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Astronomical year numbering is based on AD (Anno Domini)/CE (Common Era) 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.[1] 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),[2] Simon Newcomb (1898)[3] and Fred 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 year n 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 general n AD/CE is simply n or +n.[1] 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, 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.

A zero year was first used by the eighteenth century French astronomers Philippe de La Hire (1702) and Jacques Cassini (1740). However, both of these astronomers used the applicable AD/BC designations of Latin and French with their year zero, thus near the epoch the years were designated 2 BC, 1 BC, 0, 1 AD, 2 AD, etc. They did not use −/0/+. During the nineteenth century, astronomers designated years with either BC/0/AD or −/0/+. Astronomers did not exclusively use the −/0/+ system until the mid twentieth century.

Cassini gave the following reasons for using a year 0:[5]

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.

Superficially similar system

The XML Schema language, sometimes used in connection with representing data for storage in computers, contains built-in primitive datatypes, date and dateTime, which do not allow a year zero, and designate years BC as negative numbers. Years contain at least four digits. Thus -0001 in that language is equivalent to 1 BC. However, the defining recommendation indicates a change to a system similar to ISO 8601 and astronomical year numbering is likely in the future.[6]

See also


  1. ^ a b c d Espenak, Fred. "Year Dating Conventions". NASA Eclipse Web Site. NASA. Retrieved 2009-02-19.  
  2. ^ Jacques Cassini, Tables Astronomiques du Soleil, de la Lune, des Planetes (1740), Explication et Usage pp. 5 (PT30), 7 (PT32), Tables pp. 10 (PT165), 22 (PT177), etc. (French)
  3. ^ 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).
  4. ^ Fred Espenak, Phases of the Moon: −99 to 0 (100 to 1 BCE) NASA Eclipse web site
  5. ^ Jacques Cassini, Tables Astronomiques, Explication et Usage 5, translated from French.
  6. ^ Biron, P.V. & Malhotra, A. (Eds.). (28 October 2004). XML Schema Part 2: Datatypes (2nd ed.). World Wide Web Consortium.


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