Year zero
The presence or absence of year 0 is determined by convention among groups such as historians or astronomers. Neither the calendar (Julian or Gregorian) nor the era (Anno Domini or Common Era) determines that. If writers do not use the convention of their group, they must explicitly state whether or not they include a year 0 in their count of years, otherwise their historical dates will be misunderstood.
No historians include a year 0 when numbering years in the current standard era. Thus, regardless of the appellation or calendar employed (Julian or Gregorian), 1 BC (or 1 BCE) always immediately precedes AD 1 (or 1 CE). Historians even refuse to use a year 0 when using negative years before our positive era, hence their −1 immediately precedes 1 (for an example, V. Grumel, La chronologie (1958), page 30).
The anno Domini method of numbering years 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 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 twentieth 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.
However, at least two groups do include a year 0 when they number the years before these eras: astronomers and some Maya historians. In addition, some calendars of South Asia begin their counts of years with a year 0.
Bede was the first historian to use a BC year and hence the first to adopt the convention of no year 0 between BC and AD, in his
Historia ecclesiastica gentis Anglorum (
Ecclesiastical history of the English people,
731). Previous Christian histories used
anno mundi (in the year of the world"), or
anno Adami ("in the year of Adam", beginning five days later, used by Africanus), or
anno Abrahami ("in the year of Abraham", beginning 3,412 years later according to the
Septuagint, 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. All began with year 1 because the counting numbers begin with one, not zero. Bede simply continued this earlier tradition relative to the AD era. The inventor of AD,
Dionysius Exiguus in
525, did not specify whether he began his count at one or zero (nor did he mention BC).
If they wanted to indicate
zero, Bede and Dionysius used the
Latin word meaning "nothing",
nulla. It was used whenever zero was a member of a series of numbers, whether the other numbers were
Roman numerals or Latin words. Dionysius used this Latin zero in the very same table wherein he introduced his
anno Domini era, but in a neighboring column—it was the first
epact of the 19-year cycle used to
calculate Easter (see the
nineteen year cycle of Dionysius). Bede continued to use this zero epact in his
De temporum ratione (
On the reckoning of time,
725), but did not use it between BC and AD.
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" [
1]. 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. Also, the English term "before Christ" (BC) is
not a direct translation of the Latin term "before the incarnation of our Lord" (itself never abbreviated), but is only a rough equivalent:
Incarnation means the conception of
Christ, which since the
4th century has been celebrated on
25 March, which was nine months before the date on which his birth is celebrated,
25 December. Bede's singular use of 'BC' continued to be used sporadically throughout the
Middle Ages (albeit with a correct year). The first extensive use of 'BC' (hundreds of times) occurred in
Fasciculus Temporum by
Werner Rolevinck in
1474, alongside years of the world (
anno mundi).
Astronomers include a year 0 immediately before year 1. The first use of an astronomical year 0 is traditionally attributed to
Jacques Cassini in his
Tables astronomiques (
Astronomical Tables,
1740) wherein he explained his reasons for doing so. But
Phillipe de La Hire had used it earlier in
1702 in his
Tabulæ Astronomicæ (
Astronomical Tables) in the form
Christum o. ("Christ 0"), without explanation. Both Cassini and La Hire used BC years before their year 0 and AD years thereafter (hence the sequence 1 BC, 0, AD 1). Beginning in the
19th century, some astronomers began to use negative years before their year 0, while other astronomers continued to use BC years before their year 0. By the mid
20th century, all astronomers were using negative years before year 0 (hence the sequence −1, 0, 1). Although 'AD' is omitted from later years, leaving a bare number, a positive sign (+) is sometimes prefixed to the number. Because of possible confusion with the earlier use of an astronomical BC, only in the modern version can it be said that astronomical year 0 equals the historical year 1 BC.
ISO 8601:2004 and ISO 8601:2000, but not ISO 8601:1988, explicitly use astronomical year numbering in their date reference systems. Because they also specify the use of the
proleptic Gregorian calendar for all years before
1582, some readers erroneously conclude that a year zero is always included in that calendar, whereas that is unusual. The "basic" format for year 0 is the four-digit form 0000, which equals the historical year 1 BC. Several "expanded" formats are possible: -0000 and +0000, as well as five- and six-digit versions. Earlier years are also negative four-, five- or six-digit years, which have an
absolute value one less than the equivalent BC year, hence -0001 = 2 BC. Because only
ISO 646 (7-bit
ASCII) characters are allowed by ISO 8601, the minus signs are hyphens.
All eras used with
Hindu and
Buddhist calendars, such as the
Saka era or the
Kali Yuga, begin with a year 0 because all of these calendars use elapsed, expired, or complete years, in contrast with most other calendars which use current years. A complete year had not yet elapsed for any date in the year beginning at the epoch, thus that could not be year 1 — instead, it was year 0. This is similar to the Western method of stating a person's age — people do not reach age one until one year has elapsed since birth (but their age during the year beginning at birth is specified in months, not usually as age zero; however if ages were specified in years and months, such a person would be said to be, for example, 0 years and 6 months old).
Many
Maya historians, but not all, assume (or used to assume) that a year 0 exists in the modern calendar and thus specify that the epoch of the Long Count of the
Maya calendar occurred in
3113 BC rather than
3114 BC. This would require the sequence 1 BC, 0, AD 1 as in early astronomical years.
Historians note that the
3rd millennium began on
1 January 2001 because they regard the Christian Era as begining with year 1, whereas many people assumed that it began on
1 January 2000 because that was when the most significant digit of the year changed (1 → 2).
Astronomical year numbering cannot be used to support year 2000 as the first year of the 3rd millennium because of uncertainty regarding astronomical millennia. Including year 0 in the first positive millennium (0 to 999) while excluding it from the first negative millennium (−1000 to −1) would be inconsistent. But consistency produces unusual results: either year 0 separates the first positive millennium (1 to 1000) from the first negative millennium (−1000 to −1) or it is included in both (−999 to 0 to 999).
In the movie
Back to the Future Dr. Emmett Brown, the inventor of a time machine, enters the input date of the "birth of
Christ" on a keypad as
December 25, 0000. Not only does the entered input date use the astronomical year number, but the entered input date implies that
Jesus was actually born in that year (he was born between 8 BC and AD 9, according to different sources), and also that Christianity's celebration of
Christmas on
December 25 is a true historical anniversary rather than a traditional date (biblical clues indicate that Jesus was born during spring time). The novel of the film has Doc saying that "assuming that this date is indeed correct, all we'd need to do is find our way to Bethlehem!" - so it's possible that Dr. Emmett Brown was just showing off and/or being funny (both to
Marty McFly and the audiences of the movie).
The famous and fictitious theologian
Franz Bibfeldt's most famous work relates to the year 0: a 1927 dissertation submission to the University of Worms entitled "
The Problem of the Year 0".
