Information about Year Zero

The year zero does not exist in the predominant Western Gregorian calendar nor its predecessor, the Julian calendar. However, it does exist in ISO 8601:2004 and astronomical year numbering (where it is equal to the Gregorian year 1 BC) as well as in all Buddhist and Hindu calendars.

Julian and Gregorian calendars

The Roman numeral system has no symbol for null (zero), but instead the Latin words nulla and nihil, which in normal speech meant nothing, were used. Nulla was used whenever zero was a member of a series of numbers, whether the other numbers were Roman numerals or Latin words.

Dionysius Exiguus (c.470–c.544) used the 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 (c.672–735) was the first historian to use a BC year, and hence the first to adopt the convention of no year 0 between the BC and AD epochs, 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. Bede continued to use this zero epact in his De temporum ratione (On the reckoning of time, 725), but did not use it between dates BC and AD. Nihil was used for a remainder of zero when a number was evenly divisible (see arguments two and five in the nineteen year cycle of Dionyius just cited).

Bede did not sequentially number any other calendar units (days of the month, weeks of the year, or months of the year — but he was aware of the Jewish days of the week which were numbered beginning with one (except for the seventh which was called the Sabbath) and partially numbered the days of his Christian week accordingly (Lord's day, second day, …, sixth day, Sabbath in English translation).

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. 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 lordly incarnation"), which was itself never abbreviated. Incarnation means the conception, not the birth, of Christ, which since the 4th century has been celebrated on 25 March, nine months before the date on which the celebration of his birth at Christmas (25 December). Bede's singular use of 'BC' continued to be used sporadically throughout the Middle Ages (albeit with a correct year).

It is often argued that Bede did not use a year zero because he did not know about the number zero. Although the Arabic numeral (symbol) for zero (0) did not enter Europe until the eleventh century, Bede and Dionysius were well aware of the concept of zero; two words for zero, the Latin words nulla and nihil, were normal in Latin speech, meaning nothing. Nulla was used whenever zero was a member of a series of numbers, whether the other numbers were Roman numerals or Latin words. The first documented use of zero in the Hindu-Arabic numeral system occurred towards the end of the 9th century. However, the Arabic numerals were totally unknown in Europe until about the end of the first millennium and were not generally used before Renaissance time.

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).

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.

Historians

Since Bede, historians have not counted with a year zero. This means that between, for example, 500 BC, January 1 and AD 500, January 1 there are counter-intuitively only 999 years: 500 for the time taking place BC, and 499 for the part of AD. Still, on further scrutiny one realizes that there are indeed 1000 years between the beginning of the 500 BC and the END of the year AD 500. This is reflected by the fact that the number 500 on the straight line coincides with the completion of the AD 500, rather than with its beginning, whereas the number 0 indicates the point in time in which the first year BC and the first year AD converge - midnight of December 31st of 1 BC.

In common usage Anno Domini 1 is preceded by the year 1 BC, without an intervening year zero.^[2] 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. Thus, regardless of the appellation or calendar employed (Julian or Gregorian), 1 BC always immediately precedes AD 1. Historians even refuse to use a year 0 when using negative years before our positive era, hence their −1 immediately precedes 1 (for example, V. Grumel, La chronologie (1958), page 30).

Astronomical year numbering

Astronomers

Astronomers, for whom ease of mathematical calculation is more important, have used for several centuries a defined leap year zero equal to BC 1 of the traditional Christian era. The first use of an astronomical year 0 is traditionally attributed to Jacques Cassini whose stated reasons for including a year zero were:

But Philippe de La Hire had used a year zero 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). That is why Cassini stated that their sum yielded the interval. For example, 1 + 1 = 2. 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). Thus modern astronomers would state that the years' difference yields the interval, just as it does if the years are both positive or both negative. For example, 1 − (−1) = 2, and 2000 − 1999 = 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

ISO 8601:2004 (and previously ISO 8601:2000, but not ISO 8601:1988) explicitly uses astronomical year numbering in its date reference systems. Because it also specifies the use of the proleptic Gregorian calendar for all years before 1582, some readers incorrectly assume that a year zero is also included in that proleptic 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 sign is represented by a hyphen-minus.

Other year zero traditions

South Asian moon calendars

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.

Maya historians

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 Mesoamerican Long Count calendar occurred in 3113 BC rather than 3114 BC. This would require the sequence 1 BC, 0, AD 1 as in early astronomical years.

Numerical explanation

Historians use ordinal numbers (e.g. first, second, third, ...) to label years, centuries, and millennia, whereas astronomical year numbering, Hindu and Buddhist calendars all use cardinal numbers (e.g. 1, 2, 3, ...), which measure the elapsed time from a starting point.

Ordinal numbers give a position in a sequence (e.g. first, second, third, ...). When using ordinal numbers to label years, the first year after the starting point necessarily comes immediately after the first year before the starting point. Thus 1 AD comes immediately after 1 BC.

Cardinal numbers mark the integer positions on a continuous scale, in this case a time axis. When using cardinal numbers to measure elapsed time year 1 begins exactly one year after the starting point. Thus, with cardinal numbers, the first year is year 0 (meaning that zero years have elapsed since the starting point).

Normally there is no confusion between ordinal and cardinal numbers, but if one uses the numeral 1 to stand for the first in a sequence, and "2" for the second, and so forth, then ambiguity can result. There is no way to tell whether "year 1" is ordinal (using 1 to mean "first"), or cardinal (using 1 to mean one year after the starting point).

A similar distinction occurs in numbering the floors of a building, and the exits of a highway. See Floor numbering and Exit 0.

Third millennium

The 3rd 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 Common 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.

In popular culture

• 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, implying that he uses the astronomical year numbering (and ignoring questions of whether Jesus was actually born on that year or date). It should be noted that the DVD commentary clearly points out that the date was a joke.
• In the movie, The Beach, Leonardo DiCaprio is, during his mental instability, crazed about the term Year 0.
• 30 Seconds to Mars have a song entitled "Year Zero" on their debut album.
• The sixth studio album from Nine Inch Nails is also named "Year Zero". It follows a viral marketing campaign, part of the Year Zero Alternate Reality Game.
• The 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".
• The Zork timeline included with the comedy game Zork Grand Inquisitor features the year 0 GUE with the annotation: "As the year zero begins, people feel fairly confident that something big is about to happen."

Notes