Astronomical year numbering

Astronomical year numbering is a calendar-year system that includes year 0, corresponding to 1 BCE; negative numbers denote earlier years.

Year zero: A numbered year labeled 0, positioned between year −1 and year 1 in astronomical year numbering. It is the system’s defining feature and makes intervals across the era boundary straightforward to calculate.
Julian day: A Julian day is a continuous count of days used in astronomy and chronology, measured from a fixed epoch. Astronomical year numbers provide civil-calendar dates that can be converted to this continuous day count.
Anno Domini: Anno Domini is a year-counting convention that numbers years from the traditional date of Jesus’s birth. Its conventional sequence has no year 0, unlike astronomical year numbering.
Joseph Justus Scaliger: Joseph Justus Scaliger was a French scholar whose work shaped early modern chronology and classical studies. His chronological methods helped establish mathematical approaches to historical dating.
BCE: BCE is a year designation counting backward before the Common Era, with 1 BCE immediately preceding 1 CE. Astronomical year 0 corresponds to 1 BCE, rather than introducing a separate year between them.
ISO 8601: ISO 8601 is an international standard for representing dates and times in a consistent numeric format. Its expanded date representations use astronomical numbering for years before 0001.
Dionysian era: The Dionysian era is the year-numbering system devised by Dionysius Exiguus that underlies the familiar AD and BC sequence. Its original year sequence distinguishes the traditional system from astronomical numbering’s added year 0.
Julian period: The Julian period is a 7,980-year cycle formed by combining the Julian, solar, and indiction cycles. Scaliger introduced it as a framework for aligning dates across different calendar eras.
Common Era: The Common Era is a year-counting convention whose years are numerically equivalent to those of the Anno Domini system. Its positive-year sequence supplies the numbering that continues after astronomical year 0.
Proleptic Julian calendar: The proleptic Julian calendar applies Julian calendar rules to dates before the calendar’s historical introduction. Astronomical numbering helps label its early years, including dates before the Common Era.

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