Year zero is not used in the widely used
Gregorian calendarThe Gregorian calendar is the internationally accepted civil calendar. It was first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, after whom the calendar was named, on 24 February 1582 by the papal bull Inter gravissimas...
, nor in its predecessor, the
Julian calendarThe Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus...
. Under those systems, the year
1 BC1 BC was a leap year starting on Thursday of the Julian calendar. It was preceded by 2 BC and followed by AD 1 , since there was no year zero.-Religion:...
is followed by
AD 1Year 1 was a common year starting on Saturday of the Julian calendar. The preceding year is 1 BC in the widely used Gregorian calendar or in its predecessor, the Julian calendar, neither of which has a "year zero".-Roman Empire:...
. However, there is a year zero in
astronomical year numberingAstronomical 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...
(where it coincides with the Julian year 1 BC) and in
ISO 8601:2004ISO 8601 is an international standard covering the exchange of date and time-related data. The standard is titled “Data elements and interchange formats — Information interchange — Representation of dates and times” and was issued by the International Organization for Standardization...
(where it coincides with the Gregorian year 1 BC) as well as in all
BuddhistThe Buddhist calendar is used on mainland Southeast Asia in the countries of Cambodia, Laos, Thailand, Burma and Sri Lanka in several related forms. It is a lunisolar calendar having months that are alternately 29 and 30 days, with an intercalated day and a 30-day month added at regular intervals...
and
Hindu calendarThe Hindu calendar used in ancient times has undergone many changes in the process of regionalization, and today there are several regional Indian calendars, as well as an Indian national calendar....
s.
Numerical background
There are two different ways of reckoning time:
The first way of reckoning is the traditional one in
historiographyHistoriography is the history of history, the aspect of history and of semiotics that considers how knowledge of the past, either recent or distant, is obtained and transmitted...
and in common usage to label years, centuries, and millennia via a counting method. The second is used, for example, with a person's age which reckons time according to a measuring system.
Counting
One way is to use cardinal numbers (e.g. one, two, three, ...) or ordinal numbers (e.g. first, second, third, ...) This corresponds to treating time as a discrete variable, and the labels as
countsCounting is the mathematical action of repeatedly adding one, usually to find out how many objects there are or to set aside a desired number of objects , or for well-ordered objects, to find the ordinal number of a...
. Under this point of view, the first year counted after the starting point will come immediately after the first year counted before the starting point.
Measuring
In some contexts, however, such as
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
, it can be more convenient to regard time as a continuous variable, and label time periods as
intervalsWith regards to time, an interval or period is the duration between two events or occurrences of similar events. It is related to the mathematical concept of interval in that the interval contains all of the points of time between the two events....
on a continuous scale, that is, as measurements of the total time elapsed since the start of the
eraAn era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in...
. According to this interpretation, elapsed time year 1 begins exactly one full year
after the starting point, and the first year is year 0 (meaning that zero full years have elapsed since the starting point).
Third millennium
According to the normal historians' usage, the third millennium of the Gregorian calendar began on 1 January 2001, rather than the popularly-celebrated 1 January 2000. This is a direct consequence of the absence of a year zero in the
anno Domini era. Had there been a year zero, which might be considered part of the first millennium, then 1 January 2000 would indeed mark 2000 years since the year numbering datum and be the start of the third millennium.
Note that this also applies to centuries. Thus, the 20th century actually began on 1 January 1901; and the 21st century actually began on 1 January 2001.
Historians
Dionysius Exiguus (c.
470-Europe:*Euric, king of the Visigoths, defeats an attempted invasion of Gaul by the Celtic military leader Riothamus.-Births:*Buddhapalita, Indian scholar*Dionysius Exiguus, inventor of the Anno Domini era...
–c.
544-Byzantine Empire:* Belisarius is sent back to Italy to once more fight the Ostrogoths.* Pope Vigilius is ordered to Constantinople.* Khosrau I of Persia unsuccessfully attacks the Byzantine fortress of Dara.-Southeast Asia:...
) introduced the
anno Domini eraAn era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in...
, which he used to identify the several Easters in his
Easter tableDionysius Exiguus' Easter table was constructed in the year 525 by Dionysius Exiguus for the years 532–626. He obtained it from an Easter table attributed to Patriarch Cyril of Alexandria for the years 437–531. The latter was constructed around the year 440 by means of extrapolation from an...
, but did not use it to date any historical event. When he devised his table, Julian calendar years were identified by naming the
consul-Ancient Rome:During the time of ancient Rome as a Republic, the consuls were the highest civil and military magistrates, serving as the heads of government for the Republic. New consuls were elected every year. There were two consuls, and they ruled together...
s who held office that year — he stated that the "present year" was "the consulship of Probus Junior [Flavius Probus]", which he also stated was 525 years "since the incarnation of our Lord Jesus Christ". How he arrived at that number is unknown. He invented a new system of numbering years to replace the
DiocletianGaius Aurelius Valerius Diocletianus , born Diocles and commonly known as Diocletian , was Roman Emperor from 20 November 284 to 1 May 305. Born to a Dalmatian family of low status, he rose through the ranks of the military to become cavalry commander to the emperor Carus...
years that had been used in an old Easter table because he did not wish to continue the memory of a tyrant who persecuted Christians.
BedeBede , also Saint Bede, the Venerable Bede, or Beda , was a monk at the Northumbrian monastery of Saint Peter at Monkwearmouth, today part of Sunderland, England, and of its companion monastery, Saint Paul's, in modern Jarrow , both in the Kingdom of Northumbria.He is well known as an author and...
(c.672–735) was the first historian to use a BC year, and hence the first one to choose 1 as the origin of the BC era, thus 1 BC, in his
Historia ecclesiastica gentis AnglorumThe Historia ecclesiastica gentis Anglorum is a work in Latin by Bede on the history of the Church in England, and of England generally; its main focus is on the conflict between Roman and Celtic Christianity.It is considered to be one of the most important original references on Anglo-Saxon history...
(
Ecclesiastical history of the English people, 731). Bede did not sequentially number days of the month, weeks of the year, or months of the year, but he did number many of the days of the week using a counting origin of one in
Ecclesiastical LatinEcclesiastical Latin is the Latin used by the Latin Rite of the Catholic Church in all periods for ecclesiastical purposes...
. Previous Christian histories used
anno mundi' abbreviated as AM or A.M., refers to a Calendar era counting from the Biblical creation of the world.-Jewish computation:Years in the Hebrew calendar are counted from the Creation year...
("in the year of the world") beginning on the first day of
CreationCultures throughout history have believed the world formed or was formed at some time in the past, so methods of dating Creation have involved analysing scriptures.-Creation dates:...
, or
anno Adami ("in the year of Adam") beginning at the creation of Adam five days later (the sixth day of Creation week), used by Africanus, or
anno Abrahami ("in the year of
AbrahamAbraham is the founding patriarch of the Israelites, Ishmaelites, Midianites and Edomite peoples, as described in the book of Genesis. He is widely regarded as the patriarch of Jews, Christians, and Muslims....
") beginning 3,412 years after Creation according to the
SeptuagintThe Septuagint , or simply "LXX", referred to in critical works by the abbreviation ...
, used by Eusebius, all of which assigned "one" to the year beginning at Creation, or the creation of Adam, or the birth of Abraham, respectively. Bede continued this earlier tradition relative to the AD era.
In chapter II of book I of
Ecclesiastical history, Bede stated that
Julius CaesarGaius Julius Caesar , , was a Roman military and political leader. He played a critical role in the transformation of the Roman Republic into the Roman Empire....
invaded Britain "in the year 693 after the building of
RomeRome is the capital of Italy and the country's largest and most populated municipality , with over 2.7 million residents in , while the population of the urban area is estimated by Eurostat to be 3.46 million. The metropolitan area of Rome is estimated by OECD to have a population of 3.7 million...
, but the sixtieth year before the incarnation of our Lord", while stating in chapter III, "in the year of Rome 798,
ClaudiusTiberius Claudius Caesar Augustus Germanicus was the fourth Roman Emperor, a member of the Julio-Claudian dynasty, ruling from 24 January AD 41 to his death in AD 54...
" also invaded Britain and "within a very few days … concluded the war in … the fortysixth [year] from the incarnation of our Lord". Although both dates are wrong, they are sufficient to conclude that Bede did not include a year zero between BC and AD: 798 − 693 + 1 (because the years are inclusive) = 106, but 60 + 46 = 106, which leaves no room for a year zero. The modern English term "before Christ" (BC) is only a rough equivalent,
not a direct translation, of Bede's Latin phrase
ante incarnationis dominicae tempus ("before the time of the lord's incarnation"), which was itself never abbreviated. Bede's singular use of 'BC' continued to be used sporadically throughout the
Middle AgesThe Middle Ages of European history is a period of European history covering roughly a millennium in the 5th century through 16th centuries. More specific starting and ending points are sometimes adopted by scholars to suit their respective specializations or current focus...
(albeit with a correct year).
It is often stated that Bede did not use a year zero because he did not know about the number zero. Although the
Arabic numeralThe Arabic numerals are the ten digits . They are descended from Indian numerals and the Hindu-Arabic numeral system developed by Indian mathematicians, by which a sequence of digits such as "975" is read as a whole number...
for zero (
00 is both a number and the numerical digit used to represent that number in numerals. It plays a central role in mathematics as the additive identity of the integers, real numbers, and many other algebraic structures. As a digit, zero is used as a placeholder in place value systems...
) did not enter Europe until the eleventh century, and
Roman numeralsRoman numerals are a numeral system of ancient Rome based on letters of the alphabet, which are combined to signify the sum of their values. The first ten Roman numerals are:...
had no symbol for zero, Bede and Dionysius Exiguus did use a
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
word,
nulla meaning "nothing", alongside Roman numerals or Latin number words wherever a modern zero would have been used.
The first extensive use (hundreds of times) of 'BC' occurred in
Fasciculus Temporum by
Werner RolevinckWerner Rolevinck was a Carthusian monk and historian who wrote about 50 titles. He was born near Laer, Westphalia, the son of a wealthy farmer. In 1447 he entered the Carthusian Monastery of Santa Barbara in Cologne where he died...
in 1474, alongside years of the world (
anno mundi). The
anno Domini nomenclature was not widely used in
Western EuropeWestern Europe is the collection of countries in the westernmost region of Europe, though this definition is context-dependent and carries cultural and political connotations. One definition describes Western Europe as a cultural entity—the region lying west of Central Europe...
until the 9th century, and the 1 January to 31 December historical year was not uniform throughout Western Europe until 1752. The terms
anno Domini,
Dionysian era,
Christian era,
vulgar era, and
common era were used interchangeably between the
RenaissanceThe Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Florence in the Late Middle Ages and later spreading to the rest of Europe...
and the 19th century, at least in
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
. But
vulgar era was suppressed in
EnglishEnglish is a West Germanic language that developed in England during the Anglo-Saxon era. As a result of the military, economic, scientific, political, and cultural influence of the British Empire during the 18th, 19th, and early 20th centuries, and of the United States since the mid 20th century,...
at the beginning of the 20th century after
vulgar acquired the meaning of "offensively coarse", replacing its original meaning of "common" or "ordinary". Consequently, historians regard all these eras as equal.
Since Bede, historians have not counted with a year zero. This means that between, for example, January 1, 500 BC and January 1,
AD 500Year 500 was a leap year starting on Saturday of the Julian calendar.-Europe:* Possible date for the Battle of Mons Badonicus: Romano-British and Celts defeat an Anglo-Saxon army that may have been led by the bretwalda Aelle of Sussex or possibly Cerdic of Wessex...
, there are 999 years: 500 for BC years, and 499 for AD years preceding 500. In common usage
anno Domini 1 is preceded by the year 1
BCAnte Christum Natum , usually abbreviated to A.C.N., a.C.n., a.Ch.n. or ACN, denotes the years before the birth of Jesus Christ. It is the modern Latin equivalent to the English term "BC" and "BCE"...
, without an intervening year zero. Thus the year 2006 actually signifies "the 2006th year". Neither the choice of calendar system (whether
JulianThe Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus...
or
GregorianThe Gregorian calendar is the internationally accepted civil calendar. It was first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, after whom the calendar was named, on 24 February 1582 by the papal bull Inter gravissimas...
) nor the era (
Anno Domini or
Common EraCommon Era, abbreviated as CE, is a designation for the calendar system most commonly used world-wide for numbering the year part of the date...
) determines whether a year zero will be used. If writers do not use the convention of their group (historians or astronomers), they must explicitly state whether they include a year 0 in their count of years, otherwise their historical dates will be misunderstood. No historian includes a year 0 when numbering years in the current standard era. Historians even refuse to use a year 0 when using negative years before our positive era, hence their −1 immediately precedes 1.
Astronomers
To simplify calculations,
astronomerAn astronomer is a scientist who studies celestial bodies such as planets, stars, and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s have used a defined leap year zero equal to of the traditional Christian era since the Modern astronomers do not use years for intervals because years do not distinguish between common years and leap years, causing the resulting interval to be inaccurate.
In astronomy, the numbering of all years labeled
Anno Domini remain unchanged. However, the numerical value of years labeled
Before Christ are reduced by one by the insertion of a year 0 before . Thus, astronomical BC years and historical BC years are not equivalent. To avoid this confusion, modern astronomers label years as positive or negative, instead of BC or AD.
The current method was created by
Jacques CassiniJacques Cassini was a French-Italian astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini....
, who explained:
In this quote, Cassini used "year" as both a calendar year and as an instant before a year. He identified the calendar year 0 as the year during which Jesus Christ was born (on the traditional date of , and as calendar leap years divisible by 4 (having an extra day in February). But "the sum of years before and after Jesus Christ" referred to the years between a number of instants at the beginning of those years, including the beginning of year 0, identified by Cassini as "Jesus Christ", virtually identical to Kepler's "Christi". Consider the three instants ('years') labeled by Cassini, which modern astronomers would label . Cassini specified that his end years must be added, so the interval between the instants (noon ) and is , but modern astronomers would subtract their 'years', , which agrees with Cassini. The calendar years between these two instants would be and , leaving the calendar year beginning at +1.0 outside the interval.
Astronomical notation
Astronomical years can be used to identify a calendar year (when placed alongside a month and a day) or to identify a certain instant (known in astronomy as an
epochIn astronomy, an epoch is a moment in time used as a reference for the orbital elements of a celestial body. Typically, the epoch is either the moment an observation was made or the moment for which a prediction was calculated.-Epoch versus equinox:...
). Modern astronomers identify an instant with a small number of fractional decimal digits after the year, unless higher precision is necessary: 2000.0 is understood as noon , 1992.5 is , and 1996.25 is an instant one-quarter of a year after the beginning of 1996. Near year 0, −1.0 is noon , 0.0 is noon , 1.0 is noon , and 2.0 = noon .
During the astronomers began to change from named eras to numerical signs, with some astronomers using BC/0/AD years while others used years. By the mid all astronomers were using years. Numerical signs effectively form a new era, reducing the confusion inherent in any date which uses an astronomical year with an era named Before Christ.
Before 1925, all astronomical years began at noon at the
meridianA meridian is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations running along it with a given longitude. The position of a point on the meridian is given by the latitude. Each meridian is perpendicular to all circles of latitude at the...
of some
observatoryAn observatory is a location used for observing terrestrial and/or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed...
—Kepler used the meridian of
UraniborgUranienborg was an astronomical/astrological observatory operated by Tycho Brahe; built circa 1576-1580 on Hven , an island in the Øresund between Zealand and Scania, at that time belonging to Denmark.-History:...
(
Tycho BraheTycho Brahe, born Tyge Ottesen Brahe , was a Danish nobleman known for his accurate and comprehensive astronomical and planetary observations...
's observatory), La Hire and Cassini used the meridian of the
Paris ObservatoryThe Paris Observatory is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world.-Constitution:...
, whereas modern astronomers use the meridian of the
Royal Observatory, GreenwichThe Royal Observatory, Greenwich was commissioned in 1675 by King Charles II, with the foundation stone being laid on 10 August...
.
History of astronomical usage
In 1849 the English astronomer
John HerschelSir John Frederick William Herschel, 1st Baronet KH, FRS was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work. He was the son of astronomer Sir William Herschel and the father of 12 children.Herschel originated...
invented Julian dates, which are a sequence of numbered days and fractions thereof since noon , which was Julian date 0.0. Julian dates count the days between two instants, automatically accounting for years with different lengths, while allowing for any arbitrary precision by including as many fractional decimal digits as necessary. The modern mathematical astronomer
Jean MeeusJean Meeus is a Belgian astronomer specializing in celestial mechanics. The asteroid 2213 Meeus is named after him.Jean Meeus studied mathematics at the University of Leuven in Belgium, where he received the Degree of Licentiate in 1953...
no longer mentions determining intervals via years, stating:
In 1627 the German astronomer
Johannes KeplerJohannes Kepler was a German mathematician, astronomer and astrologer, and key figure in the 17th century scientific revolution. He is best known for his eponymous laws of planetary motion, codified by later astronomers based on his works Astronomia nova, Harmonices Mundi, and Epitome of...
first used an astronomical year which was to become year zero in his
Rudolphine TablesThe Rudolphine Tables consist of a star catalogue and planetary tables published by Johannes Kepler in 1627 using data from Tycho Brahe's observations.-Previous tables:...
. He labeled the year
Christi and inserted it between years labeled
Ante Christum (BC) and
Post Christum (AD) on the mean motion pages of the Sun, Moon, and planets. Then in 1702 the French astronomer
Philippe de la HirePhilippe de La Hire , was a French mathematician and astronomer. According to Bernard le Bovier de Fontenelle he was an "academy unto himself"....
used a year he labeled at the end of years labeled
ante Christum (BC), 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 CassiniJacques Cassini was a French-Italian astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini....
, 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 years labeled
avant Jesus-Christ (BC), immediately before years labeled
après Jesus-Christ (AD).