Astronomical year numbering

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

The system is so named due to its use in astronomy. Few other sciences outside history deal with the time before year 1, exceptions being dendrochronology, archaeology and geology, the latter two of which use 'years from the present'. And the overlap between astronomy and history (where confusion might arise) is negligible, since astronomy deals with a much longer timespan - history deals with up to thousands of years while astronomy deals with up to billions of years. One possible cause for confusion is where a historical record exists of an astronomical event, such as a (super)nova. And even then our understanding of both the record and the event would have to be precise to the year for a discrepancy to be meaningful. And in history a precision of one year rarely exists before a few hundred years before year 1.

A zero year was first used by the eighteenth century French astronomers Philippe de La Hire (1702) and Jacques Cassini (1740). However, both of these astronomers used the applicable BC/AD designations of Latin and French with their year zero, thus near the epoch the years were designated 2 BC, 1 BC, 0, AD 1, AD 2, 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.

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