{{calendars}}'''Astronomical year numbering''' is based on AD ([[Anno Domini]])/CE ([[Common Era]]) year numbering, but follows normal [[decimal]] [[integer]] numbering more strictly. Thus, it has a [[year zero|year 0]] and the years before that are designated with a minus sign '−'. The era designations BC ([[Before Christ]])/BCE ([[Before Common Era]]) are dropped. So the year 1 BC/BCE is numbered 0, the year 2 BC is numbered −1, and in general the year ''n'' BC/BCE is numbered (1−''n''). The numbers of AD/CE years are not changed, but AD/CE is not used, being replaced by either no sign or a positive sign; thus in general ''n'' AD/CE is simply (+) ''n''. For normal calculation a [[0 (number)|number zero]] is often needed, here most notably when calculating the number of years in a period that spans the [[epoch (astronomy)|epoch]]; the end years need only be subtracted from each other.

The system is so named due to its use in [[astronomy]]. Few other disciplines outside [[history]] deal with the time before year 1, exceptions being [[dendrochronology]], [[archaeology]] and [[geology]], the latter two of which use 'years before the present'. Although the absolute numerical values of astronomical and historical years only differ by one before year 1, this difference is critical when calculating astronomical events like eclipses or planetary conjunctions to determine when historical events which mention them occurred.

A zero year was first used by the [[eighteenth century]] [[France|French]] [[astronomer]]s [[Philippe de La Hire]] (1702) and [[Jacques Cassini]] (1740). However, both of these astronomers used the applicable AD/BC designations of [[Latin]] and [[French language|French]] with their year zero, thus near the epoch the years were designated 2 BC, 1 BC, 0, 1 AD, 2 AD, etc. They did not use −/0/+. During the [[nineteenth century]], astronomers designated years with either BC/0/AD or −/0/+. Astronomers did not exclusively use the −/0/+ system until the mid [[twentieth century]].

==See also==
* [[Terrestrial time]] (abbreviated: [[TT]])
* [[ISO 8601]]

{{Chronology Topics}}

[[Category:Chronology]]
[[Category:Specific calendars]]
[[Category:Time in astronomy]]

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