B.C. (or BC) - Counting and Numbering Pre-Roman History

Where Did the BC/AD Designations Come From - and How Did We Get There?

The term BC (or B.C.) is used by most people in the west to refer to pre-Roman dates in the Gregorian Calendar (our current calendar of choice). "BC" refers to "Before Christ", meaning before the putative birth year of the prophet/philosopher Jesus Christ, or at least before the date once thought to be that of Christ's birth (the year AD 1).

The first surviving use of the BC/AD convention was by the Carthaginian bishop Victor of Tunnuna [died AD 570).

Victor was working on a text called Chronicon, a history of the world begun by Christian bishops in the 2nd century AD. BC/AD was also used by the British monk the "Venerable Bede", who wrote over a century after Victor's death. The BC/AD convention was probably established as early as the first or second century AD, if not widely used until much later.

But the decision to mark years AD/BC at all is only the most prevalent convention of our current western calendar in use today, and it was devised only after some tens of thousands of years of mathematical and astronomical investigations.

Calendars BC

The people who likely devised the earliest calendars are thought to have been motivated by food: the need to track seasonal growth rates in plants and migrations in animals. These early astronomers marked time by the only way possible: by learning the motions of celestial objects such as the sun, moon, and stars.

These earliest calendars were developed all over the world, by hunter-gatherers whose lives depended on knowing when and where the next meal was coming from. Artifacts that may represent this important first step are called tally sticks, bone and stone objects which bear incised marks that may refer to the numbers of days between moons.

The most elaborate of such objects is the (somewhat controversial of course) Blanchard Plaque, a 30,000-year old piece of bone from the Upper Paleolithic site of Abri Blanchard, in the Dordogne valley of France; but there are tallies from much older sites that may or may not represent calendrical observations.

The domestication of plants and animals brought an additional layer of complexity: people were dependent on knowing when their crops would ripen or when their animals would gestate. Neolithic calendars must include the stone circles and megalithic monuments of Europe and elsewhere, some of which mark the important solar events such as solstices and equinoxes. The earliest possible first written calendar identified to date is the Gezer calendar, inscribed in ancient Hebrew and dated to 950 BC. Shang dynasty oracle bones [ca 1250-1046 BC] may also have had a calendrical notation.

Counting and Numbering Hours, Days, Years

While we take it for granted today, the crucial human requirement of capturing events and predicting future events based on your observations is a truly mind-blowing problem. It seems quite likely that much of our science, mathematics, and astronomy are a direct outgrowth of our attempts to make a reliable calendar.

And as scientists learn more about measuring time, it becomes clear how enormously complex the problem truly is. For example, you'd think figuring out how long a day was would be simple enough--but we now know that the sidereal day--the absolute chunk of the solar year--lasts 23 hours, 56 minutes, and 4.09 seconds, and is gradually lengthening. According to growth rings in mollusks and corals, 500 million years ago there may have been as many as 400 days per solar year.

Our astronomical geek ancestors had to figure out how many days there were in a solar year when the "days" and "years" varied in length. And in an attempt to know enough about the future, they did the same for a lunar year--how often did the moon wax and wane and when does it rise and set. And those kinds of calendars aren't really migratable: sunrise and sunset occur at different times at different parts of the year and different places in the world, and the moon's location in the sky is different for different people.

Really, the calendar on your wall is a remarkable feat.

How Many Days?

Fortunately, we can track the failures and successes of that process through surviving, if patchy historical documentation. The earliest Babylonian calendar reckoned the year to be 360 days long--that's why we have 360 degrees in a circle, 60 minutes to an hour, 60 seconds to the minute. By about 2,000 years ago, societies in Egypt, Babylon, China and Greece had figured out that the year was actually 365 days and a fraction. The problem became--how do you deal with a fraction of a day? Those fractions built up over time: eventually, the calendar that you were relying on to schedule events and tell you when to plant became off by several days: a disaster.

In 46 BC, the Roman ruler Julius Caesar established the Julian calendar, which was built solely on the solar year: it was instituted with 365.25 days and ignored the lunar cycle entirely. A leap day was built in every four years to account for the .25, and that worked pretty well. But today we know our solar year is actually 365 days, 5 hours, 48 minutes and 46 seconds long, which is not (quite) 1/4 of a day. The Julian calendar was off by 11 minutes per year, or a day every 128 years. That doesn't sound too bad, right? But, by 1582, the Julian calendar was off by 12 days and cried out to be corrected. But that's another story.

Other Common Calendar Designations

• A.D.
• B.P.
• RCYBP
• cal BP
• A.H.
• B.C.E.
• C.E.

Sources

In general, calendars and timekeeping are vastly complicated topics which cross the fields of astronomy and mathematics, not to mention philosophy and religion.

I've barely scraped the surface here.

This glossary entry is part of the About.com Guide to Calendar Designations and the Dictionary of Archaeology.

Dutka J. 1988. On the Gregorian revision of the Julian calendar. The Mathematical Intelligencer 30(1):56-64.

Marshack A, and D'Errico F. 1989. On Wishful Thinking and Lunar "Calendars".Current Anthropology 30(4):491-500.

Peters JD. 2009. Calendar, clock, tower. MIT6 Stone and Papyrus: Storage and Transmission. Cambridge: Massachusetts Institute of Technology.

Richards EG. 1999. Mapping Time: The Calendar and its History. Oxford: Oxford University Press.

Sivan D. 1998. The Gezer Calendar and Northwest Semitic Linguistics. Israel Exploration Journal 48(1/2):101-105.

Taylor T. 2008. Prehistory vs. Archaeology: Terms of Engagement. Journal of World Prehistory 21:1–18.