Year zero
Encyclopedia
"Year zero" does not exist in the widely used Gregorian calendar
or in its predecessor, the Julian calendar
. Under those systems, the year 1 BC
is followed by AD 1
. However, there is a year zero in astronomical year numbering
(where it coincides with the Julian year 1 BC) and in ISO 8601:2004
(where it coincides with the Gregorian year 1 BC) as well as in all Buddhist
and Hindu calendar
s.
This rule arose because the Gregorian calendar begins with a year 1 instead of 0. Cardinal and ordinal numbering of years is therefore identical: The year 10 is the tenth year of the calendar and the end of the first decade. The year 11 is the first year of the second decade, and so on. In spite of this rule, years ending in 0, rather than 1, are commonly perceived as marking the beginning of a new decade, century, or millennium. Decades, however, are more used as a collective term (e.g. the 1910s) rather than a periodical term (e.g. 1910-1919).
era
was introduced in 525 by Dionysius Exiguus
(c.470–c.544), who used it to identify the years on his Easter table
. He introduced the new era to avoid using the Diocletian era, based on the accession of Emperor Diocletian
, as he did not wish to continue the memory of a persecutor of Christians. In the preface to his Easter table, Dionysius stated that the "present year" was "the consulship of Probus Junior [Flavius Probus]" which was also 525 years "since the incarnation of our Lord Jesus Christ". How he arrived at that number is unknown.
Dionysius did not use AD years to date any historical event. This began with the English cleric Bede
(c. 672–735), who used AD years in his Historia ecclesiastica gentis Anglorum
(731), popularizing the era. Bede also used a term similar to the English before Christ
once, but that practice did not catch on until very much later. Bede did not sequentially number days of the month, weeks of the year, or months of the year, however, he did number many of the days of the week using a counting origin of one in Ecclesiastical Latin
. Previous Christian histories used anno mundi
("in the year of the world") beginning on the first day of creation, or anno Adami ("in the year of Adam") beginning at the creation of Adam five days later (the sixth day of creation according to the Genesis creation narrative), used by Africanus, or anno Abrahami ("in the year of Abraham
") beginning 3,412 years after Creation according to the Septuagint, used by Eusebius of Caesarea
, 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 Caesar
invaded Britain "in the year 693 after the building of Rome
, but the sixtieth year before the incarnation of our Lord", while stating in chapter III, "in the year of Rome 798, Claudius
" 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 Ages
.
It is often incorrectly stated that Bede did not use a year zero because he did not know about the number zero. Although the Arabic numeral
for zero (0
) did not enter Europe until the eleventh century, and Roman numerals
had no symbol for zero, Bede and Dionysius Exiguus did use a Latin
word, nulla meaning "nothing", alongside Roman numerals or Latin number words wherever a modern zero would have been used.
The anno Domini nomenclature was not widely used in Western Europe
until the 9th century, and the 1 January to 31 December historical year was not uniform throughout Western Europe until 1752. The first extensive use (hundreds of times) of 'BC' occurred in Fasciculus Temporum by Werner Rolevinck
in 1474, alongside years of the world (anno mundi). The terms anno Domini, Dionysian era, Christian era, vulgar era, and common era were used interchangeably between the Renaissance
and the 19th century, at least in Latin
. But vulgar era was suppressed in English
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.
Historians have never included a year zero. This means that between, for example, January 1, 500 BC and January 1, AD 500, there are 999 years: 500 years BC, and 499 years AD preceding 500. In common usage anno Domini 1 is preceded by the year 1 BC
, without an intervening year zero. Thus the year 2006 actually signifies "the 2006th year". Neither the choice of calendar system (whether Julian
or Gregorian
) nor the era (Anno Domini or Common Era
) 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.
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 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.
). To identify an instant, astronomers add a number of fractional decimal digits to the year number, as required for the desired precision: thus J2000.0 designates noon , and 1992.5 is exactly 7.5 years of 365.25 days earlier, which is the instant (03 hours in the morning of July 2). Similarly, J1996.25 is 3.75 Julian years
before J2000.0, which is the instant 1996 July 2.8125 (19.5 hours in the evening), one-quarter of a year after the instant J1996.0 = . In this notation, J0000.0 is noon of (Julian), and J0001.0 is 6 pm on (Julian). This astronomical notation is called Julian epoch and was only introduced in 1976; before that time, astronomical year numbers with decimal fractions referred to Besselian years and were written without letter prefix.
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.
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 Meeus
no longer mentions determining intervals via years, stating:
In 1627 the German astronomer Johannes Kepler
first used an astronomical year which was to become year zero in his Rudolphine 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 Hire
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 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).
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
or in its predecessor, the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...
. Under those systems, the year 1 BC
1 BC
Year 1 BC was a common year starting on Friday or Saturday of the Julian calendar and a leap year starting on Thursday of the Proleptic Julian calendar...
is followed by AD 1
1
Year 1 was a common year starting on Saturday or Sunday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian calendar...
. However, there is a year zero in astronomical year numbering
Astronomical year numbering
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...
(where it coincides with the Julian year 1 BC) and in ISO 8601:2004
ISO 8601
ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization and was first published in 1988...
(where it coincides with the Gregorian year 1 BC) as well as in all Buddhist
Buddhist calendar
The Buddhist calendar is used on mainland Southeast Asia in the countries of Thailand, Cambodia, Laos, 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 calendar
Hindu calendar
The 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. Nepali calendar, Bengali calendar, Malayalam calendar, Tamil calendar, Telugu calendar, Kannada...
s.
Counting intervals without a zero
The absence of a year 0 leads to some confusion concerning the boundaries of longer decimal intervals, such as decades and centuries. For example, the third millennium of the Gregorian calendar began on Monday, 1 January, 2001, rather than the widely celebrated Saturday, 1 January, 2000. Likewise, the 20th century began on 1 January 1901.This rule arose because the Gregorian calendar begins with a year 1 instead of 0. Cardinal and ordinal numbering of years is therefore identical: The year 10 is the tenth year of the calendar and the end of the first decade. The year 11 is the first year of the second decade, and so on. In spite of this rule, years ending in 0, rather than 1, are commonly perceived as marking the beginning of a new decade, century, or millennium. Decades, however, are more used as a collective term (e.g. the 1910s) rather than a periodical term (e.g. 1910-1919).
Historians
The Anno DominiAnno Domini
and Before Christ are designations used to label or number years used with the Julian and Gregorian calendars....
era
Era
An 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...
was introduced in 525 by Dionysius Exiguus
Dionysius Exiguus
Dionysius Exiguus was a 6th-century monk born in Scythia Minor, modern Dobruja shared by Romania and Bulgaria. He was a member of the Scythian monks community concentrated in Tomis, the major city of Scythia Minor...
(c.470–c.544), who used it to identify the years on his Easter table
Dionysius Exiguus' Easter table
Dionysius Exiguus's 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...
. He introduced the new era to avoid using the Diocletian era, based on the accession of Emperor Diocletian
Diocletian
Diocletian |latinized]] upon his accession to Diocletian . c. 22 December 244 – 3 December 311), was a Roman Emperor from 284 to 305....
, as he did not wish to continue the memory of a persecutor of Christians. In the preface to his Easter table, Dionysius stated that the "present year" was "the consulship of Probus Junior [Flavius Probus]" which was also 525 years "since the incarnation of our Lord Jesus Christ". How he arrived at that number is unknown.
Dionysius did not use AD years to date any historical event. This began with the English cleric Bede
Bede
Bede , also referred to as Saint Bede or the Venerable Bede , 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...
(c. 672–735), who used AD years in his Historia ecclesiastica gentis Anglorum
Historia ecclesiastica gentis Anglorum
The Historia ecclesiastica gentis Anglorum is a work in Latin by Bede on the history of the Christian Churches 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...
(731), popularizing the era. Bede also used a term similar to the English before Christ
Anno Domini
and Before Christ are designations used to label or number years used with the Julian and Gregorian calendars....
once, but that practice did not catch on until very much later. Bede did not sequentially number days of the month, weeks of the year, or months of the year, however, he did number many of the days of the week using a counting origin of one in Ecclesiastical Latin
Ecclesiastical Latin
Ecclesiastical 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
Anno Mundi
' , abbreviated as AM or A.M., refers to a Calendar era based on the Biblical creation of the world. Numerous efforts have been made to determine the Biblical date of Creation, yielding varying results. Besides differences in interpretation, which version of the Bible is being referenced also...
("in the year of the world") beginning on the first day of creation, or anno Adami ("in the year of Adam") beginning at the creation of Adam five days later (the sixth day of creation according to the Genesis creation narrative), used by Africanus, or anno Abrahami ("in the year of Abraham
Abraham
Abraham , whose birth name was Abram, is the eponym of the Abrahamic religions, among which are Judaism, Christianity and Islam...
") beginning 3,412 years after Creation according to the Septuagint, used by Eusebius of Caesarea
Eusebius of Caesarea
Eusebius of Caesarea also called Eusebius Pamphili, was a Roman historian, exegete and Christian polemicist. He became the Bishop of Caesarea in Palestine about the year 314. Together with Pamphilus, he was a scholar of the Biblical canon...
, 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 Caesar
Julius Caesar
Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....
invaded Britain "in the year 693 after the building of Rome
Rome
Rome is the capital of Italy and the country's largest and most populated city and comune, with over 2.7 million residents in . The city is located in the central-western portion of the Italian Peninsula, on the Tiber River within the Lazio region of Italy.Rome's history spans two and a half...
, but the sixtieth year before the incarnation of our Lord", while stating in chapter III, "in the year of Rome 798, Claudius
Claudius
Claudius , was Roman Emperor from 41 to 54. A member of the Julio-Claudian dynasty, he was the son of Drusus and Antonia Minor. He was born at Lugdunum in Gaul and was the first Roman Emperor to be born outside Italy...
" 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 Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...
.
It is often incorrectly stated that Bede did not use a year zero because he did not know about the number zero. Although the Arabic numeral
Arabic numerals
Arabic numerals or Hindu numerals or Hindu-Arabic numerals or Indo-Arabic numerals are the ten digits . They are descended from the Hindu-Arabic numeral system developed by Indian mathematicians, in which a sequence of digits such as "975" is read as a numeral...
for zero (0
0 (number)
0 is both a numberand the numerical digit used to represent that number in numerals.It fulfills a central role in mathematics as the additive identity of the integers, real numbers, and many other algebraic structures. As a digit, 0 is used as a placeholder in place value systems...
) did not enter Europe until the eleventh century, and Roman numerals
Roman numerals
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...
had no symbol for zero, Bede and Dionysius Exiguus did use a Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
word, nulla meaning "nothing", alongside Roman numerals or Latin number words wherever a modern zero would have been used.
The anno Domini nomenclature was not widely used in Western Europe
Western Europe
Western Europe is a loose term for the collection of countries in the western most region of the European continents, though this definition is context-dependent and carries cultural and political connotations. One definition describes Western Europe as a geographic entity—the region lying in the...
until the 9th century, and the 1 January to 31 December historical year was not uniform throughout Western Europe until 1752. The first extensive use (hundreds of times) of 'BC' occurred in Fasciculus Temporum by Werner Rolevinck
Werner Rolevinck
Werner 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 terms anno Domini, Dionysian era, Christian era, vulgar era, and common era were used interchangeably between the Renaissance
Renaissance
The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...
and the 19th century, at least in Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
. But vulgar era was suppressed in English
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...
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.
Historians have never included a year zero. This means that between, for example, January 1, 500 BC and January 1, AD 500, there are 999 years: 500 years BC, and 499 years AD preceding 500. In common usage anno Domini 1 is preceded by the year 1 BC
Ante Christum Natum
Ante 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 Julian
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...
or Gregorian
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
) nor the era (Anno Domini or Common Era
Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...
) 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.
Astronomers
To simplify calculations, astronomerAstronomer
An 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 Cassini
Jacques Cassini
Jacques Cassini 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 Academy of Sciences, he was elected in 1696 a fellow of the Royal Society of London, and...
, 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
Astronomers use year numbers not only to identify a calendar year (when placed alongside a month and a day number) but also to identify a certain instant (known in astronomy as an epochEpoch (astronomy)
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are subject to perturbations and vary with time...
). To identify an instant, astronomers add a number of fractional decimal digits to the year number, as required for the desired precision: thus J2000.0 designates noon , and 1992.5 is exactly 7.5 years of 365.25 days earlier, which is the instant (03 hours in the morning of July 2). Similarly, J1996.25 is 3.75 Julian years
Julian year (astronomy)
In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86 400 SI seconds each, totaling 31 557 600 seconds. The Julian year is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is...
before J2000.0, which is the instant 1996 July 2.8125 (19.5 hours in the evening), one-quarter of a year after the instant J1996.0 = . In this notation, J0000.0 is noon of (Julian), and J0001.0 is 6 pm on (Julian). This astronomical notation is called Julian epoch and was only introduced in 1976; before that time, astronomical year numbers with decimal fractions referred to Besselian years and were written without letter prefix.
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.
History of astronomical usage
In 1849 the English astronomer John HerschelJohn Herschel
Sir 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...
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 Meeus
Jean Meeus
Jean 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 Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A 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 Copernican...
first used an astronomical year which was to become year zero in his Rudolphine Tables
Rudolphine Tables
The 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 Hire
Philippe de La Hire
Philippe 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 Cassini
Jacques Cassini
Jacques Cassini 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 Academy of Sciences, he was elected in 1696 a fellow of the Royal Society of London, and...
, 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).