0 (year) - Historical, Astronomical and ISO Year Numbering Systems - Astronomers - History of Astronomical Usage

History of Astronomical Usage

In 1849 the English astronomer John Herschel invented Julian dates, which are a sequence of numbered days and fractions thereof since noon 1 January −4712 (4713 BC), which was Julian date 0.0. Julian dates count the days between two instants, automatically accounting for years with different lengths, while allowing for any arbitrary precision by including as many fractional decimal digits as necessary. The modern mathematical astronomer Jean Meeus no longer mentions determining intervals via years, stating:

The astronomical counting of the negative years is the only one suitable for arithmetical purpose. For example, in the historical practice of counting, the rule of divisibility by 4 revealing the Julian leap-years no longer exists; these years are, indeed, 1, 5, 9, 13, ... B.C. In the astronomical sequence, however, these leap-years are called 0, −4, −8, −12 ..., and the rule of divisibility by 4 subsists. —Jean Meeus, Astronomical algorithms

In 1627 the German astronomer Johannes Kepler first used an astronomical year which was to become year zero in his Rudolphine Tables. He labeled the year Christi and inserted it between years labeled Ante Christum (BC) and Post Christum (AD) on the mean motion pages of the Sun, Moon, and planets. Then in 1702 the French astronomer Philippe de la Hire used a year he labeled Christum 0 at the end of years labeled ante Christum (BC), immediately before years labeled post Christum (AD) on the mean motion pages in his Tabulæ Astronomicæ, thus adding the designation 0 to Kepler's Christi. Finally, in 1740 the French astronomer Jacques Cassini (Cassini II), who is traditionally credited with the invention of year zero, completed the transition in his Tables astronomiques, simply labeling this year 0, which he placed at the end of years labeled avant Jesus-Christ (BC), immediately before years labeled après Jesus-Christ (AD).