The Julian calendar was introduced in 46 BC by Julius Caesar and came into force in 45 BC (709 ab urbe condita). It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus. It has a regular year of 365 days divided into 12 months, and a leap day is added to February every four years. Hence the Julian year is on average 365.25 days long.
The notation "Old Style" (OS) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (NS), which indicates a date in the Gregorian calendar. This notation is used when there might otherwise be confusion about which date is found in a text.
Although the Julian calendar remained in use into the 20th century in some countries and is still used by many national Orthodox churches, it has generally been replaced for civil use by the modern Gregorian calendar. Orthodox Churches no longer using the Julian calendar typically use the Revised Julian calendar rather than the Gregorian calendar.
Reform was required because too many leap days are added with respect to the astronomical seasons on the Julian scheme. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian year, causing the calendar to gain a day about every 134 years. While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance. However, it accumulated significantly over time, and eventually led to the reform of 1582, which replaced the Julian calendar with the more accurate Gregorian calendar.
Before the new calendar took effect, the start of the calendar year (1 January) was realigned to the tropical year by making 46 BC 445 days long. The Julian months were formed by adding 10 days to the months of the regular pre-Julian Roman year of 355 days. Two extra days were added to January, July and December, and one extra day was added to April, June, September and November. Macrobius states that the extra days were added immediately before the last day of each month to avoid disturbing the position of the established Roman fasti (days prescribed for certain events) relative to the start of the month. However, since Roman dates after the Ides of the month counted down towards the start of the next month, the extra days had the effect of raising the initial value of the count for the day after the Ides.
The old intercalary month was abolished. The new leap day was originally inserted following February 24, a.d. VI Kal. Mar. by Roman reckoning, since this is the point at which intercalary months were inserted in the pre-Julian calendar. It was considered as extending that day to 48 hours, so it was dated as "a.d. VI bis Kal. Mar.", and is called the bissextile day. When days in the month came to be numbered in consecutive day order, however, the Leap Day was considered to be the last day in February in leap years, i.e. February 29.
The ordinary year in the previous Roman calendar consisted of 12 months, for a total of 355 days. In addition, a 27-day intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March. This intercalary month was formed by inserting 22 days after the first 23 or 24 days of February, the last five days of February becoming the last five days of Intercalaris. The net effect was to add 22 or 23 days to the year, forming an intercalary year of 377 or 378 days.
According to the later writers Censorinus and Macrobius, the ideal intercalary cycle consisted of ordinary years of 355 days alternating with intercalary years, alternately 377 and 378 days long. On this system, the average Roman year would have had 366¼ days over four years, giving it an average drift of one day per year relative to any solstice or equinox. Macrobius describes a further refinement wherein, for 8 years out of 24, there were only three intercalary years, each of 377 days. This refinement averages the length of the year to 365¼ days over 24 years. In practice, intercalations did not occur schematically according to these ideal systems, but were determined by the pontifices. So far as can be determined from the historical evidence, they were much less regular than these ideal schemes suggest. They usually occurred every second or third year, but were sometimes omitted for much longer, and occasionally occurred in two consecutive years.
If managed correctly this system allowed the Roman year, on average, to stay roughly aligned to a tropical year. However, if too many intercalations were omitted, as happened after the Second Punic War and during the Civil Wars, the calendar would drift rapidly out of alignment with the tropical year. Moreover, since intercalations were often determined quite late, the average Roman citizen often did not know the date, particularly if he were some distance from the city. For these reasons, the last years of the pre-Julian calendar were later known as years of confusion. The problems became particularly acute during the years of Julius Caesar's pontificate before the reform, 63 BC to 46 BC, when there were only five intercalary months, whereas there should have been eight, and none at all during the five Roman years before 46 BC.
The reform was intended to correct this problem permanently. Before it took effect, the missed intercalations during Julius Caesar's pontificate were made up by inserting 67 days (22+23+22) between the last pre-Julian November and December in 46 BC, in the form of two months, in addition to 23 days that had already been added to February. Thus 90 days were added to this last year of the Roman Republican calendar, giving it 445 days. Because it was the last of a series of irregular years, this extra-long year was, and is, referred to as the last year of confusion. The first year of operation of the new calendar was 45 BC.
Leap year error
Despite the new calendar being much simpler than the pre-Julian calendar, the pontifices apparently misunderstood the algorithm for leap years. They added a leap day every three years, instead of every four years. According to Macrobius, the error was the result of counting inclusively, so that the four year cycle was considered as including both the first and fourth years. This resulted in too many leap days. Augustus remedied this discrepancy after 36 years of this mistake by restoring the correct frequency. He also skipped several leap days in order to realign the year.
The historic sequence of leap years in this period is not given explicitly by any ancient source, although the existence of the triennial leap year cycle is confirmed by an inscription that dates from 9 or 8 BC. The chronologist Joseph Scaliger established in 1583 that the Augustan reform was instituted in 8 BC, and inferred that the sequence of leap years was 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12, 9 BC, AD 8, 12 etc. This proposal is still the most widely accepted solution. It has sometimes been suggested that there was an additional bissextile day in the first year of the Julian reform, i.e. that 45 BC was also a leap year.
Other solutions have been proposed from time to time. Kepler proposed in 1614 that the correct sequence of leap years was 43, 40, 37, 34, 31, 28, 25, 22, 19, 16, 13, 10 BC, AD 8, 12 etc. In 1883 the German chronologist Matzat proposed 44, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11 BC, AD 4, 8, 12 etc., based on a passage in Dio Cassius that mentions a leap day in 41 BC that was said to be contrary to (Caesar's) rule. In the 1960s Radke argued the reform was actually instituted when Augustus became pontifex maximus in 12 BC, suggesting the sequence 45, 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12 BC, AD 4, 8, 12 etc. With all these solutions, except that of Radke, the Roman calendar was not finally aligned to the Julian calendar of later times until 26 February (a.d. V Kal. Mar.) AD 4. On Radke's solution, the two calendars were aligned on 26 February 1 BC.
In 1999, an Egyptian papyrus was published that gives an ephemeris table for 24 BC with both Roman and Egyptian dates. From this it can be shown that the most likely sequence was in fact 44, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11, 8 BC, AD 4, 8, 12 etc, very close to that proposed by Matzat. This sequence shows that the standard Julian leap year sequence began in AD 4, the 12th year of the Augustan reform, and that the Roman calendar was finally aligned to the Julian calendar in 1 BC, as in Radke's model. The Roman year also coincided with the proleptic Julian year between 32 and 26 BC. This suggests that one aim of the realignment portion of the Augustan reform was to ensure that key dates of his career, notably the fall of Alexandria on 1 August 30 BC, were unaffected by his correction.
Roman dates before 32 BC were typically a day or two before the day with the same Julian date, so 1 January in the Roman calendar of the first year of the Julian reform was 31 December 46 BC (Julian date). A curious effect of this is that Caesar's assassination on the Ides (15th day) of March fell on 14 March 44 BC in the Julian calendar.
Immediately after the Julian reform, the twelve months of the Roman calendar were named Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, and December, just as they were before the reform. Their lengths were set to their modern values. The old intercalary month, the Mensis Intercalaris, was abolished and replaced with a single intercalary day at the same point (i.e. five days before the end of Februarius). The first month of the year continued to be Ianuarius, as it had been since 153 BC.
The Romans later renamed months after Julius Caesar and Augustus, renaming Quintilis (originally, "the Fifth month", with March = month 1) as Iulius (July) in 44 BC and Sextilis ("Sixth month") as Augustus (August) in 8 BC. Quintilis was renamed to honour Caesar because it was the month of his birth. According to a senatus consultum quoted by Macrobius, Sextilis was renamed to honour Augustus because several of the most significant events in his rise to power, culminating in the fall of Alexandria, fell in that month.
Other months were renamed by other emperors, but apparently none of the later changes survived their deaths. Caligula renamed September ("Seventh month") as Germanicus; Nero renamed Aprilis (April) as Neroneus, Maius (May) as Claudius and Iunius (June) as Germanicus; and Domitian renamed September as Germanicus and October as ("Eighth month") Domitianus. At other times, September was also renamed as Antoninus and Tacitus, and November ("Ninth month") was renamed as Faustina and Romanus. Commodus was unique in renaming all twelve months after his own adopted names (January to December): Amazonius, Invictus, Felix, Pius, Lucius, Aelius, Aurelius, Commodus, Augustus, Herculeus, Romanus, and Exsuperatorius.
Much more lasting than the ephemeral month names of the post-Augustan Roman emperors were the names introduced by Charlemagne. He renamed all of the months agriculturally into Old High German. They were used until the 15th century, and with some modifications until the late 18th century in Germany and in the Netherlands (January-December): Wintarmanoth (winter month), Hornung (the month when the male red deer sheds its antlers), Lentzinmanoth (Lent month), Ostarmanoth (Easter month), Wonnemanoth (love making month), Brachmanoth (plowing month), Heuvimanoth (hay month), Aranmanoth (harvest month), Witumanoth (wood month), Windumemanoth (vintage month), Herbistmanoth (autumn/harvest month), and Heilagmanoth (holy month).
A 13th century scholar, Sacrobosco, proposed an explanation for the month lengths which is still widely repeated but is certainly wrong. According to Sacrobosco, the original scheme for the months in the Julian Calendar was very regular, alternately long and short. From January through December, the month lengths according to Sacrobosco for the Roman Republican calendar were:
30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 30, 29
He then thought that Julius Caesar added one day to every month except February, a total of 11 more days, giving the year 365 days. A leap day could now be added to the extra short February:
31, 29/30, 31, 30, 31, 30, 31, 30, 31, 30, 31, 30
He then said Augustus changed this to:
31, 28/29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
so that the length of Augustus would not be shorter than (and therefore inferior to) the length of Iulius, giving us the irregular month lengths which are still in use.
There is abundant evidence disproving this theory. First, a wall painting of a Roman calendar predating the Julian reform has survived, which confirms the literary accounts that the months were already irregular before Julius Caesar reformed them:
29, 28, 31, 29, 31, 29, 31, 29, 29, 31, 29, 29
Also, the Julian reform did not change the dates of the Nones and Ides. In particular, the Ides were late (on the 15th rather than 13th) in March, May, July and October, showing that these months always had 31 days in the Roman calendar, whereas Sacrobosco's theory requires that March, May and July were originally 30 days long and that the length of October was changed from 29 to 30 days by Caesar and to 31 days by Augustus. Further, Sacrobosco's theory is explicitly contradicted by the third and fifth century authors Censorinus and Macrobius, and it is inconsistent with seasonal lengths given by Varro, writing in 37 BC, before the Augustan reform, with the 31-day Sextilis given by the new Egyptian papyrus from 24 BC, and with the 28-day February shown in the Fasti Caeretani, which is dated before 12 BC.
The dominant method that the Romans used to identify a year for dating purposes was to name it after the two consuls who took office in it. Since 153 BC, they had taken office on 1 January, and Julius Caesar did not change the beginning of the year. Thus this consular year was an eponymous or named year. Roman years were named this way until the last consul was appointed in 541. In addition to consular years, the Romans sometimes used the regnal year of the emperor, and by the late fourth century documents were also being dated according to the 15-year cycle of the indiction. In 537, Justinian required that henceforth the date must include the name of the emperor, in addition to the indiction and the consul (the latter ending only four years later).
Only rarely did the Romans number the year from the founding of the city (of Rome), ab urbe condita (AUC). This method was used by Roman historians to determine the number of years from one event to another, not to date a year. Different historians had several different dates for the founding. The Fasti Capitolini, an inscription containing an official list of the consuls which was published by Augustus, used an epoch of 752 BC. The epoch used by Varro, 753 BC, has been adopted by modern historians. Indeed, Renaissance editors often added it to the manuscripts that they published, giving the false impression that the Romans numbered their years. Most modern historians tacitly assume that it began on the day the consuls took office, and ancient documents such as the Fasti Capitolini which use other AUC systems do so in the same way. However, Censorinus, writing in the third century AD, states that, in his time, the AUC year began with the Parilia, celebrated on 21 April, which was regarded as the actual anniversary of the foundation of Rome. Because the festivities associated with the Parilia conflicted with the solemnity of Lent, which was observed until the Saturday before Easter Sunday, the early Roman church did not celebrate Easter after 21 April.
While the Julian reform applied originally to the Roman calendar, many of the other calendars then used in the Roman Empire were aligned with the reformed calendar under Augustus. This led to the adoption of several local eras for the Julian calendar, such as the Era of Actium and the Spanish Era, some of which were used for a considerable time. Perhaps the best known is the Era of Martyrs, sometimes also called Anno Diocletiani (after Diocletian), which was often used by the Alexandrian Christians to number their Easters during the fourth and fifth centuries and continued to be used by the Coptic and Abyssinian churches.
In the Eastern Mediterranean, the efforts of Christian chronographers such as Annianus of Alexandria to date the Biblical creation of the world led to the introduction of Anno Mundi eras based on this event. The most important of these was the Aetos Kosmou, used throughout the Byzantine world from the 10th century and in Russia till 1700. In the West, Dionysius Exiguus proposed the system of Anno Domini in 525. This era gradually spread through the western Christian world, once the system was adopted by Bede.
New Year's Day
The Roman calendar began the year on 1 January, and this remained the start of the year after the Julian reform. However, even after local calendars were aligned to the Julian calendar, they started the new year on different dates. The Alexandrian calendar in Egypt started on 29 August (30 August after an Alexandrian leap year). Several calendars were aligned to start on the birthday of Augustus, 23 September. The indiction caused the Byzantine year to begin on 1 September, which is still used in the Eastern Orthodox Church for the beginning of the liturgical year.
During the Middle Ages 1 January retained the name New Year's Day (or an equivalent name) in all Western European countries (affiliated with the Roman Catholic Church), since the medieval calendar continued to display the months from January to December (in twelve columns containing 28 to 31 days each), just as the Romans had. However, most of those countries began their numbered year on 25 December (the Nativity of Jesus), 25 March (the Incarnation of Jesus), or even Easter, as in France (see the Liturgical year article for more details).
Most Western European countries, except for a few Italian states, shifted the first day of their numbered year to 1 January while they were still using the Julian calendar, before they adopted the Gregorian calendar, many during the sixteenth century. The following table shows the years in which various countries adopted 1 January as the start of the year.
|Country||Year starting 1 January|
|Holy Roman Empire]]||1544|
|Spain, Portugal, and Roman Catholic Netherlands||1556|
|Prussia, Denmark and Norway||1559|
From Julian to Gregorian
The Julian calendar was in general use in Europe and Northern Africa from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian Calendar, which was soon adopted by most Catholic countries. Protestant countries followed later, and the countries of Eastern Europe even later. In the British Empire (including the American colonies), Wednesday 2 September 1752 was followed by Thursday 14 September 1752. For 12 years from 1700 Sweden used a modified Julian Calendar, and adopted the new-style calendar in 1753, but Russia remained on the Julian calendar until February 1918, after the Russian Revolution of 1917 (which is thus called the 'October Revolution', but occurred in Gregorian November), while Greece continued to use it until 1923.
Although all Eastern Orthodox countries (most of them in Eastern or Southeastern Europe) had adopted the Gregorian calendar by 1927, their national churches had not. A revised Julian calendar was proposed during a synod in Constantinople in May 1923, consisting of a solar part which was and will be identical to the Gregorian calendar until the year 2800, and a lunar part which calculated Easter astronomically at Jerusalem. All Orthodox churches refused to accept the lunar part, so almost all Orthodox churches continue to celebrate Easter according to the Julian calendar (the Finnish Orthodox Church uses the Gregorian Easter).
The solar part of the revised Julian calendar was accepted by only some Orthodox churches. Those that did accept it, with hope for improved dialogue and negotiations with the Western denominations, were the Ecumenical Patriarchate of Constantinople, the Patriarchates of Alexandria, Antioch, the Orthodox Churches of Greece, Cyprus, Romania, Poland, Bulgaria (the last in 1963), and the Orthodox Church in America (although some OCA parishes are permitted to use the Julian calendar). Thus these churches celebrate the Nativity on the same day that Western Christians do, 25 December Gregorian until 2800. The Orthodox Churches of Jerusalem, Russia, Macedonia, Serbia, Georgia, Ukraine, and the Greek Old Calendarists continue to use the Julian calendar for their fixed dates, thus they celebrate the Nativity on 25 December Julian (which is 7 January Gregorian until 2100).
In Northern Africa, the Julian calendar (the Berber calendar) is still in use for agricultural purposes, and is called فلاحي fellāhī "peasant" or sاﻋﺠﻤﻲ acjamī "not Arabic". Since it did not follow the Gregorian reform, its beginning slowly shifted and now the first of yennayer corresponds to January 14.
- Gregorian calendar
- Julian day
- Julian year
- Old Style and New Style dates
- Proleptic Julian calendar
- Roman calendar
- Roman Republican calendar
- Charles W. Jones, "Development of the Latin Ecclesiastical calendar", Bedae Opera de Temporibus (1943), 1-122, p.28.
- Mike Spathaky Old Style and New Style Dates and the change to the Gregorian Calendar: A summary for genealogists
- The source has Germany, whose current area during the sixteenth century was a major part of the Holy Roman Empire, a religiously divided confederation. The source is unclear as to whether all or only parts of the country made the change. In general, Roman Catholic countries made the change a few decades before Protestant countries did.
- Calendars through the ages on WebExhibits.
- Calendar FAQ
- Roman Dates
- The Roman Calendar
- Synoptical Julian-Gregorian Calendar - compare the Julian and Gregorian calendars for any date between 1582 and 2100 using this side-by-side reference.
- Date Conversion
- Calendar Converter — converts between several calendars, for example Gregorian, Julian, Mayan, Persian, Hebrew
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