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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. Astronomers use the Julian calendar for years before 1582, including this year 0, and the Gregorian calendar for years after 1582 as exemplified by Jacques Cassini (1740),〔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). 〕 Simon Newcomb (1898)〔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 (PT36), 34–35 (PT43–PT44).〕 and Fred Espenak (2007).〔Fred Espenak, (Phases of the Moon: −99 to 0 (100 to 1 BCE) ) ''NASA Eclipse web site''〕 The prefix AD and the suffixes CE, BC or BCE (Common Era, Before Christ or Before Common Era) are dropped.〔 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)"〔 (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''.〔 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. ==Year zero usage== (詳細は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, ''(Tabulae Rudolphinae )'' (1627) Pars secunda, 42 (Zu Seite 191), 48 (197), 54 (203), 60 (209), 66 (215), 72 (221), 78 (227). (Latin)〕 Then in 1702 the French astronomer Philippe de la Hire used a year he labeled 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''.〔(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 , who is traditionally credited with the invention of year zero,〔Robert Kaplan, (''The nothing that is'' ) (Oxford: Oxford University Press, 2000) 103.〕〔Dick Teresi, ("Zero" ), ''The Atlantic'', July 1997 (see under Calendars and the Cosmos).〕〔L. E. Doggett, ("Calendars" ), ''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, ''(Tables Astronomiques )'', Explication et Usage 5, translated by Wikipedia from the French: "L'année 0 est celle dans laquelle on suppose qu'est né J. C. 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 disibles par 4 marquent les années bissextiles tant avant qu'après J. C."〕 Fred Espanak 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'' (Richmod, Virginia: Willmann-Bell, 1991) 60.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Astronomical year numbering」の詳細全文を読む スポンサード リンク
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