering years “AD”. How then did the Romans, and other early civilizations, number the years?

What Have the Romans Ever Done for Us?

Given that so many aspects of our civilization can be traced back to the Classical times of Greece and Rome, it’s reasonable to assume that our system for numbering years would have Roman origins. However, thanks to an attempt to “Christianize” the Roman system, our current system has some strange quirks that continue to cause confusion.

Recently, during a news report, I heard the announcer state that an event that had occurred during the 1920s took place in the “Nineteenth Century”. In fact, as most people know, the period from 1901 through 2000 was the Twentieth Century, but exactly why is that? Why aren’t years from 1900 through 1999 referred to as the Nineteenth Century, and why did the Twentieth Century start in 1901?

I’ll answer those questions here, but I’m afraid that the answers may be rather uninspiring!

Trick Questions

When I was growing up, I used to spend quite a lot of time watching quiz shows on TV. (Well, we only had two channels, so there wasn’t much to watch!) Most of these were “trivia” contests, where the participants were asked questions to test their knowledge of a wide variety of subjects.

I’m not sure how educational those experiences were, but I do recall one type of “trick question” that was often asked on these shows. The quizmaster would ask something like, “What is the first year of the Twentieth Century?” The correct answer is 1901, but the anticipated wrong answers would be either “1900” or even “2000”.

These oddities sparked my curiosity, leading me to ask the questions listed above. I asked my schoolteachers about it, but they seemed unable to give a rational answer. Eventually, as an adult, I discovered that the reason for this mystery is, in part at least, ancient ignorance!

Year Zero

Our current “Common Era”/”Before Common Era” year numbering system (abbreviated to “CE/BCE”, and previously called the “AD/BC” system) is one among several systems that have been used during the past few thousand years. Some other systems are still in use now.

A common feature of all these systems is that the starting year is completely arbitrary. Some systems attempted to number years from the supposed creation of the Earth, but their estimates of Earth’s age were wildly inaccurate, so in effect the starting year was still arbitrary.

In fact, it may be just as well that we do have an arbitrary datum year, and that we don’t attempt to number years from the date of Earth’s creation. According to the latest research, Earth is about 4.54 billion years old. Every time you sign a check or contract, would you want to have to write out the year as something like “4,542,311,018”? I don’t think so!

The Years AUC

When Romans in the West wanted to provide a numeric year date, they sometimes numbered the years from the supposed “Founding of Rome”, which they named Ab Urbe Condita (AUC).

This date again was arbitrary, in the sense that there was no independent confirmation that Rome had really been founded in that year. In our current CE/BCE system, the year 1 AUC corresponds to 753 BCE.

The AUC system was not used consistently, and such dates may have been added in surviving literature.

The Years AM

The Byzantine Empire (which was the development of the Eastern Roman Empire) used a year numbering system known as AM (for Anno Mundi), which supposedly numbered the years from the date of Earth’s creation. The Year One AM was from September 1, 5509 BCE, to August 31, 5508 BCE.

This also illustrates the point that, even in our own calendar, the start of a year hasn’t always been on January 1st. This fact makes it more difficult to align year computations in different calendar systems.

In about 525 CE, a Christian monk called Dionysius Exiguus was once again engaged in an attempt to determine the age of the Earth from the Bible, and this led him to develop the CE/BCE (or AD/BC, as he called it) year numbering system that we now use.

Ancient Blunders

Frankie Howerd in a Funny Thing Happened on the Way to the Forum

Frankie Howerd in a Funny Thing Happened on the Way to the Forum

My illustration above shows Frankie Howerd performing in the stage play version of A Funny Thing Happened on the Way to the Forum. His expression seems appropriate in this context, because some details of the CE/BCE system seem like something that Howerd’s Roman slave character might have concocted!

The CE/BCE system also has an arbitrary start year; in this case the supposed birthdate of someone who may never have existed (and if he did exist, probably wasn’t born in that year)!

If you’ve learned about the history of arithmetic, then you’ll know that the concept of zero was introduced to Europe during the Middle Ages from the Arabic world, and was previously unknown to Western civilizations. Dionysius Exiguus apparently wasn’t familiar with the concept, so he omitted it from his year numbering system.

As a result of that ancient ignorance, there is no Year Zero in the CE/BCE system. The year numbering goes straight from 1 BCE to 1 CE. This is part of the reason why century numbering in this system seems so confusing.

The system also has no concept of a “Zeroth Century”, either BCE or CE. Thus, the years from 1 CE through 100 CE are the “First Century CE”, which accounts for the correct answer to that “trick question” about why the years from 1901 CE through 2000 CE are the Twentieth Century CE.

Below, I’ve created a table showing how the years AUC, CE and BCE align with century numbering. This should be a helpful reference, at least for people like me, who are more visually-oriented.

Centuries BCE and CE

Centuries BCE and CE

The general algebraic formulae for the centuries are as follows:

The Nth Century CE runs from [(N-1)*100 + 1] CE through N*100 CE

The Nth Century BCE runs from N*100 BCE through [(N-1)*100 + 1] BCE

Of course, as I mentioned above, years didn’t always start on January 1st, so this table is only a rough guide. There have also been date adjustments over the centuries, such as when the Gregorian calendar superseded the Julian calendar, so, before asserting the exact equivalence of two dates, further checking would be wise.

Arch of Titus, Via Sacra, Rome

Arch of Titus, Via Sacra, Rome

One thought on “The Century Numbering Mystery

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