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.
Some historians put the actual birth of Jesus 4 years earlier (4 BC), but it WAS thought to be exactly at 1 AD.
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.
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.