Year Dating Conventions

Fred Espenak

The western-style year dating convention commonly used in many parts of the world was created by the monk Dionysius Exiguus in or about the year AD 532. The convention is based on Exiguus' determination of the year in which Jesus Christ was born. For instance, in the date AD 2001, the prefix "AD" stands for "Anno Domini" which is Latin for "the year of our Lord." Similarly, in the date 500 BC, the suffix "BC" stands for "Before Christ."

In sixth century Europe, the concept of "zero" was still unknown. Thus, the year 1 BC was followed by the year AD 1. Furthermore, modern scholars believe Christ's birth was actually four years earlier than Exiguus thought. In spite of these deficiencies, the dating system devised by Exiguus is now too deeply ensconced in the Western world to easily change.

Perhaps the most unfortunately characteristic of this convention is that "BC" is a suffix (used after the year) while "AD" is a prefix (used before the year). This is inconvenient when generating computerized lists because extra columns must be reserved for both prefixes and suffixes.

In recent years, some historical scholars have advocated the use of the religiously neutral abbreviations BCE (for "Before Common Era") to substitute for "BC," and "CE" (for "Common Era") to replace "AD." These secular terms are both used as suffixes making them better suited to computer generated tables. Consequently, the NASA Eclipse Home Page adopts the "BCE/CE" dating convention whenever the terminology is required.

However, Exiguus' dating system still lacks a "0" year which makes calendrical calculations awkward. The "astronomical" dating system refers to an alternative method of numbering years. It includes the year "0" and eliminates the need for any prefixes or suffixes by attributing the arithmetic sign to the date. Thus, the astronomical date for 2000 CE is simply +2000 or 2000. The astronomical year 0 corresponds to the year 1 BCE, while the astronomical year -1 corresponds to 2 BCE. In general, any given year "x BCE" becomes "-(x-1)" in the astronomical year numbering system. Historians should take care to note the numerical difference of one year between "BCE" dates and astronomical dates.

Astronomical date numbering was developed for astronomical calculations and is used extensively throughout this web site. The opinions expressed here are those of the author and he assumes full responsibility for their accuracy.

For more information on the subject of dating, see:

2008 Feb 25