Year zero
A year zero is a date where the current year for a given calendar system is zero. In systems which include a year zero, this year would be the epoch. Year zero does not exist in the Anno Domini calendar year system commonly used to number years in the Gregorian calendar and Julian calendar. Instead, AD 1 is treated as the epoch, so that the year is followed directly by year. However, there is a year zero in both the astronomical year numbering system, and the ISO 8601:2004 system, a data interchange standard for certain time and calendar information (where year zero coincides with the Gregorian year ;. There is also a year zero in most Buddhist and Hindu calendars.
History
The Anno Domini era was introduced in 525 by Scythian monk Dionysius Exiguus, 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 Roman 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" which was also 525 years "since the incarnation of our Lord Jesus Christ". How he arrived at that number is unknown.Dionysius Exiguus did not use "AD" years to date any historical event. This practice began with the English cleric Bede, who used AD years in his Historia ecclesiastica gentis Anglorum, popularizing the era. Bede also used – only once – a term similar to the modern English term "before Christ", though the practice did not catch on for nearly a thousand years, when books by Denis Pétau treating calendar science gained popularity. 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 the counting origin one in Ecclesiastical Latin.
Previous Christian histories used several titles for dating events: anno mundi beginning on the purported first day of creation; or anno Adami beginning at the creation of Adam five days later as used by Africanus; or anno Abrahami 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 forty-sixth 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 = 106, but 60 + 46 = 106, which leaves no room for a year zero. The modern English term "before Christ" is only a rough equivalent, not a direct translation, of Bede's Latin phrase ante incarnationis dominicae tempus, which was itself never abbreviated. Bede's singular use of 'BC' continued to be used sporadically throughout the Middle Ages.
Neither the concept of nor a symbol for zero existed in the system of Roman numerals. The Babylonian system of the BC era had used the idea of "nothingness" without considering it a number, and the Romans enumerated in much the same way. Wherever a modern zero would have been used, Bede and Dionysius Exiguus did use Latin number words, or the word nulla alongside Roman numerals. Zero was invented in India in the sixth century, and was either transferred or reinvented by the Arabs by about the eighth century. The Arabic numeral for zero did not enter Europe until the thirteenth century. Even then, it was known only to very few, and only entered widespread use in Europe by the seventeenth century.
The anno Domini nomenclature was not widely used in Western Europe until the 9th century, and the to historical year was not uniform throughout Western Europe until 1752. The first extensive use of 'BC' occurred in Fasciculus Temporum by Werner Rolevinck in 1474, alongside years of the world. 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 fell out of use 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, and , 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. Neither the choice of calendar system nor the name of the era determines whether a year zero will be used. If writers do not use the convention of their group, they must explicitly state whether they include a year 0 in their count of years, otherwise their historical dates will be misunderstood.
Astronomy
In astronomy, for the year AD 1 and later it is common to assign the same numbers as the Anno Domini notation, which in turn is numerically equivalent to the Common Era notation. But the discontinuity between 1 AD and 1 BC makes it cumbersome to compare ancient and modern dates. So the year before 1 AD is designated 0, the year before 0 is −1, and so on.The letters "AD", "BC", "CE", or "BCE" are omitted. So 1 BC in historical notation is equivalent to 0 in astronomical notation, 2 BC is equivalent to −1, etc. Sometimes positive years are preceded by the + sign. This year numbering notation was introduced by the astronomer Jacques Cassini in 1740.
History of astronomical usage
In 1627, the German astronomer Johannes Kepler, in his Rudolphine Tables, first used an astronomical year essentially as a year zero. He labeled it Christi and inserted it between years labeled Ante Christum and Post Christum BC and AD today, on the "mean motion" pages of the Sun, Moon, and planets. In 1702, the French astronomer Philippe de La Hire labeled a year as and placed it at the end of the years labeled ante Christum, and immediately before the years labeled post Christum, on the mean motion pages in his Tabulæ Astronomicæ, thus adding the number designation 0 to Kepler's Christi.Finally, in 1740, the transition was completed by French astronomer Jacques Cassini, who is traditionally credited with inventing year zero. In his Tables astronomiques, Cassini labeled the year simply as 0, and placed it at the end of years labeled avant Jesus-Christ, and immediately before years labeled après Jesus-Christ.