The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, 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 Julian calendar remained in use into the 20th century in some countries as a civil calendar, but thirteen days have been excised to make the date the same as in other countries. This is described as the "New calendar". Research will be needed to establish which leap year model has been adopted (if any), [1] but the civil calendar is identical with the new calendar of fixed holy days in Orthodox countries.[2] All Orthodox churches (with the exception of Estonia and Finland) still use the Julian calendar for calculating the dates of movable feasts. Some Orthodox churches do not use the new calendar at all. [3] The Julian calendar is still used by the Berber people of North Africa, and on Mount Athos.
The notation "Old Style" (OS) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (NS), which either represents the Julian date with the start of the year as 1 January or a full mapping onto the Gregorian calendar. This notation is used in reference to dates from tsarist Russia (the country did not switch to the new calendar until 1918).
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, which counted down toward the start of March, become 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. [4]
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, since the Pontifices were often politicians, and because a Roman magistrate's term of office corresponded with a calendar year, this power was prone to abuse: a Pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power. 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, because 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, 63–46 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. For example, Caesar crossed the Rubicon on January 10, 49 BC of the official calendar, but the official calendar had drifted so far away from the seasons that it was actually mid-autumn.
The reform was intended to correct this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention.
The first step of the reform was to realign the start of the calendar year (1 January) to the tropical year by making 46 BC 445 days long, compensating for the intercalations which had been missed during Caesar's pontificate. This year had already been extended from 355 to 378 days by the insertion of a regular intercalary month in February. When Caesar decreed the reform, probably shortly after his return from the African campaign in late Quintilis (July), he added 67 (= 22 + 23 + 22) more days by inserting two extraordinary intercalary months between November and December. These months are called Intercalaris Prior and Intercalaris Posterior in letters of Cicero written at the time; there is no basis for the statement sometimes seen that they were called "Unodecember" and "Duodecember".[citation needed] Their individual lengths are unknown, as is the position of the Nones and Ides within them. Because 46 BC 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.
The Julian months were formed by adding ten days to a regular pre-Julian Roman year of 355 days, creating a regular Julian year of 365 days: Two extra days were added to Ianuarius,[5] Sextilis (Augustus) and December, and one extra day was added to Aprilis, Iunius, September and November, setting the lengths of the months to the values they still hold today:
Months Lengths before 45 BC Lengths as of 45 BCMacrobius 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 toward the start of the next month, the extra days had the effect of raising the initial value of the count of the day after the Ides. Romans of the time born after the Ides of a month responded differently to the effect of this change on their birthdays. Mark Antony kept his birthday on the 14th day of Ianuarius, which changed its date from a.d. XVII Kal. Feb. to a.d. XIX Kal. Feb., a date that had previously not existed. Livia kept the date of her birthday unchanged at a.d. III Kal. Feb., which moved it from the 28th to the 30th day of Ianuarius, a day that had previously not existed. Augustus kept his on the 23rd day of September, but both the old date (a.d. VIII Kal. Oct.) and the new (a.d. IX Kal. Oct.) were celebrated in some places.
The old intercalary month was abolished. The new leap day was dated as ante diem bis sextum Kalendas Martias, usually abbreviated as a.d. bis VI Kal. Mart.; hence it is called in English the bissextile day. The year in which it occurred was termed annus bissextus, in English the bissextile year.
There is debate about the exact position of the bissextile day in the early Julian calendar. The earliest direct evidence is a statement of the first century jurist Celsus, who states that there were two halves of a 48-hour day, and that the intercalated day was the "posterior" half. An inscription from A.D. 168 states that a.d. V Kal. Mart. was the day after the bissextile day. The 19th century chronologist Ideler argued that Celsus used the term "posterior" in a technical fashion to refer to the earlier of the two days, which requires the inscription to refer to the whole 48-hour day as the bissextile. Some later historians share this view. Others, following Mommsen, take the view that Celsus was using the ordinary Latin (and English) meaning of "posterior". A third view is that neither half of the 48-hour "bis sextum" was originally formally designated as intercalated, but that the need to do so arose as the concept of a 48-hour day became obsolete.[6]
There is no doubt that the bissextile day eventually became the earlier of the two days. In 238 Censorinus stated that it was inserted after the Terminalia (23 February) and was followed by the last five days of February, i.e. a. d. VI, V, IV, III and prid. Kal. Mart. (which would be the 24th to 28th days of February in a common year and the 25th to the 29th days in a leap year). Hence he regarded the bissextum as the first half of the doubled day. All later writers, including Macrobius about 430, Bede in 725, and other medieval computists (calculators of Easter) followed this rule, as did the liturgical calendar of the Roman Catholic Church until 1970.
During the late Middle Ages days in the month came to be numbered in consecutive day order. Consequently, the leap day was considered to be the last day in February in leap years, i.e. 29 February, which is its current position.
Although the new calendar was 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 by restoring the correct frequency. He also skipped several leap days in order to realign the year. Once this reform was complete—after AD 8 at the latest—the Roman calendar was the same as the Julian proleptic calendar.[7]
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, on the same material used by Scaliger, 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 25 February (a.d. V Kal. Mar.) AD 4. On Radke's solution, the two calendars were aligned on 25 February 1 BC.[8]
In 1999, an Egyptian papyrus was published that gives an ephemeris table for 24 BC with both Roman and Egyptian dates.[9] The Roman dates are not aligned with any of these solutions. One suggested resolution of this problem is a fifth triennial sequence: 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.[10] On this sequence the standard Julian leap year sequence began in AD 4, the 12th year of the Augustan reform, and the Roman calendar was finally aligned to the Julian calendar in 1 BC (with AD 1 the first full year of alignment), as in Radke's model. In it, the Roman year also coincided with the proleptic Julian year between 32 and 26 BC, suggesting 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. It also places Roman dates between 45 and 32 BC as 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.