Origin, meaning of the phrase 'BC', 'AD'

When reading about history, we often come across the abbreviations "BC" and "AD" or "BCN" and "CN" to refer to time points. So where exactly do these words come from and what do they mean?

These are terms that mark the time before and after Jesus was born. They are sometimes written as "CE" and "BCE".

What is AD?

The astronomical clock at the Mesina Cathedral, Sicily, Italy (Photo: Eye Ubiquitous/ Getty Images).

"AD" stands for "anno domini" , which is Latin for "in the year of the Lord," referring specifically to the year Jesus was born. "BC" stands for "before Christ."

In historical texts, the system of marking years is based on a popular conception of when Jesus was born. When written "AD" it means the years after Jesus' birth and "BC" refers to the years before that date.

In recent years an alternative spelling for BC and AD has emerged. Many documents use the abbreviations "CE" (Common Era) and "BCE" (before Common Era) to avoid religious overtones.

In Vietnamese, these two words are often abbreviated as "TCN" and "CN".

Before talking about the reasons and origins of this abbreviation system, let's look at some historical context.

The First Ecumenical Council aimed to standardize the timing of Easter celebrations, which led to the creation of the AD time designation system (Photo: Getty Images).

When was "AD" invented?

In the early Middle Ages of Europe, the most important calculation and one of the main motivations for conducting mathematical research was when to celebrate Easter.

In 325 AD, the First Ecumenical Council decided that Easter would fall on the Sunday following the first full moon after the vernal equinox . "Computus" in Latin means "to calculate", the process for calculating this most important day and is recorded in documents as the "Easter table".

In 525 AD, one of those Easter tables was added by a monk named Dionysius Exiguus of Scythia Minor to count the years since the birth of Christ and introduced the AD system.

The monk Dionysius does not specify how he determined the date of Jesus' birth, but he probably used surviving texts from early Christians, such as those of Clement of Alexandria or Eusebius of Caesarea, to calculate the date of Jesus' birth.

The monk Dionysius took 1 AD as the year of Jesus' birth, but he may have miscalculated by a few years, since modern estimates place Jesus' birth sometime between 4 BC and 6 AD.

What time marking system was used before "AD"?

Dionysius devised this system to replace the Diocletian system, named after the Roman Emperor (reigned from 284 AD to 305 AD). The monk made the change to avoid remembering Emperor Diocletian, who enacted changes that resulted in the killing or imprisonment of many Christians and the burning of their churches and bibles.

Two centuries after Dionysius came up with the "AD" spelling, "BC" was added to the system.

In 731, Bede the Venerable of the Kingdom of Northumbria published his book "Ecclesiastical History of the English People", which expanded the year marking system to include years before 1 AD.

The years before year 1 are numbered backwards when referring to events that occurred before Christ and are abbreviated as "BC", in Vietnamese they are often written as "TCN".

Bede the Venerable was the first to use "BC" to count years before the birth of Jesus (Photo: Getty Images).

Why is there no year 0?

In the updated system of Saint Bede, there is no year 0 because the concept of "zero" did not exist in Western Europe. Professor Charles Seife - Director of the Arthur L. Carter Journalism Institute, New York University, USA - said: "Saint Bede did not know the number 0 and for him, the previous year of the year 1 AD was the year 1 BC. There was no year 0. For him, the number 0 did not exist."

However, zero did exist elsewhere. The concept of zero first appeared in a 628 AD publication by the Indian scholar Brahmagupta, although earlier cultures also had concepts of zero, including the Babylonians and the Maya.

It was not until the 11th to 13th centuries that the concept became popular in medieval European Christianity.

When did AD and BC become popular?

The AD/BC system came into greater use from the 9th century, after Holy Roman Emperor Charlemagne introduced it to date state activities throughout Europe.

By the 15th century, all of Western Europe was using the AD/BC system. The Gregorian calendar, which was introduced in the 16th century, also used this system, and since 1998, when the International Organization for Standardization (ISO) issued ISO 8601 describing how to represent dates and times, this system has become an internationally accepted standard.

Holy Roman Emperor Charlemagne introduced the AD/BC system in the 9th century (Photo: Getty Images).

How are BC and BCE different?

Some organizations and individuals have switched to using the terms "BCE" (Before Common Era) and CE (Common Era) instead of BC and AD on the grounds that it is to avoid sensitivity to non-Christians and because BC and AD do not accurately correspond to the time of Jesus' birth.

The terms CE and BCE appeared in a 1795 book on astronomy and were used interchangeably with the term "Vulgar Era" . At that time, "vulgar" meant "normal" rather than "vulgar".

The term "Vulgar Era" was used even earlier, in the book of the famous astronomer Johannes Kepler, who lived during the time of the Holy Roman Empire.