Astronomical year numbering#Usage of the year zero

{{Main|Year zero}}

In his Rudolphine Tables (1627), Johannes Kepler used a prototype of year zero which he labeled Christi (Christ's) between years labeled Ante Christum (Before Christ) and Post Christum (After Christ) on the mean motion tables for the Sun, Moon, Saturn, Jupiter, Mars, Venus and Mercury.Johannes Kepler, [http://www.ub.uni-kiel.de/digiport/bis1800/Arch3_436.html Tabulae Rudolphinae] (1627) Pars secunda, 42 (Zu Seite 191), 48 (197), 54 (203), 60 (209), 66 (215), 72 (221), 78 (227). (Latin) In 1702, the French astronomer Philippe de la Hire used a year he labeled {{nowrap|Christum 0}} at the end of years labeled ante Christum (BC), and 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.[https://books.google.com/books?id=BjygAAAAMAAJ Tabulae Astronomicae – Philippo de la Hire] (1702), Tabulæ 15, 21, 39, 47, 55, 63, 71; Usus tabularum 4. (Latin) Finally, in 1740 the French astronomer Jacques Cassini {{nowrap|(Cassini II)}}, who is traditionally credited with the invention of year zero,Robert Kaplan, [https://books.google.com/books?id=Bn0EBVsfi1YC&pg=PA103 The nothing that is] (Oxford: Oxford University Press, 2000) 103.Dick Teresi, [https://www.theatlantic.com/past/docs/issues/97jul/zero.htm "Zero"], The Atlantic, July 1997 (see under Calendars and the Cosmos).L. E. Doggett, [http://charon.nmsu.edu/~lhuber/leaphist.html "Calendars"] {{webarchive|url=https://web.archive.org/web/20120210184836/http://charon.nmsu.edu/~lhuber/leaphist.html |date=10 February 2012 }}, Explanatory Supplement to the Astronomical Almanac, ed. P. Kenneth Seidelmann, (Sausalito, California: University Science Books, 1992/2005) 579. completed the transition in his Tables astronomiques, simply labeling this year 0, which he placed at the end of Julian years labeled avant Jesus-Christ (before Jesus Christ or BC), and immediately before Julian years labeled après Jesus-Christ (after Jesus Christ or AD).

Cassini gave the following reasons for using a year 0:Jacques Cassini, [https://books.google.com/books?id=fVIVAAAAQAAJ&pg=RA1-PA3 Tables astronomiques], 1740, Explication et Usage p. 5, translated by Wikipedia from the French:
"L'année 0 est celle dans laquelle on suppose qu'est né Jesus-Christ & que plusieurs Chronologistes marquent 1 avant la naissance de J. C. & que nous avons marquée 0, afin que la somme des années avant & après J. C. donne l'intervalle qui est entre ces années, & que les nombres divisibles par 4 marquent les années bissextiles tant avant qu'après Jesus-Christ."

{{Blockquote|The year 0 is that in which one supposes that Jesus Christ was born, which several chronologists mark 1 before the birth of Jesus Christ and which we marked 0, so that the sum of the years before and after Jesus Christ gives the interval which is between these years, and where numbers divisible by 4 mark the leap years as so many before or after Jesus Christ.|Jacques Cassini}}

Fred Espenak of NASA lists 50 phases of the Moon within year 0, showing that it is a full year, not an instant in time. Jean Meeus gives the following explanation:Jean Meeus, Astronomical Algorithms (Richmond, Virginia: Willmann-Bell, 1991) 60.

{{Blockquote|There is a disagreement between astronomers and historians about how to count the years preceding year 1. In [Astronomical Algorithms], the 'B.C.' years are counted astronomically. Thus, the year before the year +1 is the year zero, and the year preceding the latter is the year −1. The year which historians call 585 B.C. is actually the year −584.

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 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}}

Although he used the usual French terms "avant J.-C." (before Jesus Christ) and "après J.-C." (after Jesus Christ) to label years elsewhere in his book, the Byzantine historian Venance Grumel (1890–1967) used negative years (identified by a minus sign, −) to label BC years and unsigned positive years to label AD years in a table. He may have done so to save space and he put no year 0 between them.V. Grumel, La chronologie (Paris: Presses Universitaires de France, 1958) 30. {{in lang|fr}}

Version 1.0 of the XML Schema language, often used to describe data interchanged between computers in XML, includes built-in primitive datatypes date and dateTime. Although these are defined in terms of ISO 8601 which uses the proleptic Gregorian calendar and therefore should include a year 0, the XML Schema specification states that there is no year zero. Version 1.1 of the defining recommendation realigned the specification with ISO 8601 by including a year zero, despite the problems arising from the lack of backward compatibility.Biron, P.V. & Malhotra, A. (Eds.). (28 October 2004). [http://www.w3.org/TR/xmlschema-2/#dateTime XML Schema Part 2: Datatypes (2nd ed.).] World Wide Web Consortium.

• Julian day, another calendar commonly used by astronomers
• Astronomical chronology
• Holocene calendar
• ISO 8601

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Year numbering