Year numbering

From Academic Kids

Year numbering is the assignment of integers to calendar years for the purpose of uniquely identifying the years.

A calendar defines, among many other things, the length of each year. Calendars with different year lengths must use different numbering systems. However, within a single calendar it is possible to have several year numbering systems. This occurs for the Gregorian calendar currently in common use and also for the Julian calendar which preceded it. Calendars with identical year lengths can share a numbering system, as for several proposed reformed calendars based on Gregorian years.


Gregorian calendar

The average year length of the Gregorian calendar is almost exactly one tropical year. There are several systems of year numbering, and several different names for the most common system.

  • The numbering system devised by Dionysius Exiguus in AD 525 is still the most common, which is based on the year he assigned to the birth of Jesus, although it is now believed that Christ was born four to eight years earlier.
    • AD stands for Anno Domini, Latin for in the year of the Lord. The letters AD were originally placed before the year number (e.g., AD 1960), but now are increasing placed after the year number (e.g., 1960 AD).
    • BC stands for Before Christ marking the period before the Christian era. The letters BC are placed after the year number (e.g., 10 BC). The years are numbered in reverse order, 2 BC preceding 1 BC.
      • There is no year 0 in this numbering system and the year AD 1 immediately follows 1 BC.
    • CE stands for the Common Era and nowadays is being accepted as a replacement of AD. It and BCE are preferred by those, Christian or otherwise, who think the link between the standard calendar and a particular religion is inappropriate. The letters CE are placed after the year number (e.g., 1960 CE).
    • BCE stands for Before the Common Era and is identical to BC. The letters BCE are placed after the year number (e.g., 10 BCE). The numbering is again identical to BC.
      • BCE has also been claimed to stand for Before the Christian Era, but this is generally regarded as a religiously motivated attempt to defeat the intention of the use of CE and BCE.
    • NS stands for New Style and identifies Gregorian dates for events that took place while the Julian calendar was in use, which continued in some places into the 20th century. See Julian calendar below, also proleptic Gregorian Calendar.
  • Astronomical year numbering is the same as AD for positive year numbers, but includes a year zero immediately preceding AD 1 to simplify arithmetic calculations, so there is a one year difference for years BC.
  • BP stands for Before Present (specifically, before 1950), and is used for ages determined by radiocarbon dating.

See Anno Domini for a discussion of the arguments for and against various terms.

Julian calendar

The Julian calendar was in use from 45 BC to AD 1582, and in the British Empire until AD 1752, Russia until AD 1918, Greece until AD 1923 and Turkey until AD 1926. It has a slightly longer average year length than the Gregorian calendar, and therefore falls slowly out of step with the solar year. It is still used by Orthodox Churches for reckoning the date of Easter, but no longer for year numbering.

  • AUC stands for Ab Urbe Condita, Latin for from the foundation of the city (meaning Rome). Today, Varro's epoch of 1 AUC = 753 BC is usually implied, but this method of identifying years was rarely used by the Romans themselves. The dominant Roman method of identifying years was to name the two consuls who held office that year.
  • OS stands for Old Style and marks a Julian date, as opposed to NS for New Style which marks a Gregorian date.
    • AD and BC are authentic for Julian OS dates, and their synonyms will also be encountered. The meanings are the same as for similarly-marked Gregorian year numbers.
  • The Era of the Caesars (or, in modern texts, the Spanish Era) numbers the years of the Julian calendar from January 1, 38 BC. Thus it has the same day and month as the corresponding Julian AD date, but a year 38 greater. Although this epoch presumably represents Augustus's settlement of Spain, the reason for the exact choice is unclear.

In the period in which the Julian calendar was in official use, the similarly-marked Julian and Gregorian year numbers for a particular date will either be equal or the Julian year number will be one less than the Gregorian (and of course the day of the month and perhaps the month will differ, but the day of the week will be the same). Within the Christian era, there are two reasons for the difference. Firstly, as Gregorian dates were set to be identical to Julian in AD 325, the differing year length meant that by AD 1582 a ten day difference had accumulated. Secondly and more significantly, while the Julian year originally started on January 1, early in church history the numbered year began on March 25 or December 25, the traditional dates for Christ's Incarnation and Nativity. This is quite confusing for modern readers. For example, March 30, AD 901 is almost a year before March 20, AD 901 if a specific year number began on March 25. To eliminate this problem, modern authors often give the date in the equivalent historical year, which is our modern January 1 to December 31 year projected back to the date in question. Sometimes these two will be seen combined. For example, Francis Bacon's birth-date could be given as January 22nd, 1560/61 (OS). This style of year also gives a clue that a Julian date is probably intended.

Julian dates are often encountered for events before the calendar's first regular year of AD 1. Although the Julian calendar was implemented in 45 BC, the first regularly recurring Julian leap year (every four years thereafter) was AD 4, thus the first regular Julian year was AD 1. Dates before AD 1 are usually given in the proleptic Julian calendar, which is the regular Julian calendar projected back in time, regardless of the calendar within which the dates were originally recorded.

External links

Proposed reformed calendars

New calendar proposals based on the Gregorian year length, such as the World calendar, the International Fixed Calendar and the Positivist calendar, are compatible with Gregorian numbering, or could use their own schemes.

Islamic calendar

The Islamic calendar is a lunar calendar and has a normal year length of about 354 days, significantly shorter than the solar year, so there is no simple conversion between Gregorian and Islamic year numbers. Moreover, the end of each month of the Islamic calendar depends on local observations, so different countries can and do follow different calendars.

  • AH stands for Anno Hegirae, Latin for in the year of the Hijra. In AH 17, the year AH 1 was assigned to the year during which Muhammad emigrated from Mecca to the city of Medina.
  • BH refers to years before AH 1. There is no year zero.

The 1st day of Muharram (the 1st month of the year) AH 1 was the 16th of July AD 622 in the Julian calendar. The 1st of January 2000 in the Gregorian calendar corresponded to the 24th day of Ramadan (the 9th month) AH 1420 in the tabular Islamic calendar.

Hebrew Calendar

The Jewish or Hebrew calendar is lunisolar. Its year length varies from 353 to 385 days. Its average year length is close to the average Gregorian year length.

  • AM stands for Anno Mundi meaning in the year of the world. It is placed before the number.

The Hebrew year begins in September or October of the Gregorian year, with AM 1 beginning in October, 3761 BC. Allowance must also be made for the absence of a year 0 from the normal Gregorian numbering.

French Revolutionary Calendar

The French Revolutionary Calendar was a solar calendar in force in France for a little more than 12 years from late 1793 to the end of 1805 and again briefly in 1871. Years were numbered with the roman numerals I-XIV, with each year commencing on the northern autumnal equinox. Year I was assigned to the year beginning September 22, AD 1792, over one year before the calendar was implemented.

Other calendars

Summaries still to be prepared.


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