Astronomical year numbering

Template:Short description Template:Use dmy dates Astronomical year numbering is based on AD/CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0; the years before that are designated with negative numbers and the years after that are designated with positive numbers.<ref name="nasa-eclipses">Template:Cite web</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, 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). Template:In lang</ref> Simon Newcomb (1898)<ref>Simon Newcomb, "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 VI: Tables of the Four Inner Planets, (United States Naval Observatory, 1898), pp. 27 & 34–35.</ref> and Fred Espenak (2007).<ref name=Espanak>Fred Espenak, Phases of the Moon: −99 to 0 (100 to 1 BCE) Template:Webarchive 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 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 disciplines outside history deal with the time before year 1, some exceptions being dendrochronology, archaeology and geology, the latter two of which use '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 conjunctions to determine when historical events which mention them occurred.

Usage of the year zero

Template:Main 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, 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 Template:Nowrap 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>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 Template:Nowrap, who is traditionally credited with the invention of year zero,<ref>Robert Kaplan, The nothing that is (Oxford: Oxford University Press, 2000) 103.</ref><ref>Dick Teresi, "Zero", The Atlantic, July 1997 (see under Calendars and the Cosmos).</ref><ref>L. E. Doggett, "Calendars" Template:Webarchive, 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, 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."</ref> Template:Blockquote

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.<ref name=Espanak/> Jean Meeus gives the following explanation:<ref>Jean Meeus, Astronomical Algorithms (Richmond, Virginia: Willmann-Bell, 1991) 60.</ref> Template:Blockquote

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. Template:In lang</ref>

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.<ref>Biron, P.V. & Malhotra, A. (Eds.). (28 October 2004). 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