Astronomical year numbering is based on
AD/
CE year numbering, but follows normal
decimal
The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral ...
integer
An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign (−1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
numbering more strictly. Thus, it has a
year 0
A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (nor in its predecessor, the Julian calendar); in this system, the year is followed directly by year . However, the ...
; the years before that are designated with
negative number
In mathematics, a negative number represents an opposite. In the real number system, a negative number is a number that is less than zero. Negative numbers are often used to represent the magnitude of a loss or deficiency. A debt that is owed m ...
s and the years after that are designated with positive numbers.
Astronomers use the
Julian calendar
The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandr ...
for years before 1582, including the year 0, and the
Gregorian calendar
The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years dif ...
for years after 1582, as exemplified by
Jacques Cassini
Jacques Cassini (18 February 1677 – 16 April 1756) was a French astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini.
Cassini was born at the Paris Observatory. Admitted at the age of seventeen to membership of the French ...
(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
Simon Newcomb (March 12, 1835 – July 11, 1909) was a Canadian–American astronomer, applied mathematician, and autodidactic polymath. He served as Professor of Mathematics in the United States Navy and at Johns Hopkins University. Born in Nov ...
(1898) and
Fred Espenak
Fred Espenak is a retired emeritus American astrophysicist. He worked at the Goddard Space Flight Center. He is best known for his work on eclipse predictions.
He became interested in astronomy when he was 7–8 years old, and had his first tel ...
(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
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by ...
; the end years need only be subtracted from each other.
The system is so named due to its use in
astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
. Few other disciplines outside
history
History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the History of writing#Inventions of writing, invention of writing systems is considered prehistory. "History" is an umbr ...
deal with the time before year 1, some exceptions being
dendrochronology
Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...
,
archaeology
Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscap ...
and
geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
, 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
An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...
or
planetary conjunction
In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth.
When two objects always appear close to the ecliptic—such as two ...
s to determine when historical events which mention them occurred.
Usage of the year zero
In his
Rudolphine Tables
The ''Rudolphine Tables'' ( la, Tabulae Rudolphinae) consist of a star catalogue and planetary tables published by Johannes Kepler in 1627, using observational data collected by Tycho Brahe (1546–1601). The tables are named in memory of Rudolf ...
(1627),
Johannes Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
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. 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''. Finally, in 1740 the French astronomer
Jacques Cassini
Jacques Cassini (18 February 1677 – 16 April 1756) was a French astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini.
Cassini was born at the Paris Observatory. Admitted at the age of seventeen to membership of the French ...
, who is traditionally credited with the invention of year zero, 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:
Fred Espenak of ]NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
lists 50 phases of the Moon
Concerning the lunar month of ~29.53 days as viewed from Earth, the lunar phase or Moon phase is the shape of the Moon's directly sunlit portion, which can be expressed quantitatively using areas or angles, or described qualitatively using the t ...
within year 0, showing that it is a full year, not an instant in time.[ Jean Meeus gives the following explanation:
]
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
Venance Grumel (born François Grumel on 23 May 1890, La Serraz, Le Bourget-du-Lac, Savoy, France – died 13 August 1967, Paris) was a French theologian and Byzantinist.
Biography
He was born on 23 May 1890 under the name of François Grume ...
(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.
Version 1.0 of the XML Schema language, often used to describe data interchanged between computers in XML
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...
, includes built-in primitive datatypes date and dateTime. Although these are defined in terms of ISO 8601
ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, wi ...
which uses the proleptic Gregorian calendar
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to the dates preceding its official introduction in 1582. In nations that adopted the Gregorian calendar after its official and first introduction, dates occ ...
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
Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially i ...
.[Biron, P.V. & Malhotra, A. (Eds.). (28 October 2004)]
''XML Schema Part 2: Datatypes'' (2nd ed.).
World Wide Web Consortium.
See also
* Julian day
The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g. food production date and sell by date).
...
, another calendar commonly used by astronomers
* Astronomical chronology
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
* Holocene calendar
The Holocene calendar, also known as the Holocene Era or Human Era (HE), is a year numbering system that adds exactly 10,000 years to the currently dominant ( AD/ BC or CE/BCE) numbering scheme, placing its first year near the beginning of the ...
* ISO 8601
ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, wi ...
References
{{Time measurement and standards
Calendar eras
Chronology
Specific calendars
Year numbering