Astronomical year numbering is based onAD/CE year numbering, but follows normaldecimalinteger numbering more strictly. Thus, it has ayear 0; the years before that are designated withnegative numbers and the years after that are designated with positive numbers.[1] Astronomers use theJulian calendar for years before 1582, including the year 0, and theGregorian calendar for years after 1582, as exemplified byJacques Cassini (1740),[2]Simon Newcomb (1898)[3] andFred Espenak (2007).[4]
The prefix AD and the suffixes CE, BC or BCE (Common Era, Before Christ or Before Common Era) are dropped.[1] The year 1 BC/BCE is numbered 0, the year 2 BC is numbered −1, and in general the yearn BC/BCE is numbered "−(n − 1)"[1] (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 generaln AD/CE is simplyn or +n.[1] For normal calculation anumber zero is often needed, here most notably when calculating the number of years in a period that spans theepoch; the end years need only be subtracted from each other.
The system is so named due to its use inastronomy. Few other disciplines outsidehistory deal with the time before year 1, some exceptions beingdendrochronology,archaeology andgeology, 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 likeeclipses orplanetary conjunctions to determine when historical events which mention them occurred.
In hisRudolphine Tables (1627),Johannes Kepler used a prototype of year zero which he labeledChristi (Christ's) between years labeledAnte Christum (Before Christ) andPost Christum (After Christ) on the mean motion tables for the Sun, Moon, Saturn, Jupiter, Mars, Venus and Mercury.[5] In 1702, the French astronomerPhilippe de la Hire used a year he labeledChristum 0 at the end of years labeledante Christum (BC), and immediately before years labeledpost Christum (AD) on the mean motion pages in hisTabulæ Astronomicæ, thus adding the designation0 to Kepler'sChristi.[6] Finally, in 1740 the French astronomerJacques Cassini(Cassini II), who is traditionally credited with the invention of year zero,[7][8][9] completed the transition in hisTables astronomiques, simply labeling this year0, which he placed at the end of Julian years labeledavant Jesus-Christ (before Jesus Christ or BC), and immediately before Julian years labeledaprès Jesus-Christ (after Jesus Christ or AD).[2]
Cassini gave the following reasons for using a year 0:[10]
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 ofNASA lists 50phases of the Moon within year 0, showing that it is a full year, not an instant in time.[4] Jean Meeus gives the following explanation:[11]
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 historianVenance 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.[12]
Version 1.0 of theXML Schema language, often used to describe data interchanged between computers inXML, includes built-in primitive datatypesdate anddateTime. Although these are defined in terms ofISO 8601 which uses theproleptic 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 definingrecommendation realigned the specification with ISO 8601 by including a year zero, despite the problems arising from the lack ofbackward compatibility.[13]
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.
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