Editing Astronomical year numbering

{{Short description|Year numbering system using 0 for 1 BC}}
{{Use dmy dates|date=September 2019}}
'''Astronomical year numbering''' is based on [[Anno Domini|AD]]/[[Common Era|CE]] year numbering, but follows normal [[decimal]] [[integer]] numbering more strictly. Thus, it has a [[0 (year)|year 0]]; the years before that are designated with [[negative number]]s and the years after that are designated with positive numbers.<ref name="nasa-eclipses">{{cite web|url=http://eclipse.gsfc.nasa.gov/SEhelp/dates.html|title=Year Dating Conventions|last=Espenak|first=Fred|work=NASA Eclipse Web Site|publisher=NASA|access-date=19 February 2009| archive-url= https://web.archive.org/web/20090208212742/http://eclipse.gsfc.nasa.gov/SEhelp/dates.html| archive-date= 8 February 2009 | url-status= live}}</ref> Astronomers use the [[Julian calendar]] for years before 1582, including the year 0, and the [[Gregorian calendar]] for years after 1582, as exemplified by [[Jacques Cassini]] (1740),<ref name=Cassini>Jacques Cassini, ''[https://books.google.com/books?id=fVIVAAAAQAAJ&pg=RA1-PA10 Tables Astronomiques]'' (1740), Explication et Usage pp. 5 (PA5), 7 (PA7), Tables pp. 10 (RA1-PA10), 22 (RA1-PA22), 63 (RA1-PA63), 77 (RA1-PA77), 91 (RA1-PA91), 105 (RA1-PA105), 119 (RA1-PA119). {{in lang|fr}}</ref> [[Simon Newcomb]] (1898)<ref>Simon Newcomb, [https://archive.org/details/06AstronomicalPapersPreparedForTheUse "Tables of the Motion of the Earth on its Axis and Around the Sun"] in ''Astronomical Papers Prepared for the Use of the American Ephemeris and Nautical Almanac, Volume&nbsp;VI: Tables of the Four Inner Planets'', (United States Naval Observatory, 1898), pp.&nbsp;27 &amp; 34–35.</ref> and [[Fred Espenak]] (2007).<ref name=Espanak>Fred Espenak, [http://eclipse.gsfc.nasa.gov/phase/phases-0099.html Phases of the Moon: −99 to 0 (100 to 1 BCE)] {{webarchive |url=https://web.archive.org/web/20090605101653/http://eclipse.gsfc.nasa.gov/phase/phases-0099.html |date=5 June 2009 }} ''NASA Eclipse web site''</ref>

The prefix AD and the suffixes CE, BC or BCE (Common Era, Before Christ or Before Common Era) are dropped.<ref name="nasa-eclipses" /> 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 "−(''n'' − 1)"<ref name="nasa-eclipses" /> (a negative number equal to 1 − ''n''). The numbers of AD/CE years are not changed and are written with either no sign or a positive sign; thus in general ''n'' AD/CE is simply ''n'' or +''n''.<ref name="nasa-eclipses" /> 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, some exceptions being [[dendrochronology]], [[archaeology]] and [[geology]], the latter two of which use '[[Before Present|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 conjunction]]s to determine when historical events which mention them occurred.

==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.<ref>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)</ref> 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''.<ref>[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)</ref> Finally, in 1740 the French astronomer [[Jacques Cassini]] {{nowrap|(Cassini II)}}, who is traditionally credited with the invention of year zero,<ref>Robert Kaplan, [https://books.google.com/books?id=Bn0EBVsfi1YC&pg=PA103 ''The nothing that is''] (Oxford: Oxford University Press, 2000) 103.</ref><ref>Dick Teresi, [https://www.theatlantic.com/past/docs/issues/97jul/zero.htm "Zero"], ''The Atlantic'', July 1997 (see under Calendars and the Cosmos).</ref><ref>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.</ref> 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).<ref name=Cassini/>

Cassini gave the following reasons for using a year 0:<ref>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:<br> "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."</ref> 
{{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 [[Lunar phase|phases of the Moon]] within year 0, showing that it is a full year, not an instant in time.<ref name=Espanak/> Jean Meeus gives the following explanation:<ref>Jean Meeus, ''Astronomical Algorithms'' (Richmond, Virginia: Willmann-Bell, 1991) 60.</ref>
{{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''}}

==Signed years without the year zero==
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.<ref>V. Grumel, ''La chronologie'' (Paris: Presses Universitaires de France, 1958) 30. {{in lang|fr}}</ref>

Version 1.0 of the [[XML Schema (W3C)|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 [[W3C recommendation|recommendation]] realigned the specification with ISO 8601 by including a year zero, despite the problems arising from the lack of [[backward compatibility]].<ref>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.</ref>

==See also==
* [[Julian day]], another calendar commonly used by astronomers
* [[Astronomical chronology]]
* [[Holocene calendar]]
* [[ISO 8601]]

==References==
{{Reflist|30em}}

{{Calendars}}
{{Chronology}}
{{Time measurement and standards}}

[[Category:Calendar eras]]
[[Category:Chronology]]
[[Category:Specific calendars]]
[[Category:Time in astronomy|Year numbering]]