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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.
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), Simon Newcomb
(1898) and Fred Espenak
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 −
(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, 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:
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.
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
- ^ a
Espenak, Fred. "Year Dating Conventions".
NASA Eclipse Web Site. NASA. http://eclipse.gsfc.nasa.gov/SEhelp/dates.html. Retrieved
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),
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
Fred Espenak, Phases of the Moon: −99 to 0
(100 to 1 BCE) NASA Eclipse web site
Jacques Cassini, Tables
Astronomiques, Explication et Usage 5, translated from
Biron, P.V. & Malhotra, A. (Eds.). (28 October 2004). XML Schema Part 2:
Datatypes (2nd ed.). World Wide Web Consortium.