# 1 BC
**1 BC** (also written 1 BCE) is the calendar year immediately before AD 1 in the [Anno Domini](https://www.edgepedia.org/anno-domini) and [Common Era](https://www.edgepedia.org/common-era) systems of year numbering. Because these systems have no year zero, 1 BC is followed directly by AD 1, and it is the last year of the 1st century BC in the conventional count.[1](https://eclipse.gsfc.nasa.gov/SEhelp/dates.html)
| Key fact | Detail |
|---|---|
| Position in era | The year before AD 1; there is no year zero between 1 BC and AD 1 in the traditional Christian-era count[1](https://eclipse.gsfc.nasa.gov/SEhelp/dates.html) |
| Astronomical number | Year 0; in general, any year "x BCE" becomes −(x−1) in astronomical year numbering[1](https://eclipse.gsfc.nasa.gov/SEhelp/dates.html) |
| Leap-year status | In the astronomical system year 0 (equivalent to 1 BC) is counted as a leap year, though the actual historical calendar's leap sequence in this period is disputed[2](https://www.primidi.com/proleptic_julian_calendar) |
| Roman dating practice | Romans themselves described years by the names of the consuls, not by numbered eras[3](https://time.com/4462775/bc-ad-dating-history/) |
| Documented event | Cassius Dio records, retrospectively, an Armenian rebellion supported by the Parthians in year 1 BCE[4](https://www.iranicaonline.org/articles/phraates-v/) |
| Dynastic event | Gaius Caesar, Augustus's adopted son, married Livia Julia (Livilla) in 1 BCE[5](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781444338386.wbeah30488) |
## Numbering and the missing year zero
The year numbering that places 1 BC next to AD 1 was instituted by the sixth-century Roman monk [Dionysius Exiguus](https://www.edgepedia.org/dionysius-exiguus) for his Easter tables and popularized by the Venerable Bede around 731, and the custom of omitting a year zero became well established.[6](https://deadseaquake.info/pdfs/RD2018.pdf) In sixth-century Europe the concept of zero as a number was unknown, so the count ran from 1 BC straight to AD 1.[1](https://eclipse.gsfc.nasa.gov/SEhelp/dates.html) This creates the well-known arithmetic quirk that the interval between, say, 2 BC and 5 AD spans only six years, because both halves of the era are counted with ordinals beginning at one.[1](https://eclipse.gsfc.nasa.gov/SEhelp/dates.html)
Astronomical year numbering removes the gap. In the system introduced by the French astronomer Jacques Cassini in 1740, the year 1 BC/BCE is numbered 0, 2 BC is numbered −1, and in general year n BC is numbered −(n−1).[1](https://eclipse.gsfc.nasa.gov/SEhelp/dates.html) In this system the year 0 is equivalent to 1 BC and is a leap year, although in the actual calendar of the time there were no leap years operating as intended, because of an earlier leap-year error.[2](https://www.primidi.com/proleptic_julian_calendar) The international date standard [ISO 8601](https://www.edgepedia.org/iso-8601) likewise supports a year 0000, unlike the traditional BC/AD and BCE/CE counts.[6](https://deadseaquake.info/pdfs/RD2018.pdf)
## Calendar state in 1 BC
The [Julian calendar](https://www.edgepedia.org/julian-calendar), introduced by [Julius Caesar](https://www.edgepedia.org/julius-caesar) in 46–45 BC with a leap day every four years, was not yet running correctly. The Roman habit of counting inclusively, together with an error in applying the four-year rule, meant the leap days came too early; the error was not corrected until the period of Augustus, and the Julian year began to function properly only from 4 CE.[7](https://academic.oup.com/edited-volume/43505/chapter/364127569) On the corrected retrospective pattern, 1 BC falls among the years that modern calculations treat as leap years (along with 5 BC and 9 BC), but whether an intercalary day was actually observed in that year is one of the standing disputes in Roman chronology.[8](https://en.alegsaonline.com/art/112301)
Rome itself did not number the year. Consuls took office in January for one-year terms, and events were dated by their names, so "1 BC" is a later scholarly label retrojected onto a year the Romans identified as the consulship of two named men.[3](https://time.com/4462775/bc-ad-dating-history/)
## Events and chronology
The surviving narrative sources attach comparatively few events to this year. [Cassius Dio](https://www.edgepedia.org/cassius-dio)'s account of year 1 BCE, written retrospectively, recalls the Armenian rebellion supported by the Parthians after the defeat of the pro-Roman pretender Artavasdes III.[4](https://www.iranicaonline.org/articles/phraates-v/) Armenia at this time was ruled by Tigranes IV and his sister and wife Erato (ca. 6 BCE–1 CE), who relied on Parthian backing against pro-Roman circles and against the Iberians.[4](https://www.iranicaonline.org/articles/phraates-v/) Parthian coins show the conflict's context: tetradrachms of the Parthian king Phraataces ([Phraates V](https://www.edgepedia.org/phraates-v)) are dated from Audynaios (December) 2 BCE to Hyperberetaios (September) 1 BCE.[4](https://www.iranicaonline.org/articles/phraates-v/)
At Rome, the marriage of [Gaius Caesar](https://www.edgepedia.org/gaius-caesar) to Livia Julia (Livilla), the daughter of Drusus the Elder and Antonia Minor, took place in 1 BCE.[5](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781444338386.wbeah30488) Gaius (20 BCE–4 CE) and his brother Lucius (17 BCE–2 CE) were Augustus's adopted sons and intended successors.[5](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781444338386.wbeah30488)
**Chronological debates around the year.** Two famous dates are tied to 1 BC by parts of the tradition. The December 25 birth of Jesus is the traditional date assigned by Dionysius Exiguus in constructing his Anno Domini era, though the era's year 1 does not match modern estimates of the birth year.[9](https://www.fact-archive.com/encyclopedia/1_BC) The death of [Herod the Great](https://www.edgepedia.org/herod-the-great), which anchors the chronology of the late Hasmonaean and early Herodian period, is placed in 1 BC according to some interpretations of Josephus, against the more common dating to 4 BC.[9](https://www.fact-archive.com/encyclopedia/1_BC)
## References
1. [NASA Eclipse Website, Year Dating Conventions](https://eclipse.gsfc.nasa.gov/SEhelp/dates.html)
2. [Proleptic Julian Calendar, Technology Trends](https://www.primidi.com/proleptic_julian_calendar)
3. [Now You Know: When Did People Start Saying That the Year Was 'A.D.'?, TIME](https://time.com/4462775/bc-ad-dating-history/)
4. [Phraates V, Encyclopædia Iranica (Marek Jan Olbrycht)](https://www.iranicaonline.org/articles/phraates-v/)
5. [Gaius and Lucius Caesar, Encyclopedia of Ancient History, Wiley Online Library](https://onlinelibrary.wiley.com/doi/abs/10.1002/9781444338386.wbeah30488)
6. [Calendrical Calculations (reference text, PDF)](https://deadseaquake.info/pdfs/RD2018.pdf)
7. [Theorization, Measurement, and Standardization of Calendrical Time, Oxford Handbook Topics in Classical Studies](https://academic.oup.com/edited-volume/43505/chapter/364127569)
8. [1 BC (1 BCE), Alegsa Online](https://en.alegsaonline.com/art/112301)
9. [1 BC, Fact-Archive encyclopedia entry](https://www.fact-archive.com/encyclopedia/1_BC)
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*Topic: Encyclopedia › Society and history › History and archaeology › General historical chronologies › Year articles, ancient and classical (to AD 500)*
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