The Julian Calendar was introduced in 46 BC by Julius Caesar and took force in 45 BC (709 ab urbe condita). It was developed in consultation with the Alexandrian astronomer Sosigenes and was probably designed to approximate the tropical year. 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 Calendar's year is on average 365¼ days long.
One difficulty with the Julian Calendar is that too many leap days are added with respect to the astronomical seasons. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian Calendar year, causing the Julian Calendar to drift backwards one day about every 128 years. While Sosigenes was presumably aware of this 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 calendrical reform of 1582, which introduced the more astronomically accurate Gregorian Calendar.
The notation Old Style or (OS) is sometimes used to indicate a date in the Julian Calendar, as opposed to New Style or (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 given text.
From Roman to Julian
The ordinary year in the earlier Roman calendar consisted of 12 months with a total of 355 days. In addition, an intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March. This intercalary month was formed by inserting 22 days before the last five days of February, creating a 27-day month. It began after a truncated February having 23 or 24 days, so that it had the effect of adding 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, which were 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 correct 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 Julius Caesar's pontificate (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 years before 46 BC.
The Julian 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 November and December of 46 BC in the form of two months, in addition to 23 days which had already been added to February. Thus 90 days were added to this last year of the Roman 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 years error
Despite the new calendar being much simpler than the Roman calendar, the pontifices apparently misunderstood the algorithm. 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. Caesar Augustus remedied this discrepancy by restoring the correct frequency after 36 years of this mistake. He also skipped several leap days in order to realign the year.
The historic sequence of leap years (i.e. years with a leap day) 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 also sometimes been suggested that 45 BC was 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.
In 1999, an Egyptian papyrus was published which 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 twelfth year of the Augustan reform. Also, under this sequence the actual Roman year 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 August 1, 30 BC, were unaffected by his correction.
Naming of the months
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 an 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 ("Fifth month"; Martius was originally the first month) as Iulius (July) in 44 BC and Sextilis ("Sixth month") as Augustus (August) in 8 BC. (Note that the letter J was not invented until the 17th century). Quintilis was renamed to honor Julius Caesar because it was the month of his birth. According to a senatusconsultum 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, came 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 ("Eighth month") as Domitianus. At other times, September was also renamed as Antoninus and Tacitus, and November ("Ninth month") was renamed 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 (spring), Lentzinmanoth (Lent month), Ostarmanoth (Easter month), Winnemanoth (grazing month), Brachmanoth (plowing month), Heuvimanoth (hay month), Aranmanoth (harvest month), Witumanoth (wood month), Windumemanoth (vintage month), Herbistmanoth (autumn/harvest month), and Heilagmanoth (holy month). Translations of these month names are still used to this day in some Slavic languages, such as Polish.
Lengths of the months
According to the 13th century scholar 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 antecedent Roman Republican calendar were:
- 30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 30, and 29, totaling 354 days.
He wrote 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, and 30
He then said Augustus changed this to:
- 31, 28 (29), 31, 30, 31, 30, 31, 31, 30, 31, 30, and 31
giving us the irregular month lengths which we still use today, so that the length of Augustus' named month would not be shorter than (and therefore inferior to) the length of Julius' month.
Although this theory is still widely repeated, it is certainly wrong. First, a wall painting of a Roman Republican calendar has survived , which confirms the literary accounts that the months were already irregular before Julius Caesar instituted his reform:
- 29, 28, 31, 29, 31, 29, 31, 29, 29, 31, 29, and 29
Also, the Julian reform did not change the dates of the Nones and Ides. In particular, the Ides are 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 the length of October was changed. Further, Sacrobosco's theory is explicitly contradicted by the third and fifth century authors Censorinus and Macrobius, and, finally, 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 recently-discovered 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 January 1, and Julius Caesar did not change the turn 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. Only rarely did the Romans number the year ab urbe condita or (AUC) (from the founding of the City [of Rome]). Rather, 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. However, the epoch used by Varro, 753 BC, has been adopted by modern historians. Renaissance editors often added the AUC designation to the manuscripts that they published, giving the false impression that the Romans numbered their years in this way. Most modern historians tacitly assume that a given year began on the day the consuls took office, and ancient documents such as the Fasti Capitolini that use other AUC systems do so in the same way. However, the Varronian AUC year did not formally begin on January 1, but on Founder's Day, April 21.
In addition to consular years, the Romans sometimes used the regnal year of the emperor. Anno Diocletiani, named after Diocletian, was often used by the Alexandrian Christians to number their Paschal celebrations during the fourth and fifth centuries. In AD 537, Justinian required that thenceforth the date must include the name of the emperor, in addition to the Indiction and the consul (the latter ending only four years later). The Indiction cycle caused the year to begin on September 1 in the Byzantine Empire, which date is still used in the Orthodox Church for the beginning of the liturgical year. In AD 525 Dionysius Exiguus proposed the system of anno Domini, which gradually spread through the Christian world, hastened when the system was adopted by Bede. Years were numbered variously from the date of the Feast of the Annunciation (the conception of Jesus Christ by the Theotokos) on March 25, the Feast of Christ's Nativity on December 25, and even Pascha.
From Julian to Gregorian
The Julian calendar was in general use in western Europe from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian Calendar, which was soon adopted by most Roman Catholic countries. The Protestant countries followed later, and the countries of eastern Europe even later. Russia remained on the Julian Calendar until after the Russian Revolution (which is thus called the October Revolution, although it occurred in November according to the Gregorian Calendar), in 1917, while Greece continued to use it until 1923.
Although all eastern European countries had adopted the Gregorian Calendar by 1923, the Orthodox Churches located in them had not. A Revised Julian Calendar was proposed during a synod in Constantinople in May of 1923, consisting of a solar part which was and will be identical to the Gregorian calendar until the year 2800, and a lunar part that calculated Pascha astronomically at Jerusalem. No local Orthodox Church accepted the lunar part of this proposal, so almost all Orthodox Churches continue to celebrate Pascha according to the Julian Calendar (the Churches of Finland and Estonia use the Gregorian Paschalion). The solar part has only been accepted by certain Orthodox Churches, those of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, Bulgaria, and the Orthodox Church in America (although the OCA's Diocese of Alaska and some other parishes retain the Julian Calendar). Thus, these churches celebrate the Nativity of Christ on the same day that Western Christians do because December 25 coincides on both the Gregorian and Revised Julian Calendars (until 2800 when the Revised Julian Calendar will drop one day behind the Gregorian Calendar due to differing leap year rules).
The Old Calendarists and the Orthodox Churches of Jerusalem, Russia, Serbia, Georgia, and Ukraine continue to use the Julian Calendar for their fixed dates, thus they celebrate Nativity on December 25 on the Julian Calendar, which corresponds to January 7 on the Gregorian Calendar (until 2100, when the Julian Calendar will drift back one additional day with respect to the Gregorian Calendar).