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Astronomical year numbering is a method of numbering years before the start of the anno Domini era. In astronomical year numbering, the year 1 BC is numbered 0, the year 2 BC is numbered -1, and in general the year n BC is numbered –(n–1). Years AD are unchanged. The system is so named due to its original use by astronomers. It is convenient to have a year 0 when subtracting years and computing time intervals that span the epoch. However, in order to avoid this problem when making computations for historical times, astronomers find it more convenient to use Julian days, which have a very early epoch.

The first use of the year zero, which is omitted from the traditional proleptic Gregorian calendar has been attributed to Jacques Cassini in his Tables astronomiques (Astronomical Tables, 1740) wherein he explained his reasons for doing so, and also to Phillipe de La Hire who used it earlier in 1702 in his Tabulæ Astronomicæ (Astronomical Tables) in the form Christum o. (Christ 0), without explanation.

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Topics: Astronomical Year Numbering, Astronomical Unit...

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