Julian calendar

The Julian calendar is a reform of the Roman calendar introduced by Julius Caesar in 46 BC (708 AUC). It took effect the following year, 45 BC (709 AUC). The calendar has a regular year of 365 days divided into 12 months, as listed in Table of months. A leap day is added to February every four years. The Julian year is, therefore, on average 365.25 days long.

The calendar year was probably intended to approximate the tropical (solar) year. Although Greek astronomers had known, at least since Hipparchus, that the tropical year was a few minutes shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gained about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was corrected by the Gregorian reform, introduced in 1582.

The calendar was used as the civil calendar in some countries into the 20th century, but has been replaced by the Gregorian calendar in all countries which formerly used it. The Roman Catholic Church, the Eastern Rite Catholic Churches, Anglican, Protestant and other churches have also replaced the Julian calendar with the Gregorian calendar. However, most branches of the Orthodox Church still use the Julian calendar for calculating the dates of moveable feasts, including Easter or Pascha. Some Orthodox churches have adopted the Revised Julian calendar for the observance of fixed feasts, while other Orthodox churches retain the Julian calendar for all purposes. The Julian calendar is still used by the Berber people of North Africa, and on Mount Athos.

Table of months

Months (Roman)	Lengths before 45 BC	Lengths as of 45 BC	Months (English)
Ianuarius	29	31	January
Februarius	28 (leap years: 23 or 24)	28 (leap years: 29)	February
Mercedonius/Intercalaris	0 (leap years: 27)	(abolished)	—
Martius	31	31	March
Aprilis	29	30	April
Maius	31	31	May
Iunius	29	30	June
Quintilis (Iulius)	31	31	July
Sextilis (Augustus)	29	31	August
September	29	30	September
October	31	31	October
November	29	30	November
December	29	31	December

Motivation

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, became 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, 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.

The reform was intended to correct this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. This feature proved useful very soon after the new calendar came into effect. Varro used it in 37 BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier. A century later, when Pliny gave the distances between the dates of the solstices and the equinoxes, he dated the winter solstice to 25 December (when the sun entered the 8th degree of Capricorn); this stability had become an ordinary fact of life.

Context of the reform

The approximation of 365¼ days for the tropical year had been known for a long time but was not used directly since ancient calendars were not solar (except in Egypt). It was the mean length of the year in the octaeteris, a cycle of 8 lunar years popularized by Cleostratus (and also commonly attributed to Eudoxus) which was used in some early Greek calendars, notably in Athens. The same value was the basis of the 76 years cycle devised by Callippus (a student under Eudoxus) to improve the Metonic cycle. The length of the year in the cycle of Meton was less accurate: about 30 minutes too long (1 day in 48 years).

In Egypt, a fixed year of 365 days was in use, drifting by one day against the sun in four years. An unsuccessful attempt to add an extra day every fourth year was made in 238 BC (Decree of Canopus). Caesar probably experienced the solar calendar in that country. He landed in the Nile delta in October 48 BC and soon became embroiled in the Ptolemaic dynastic war, especially when Cleopatra managed to be "introduced" to him in Alexandria. Caesar imposed a peace, and a banquet was held to celebrate the event. Lucan depicted Caesar talking to a wise man called Acoreus during the feast, stating his intention to create a calendar more perfect than that of Eudoxus (Eudoxus was popularly credited with having determined the length of the year to be 365¼ days). But the war soon resumed and Caesar was attacked by the Egyptian army for several months until he achieved victory. He then enjoyed a long cruise on the Nile with Cleopatra before leaving the country in June 47 BC.

Caesar returned to Rome in 46 BC and, according to Plutarch, called in the best philosophers and mathematicians of his time to solve the problem of the calendar. Pliny says that Caesar was aided in his reform by the astronomer Sosigenes of Alexandria who is generally considered the principal designer of the reform. Sosigenes may also have been the author of the astronomical almanac published by Caesar to facilitate the reform. Eventually, it was decided to establish a calendar that would be a combination between the old Roman months, the fixed length of the Egyptian calendar, and the 365¼ days of the Greek astronomy. According to Macrobius, Caesar was assisted in this by a certain Marcus Flavius.

Julian reform

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 (708 AUC) 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 "Undecember" and "Duodecember". 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, Sextilis (later changed in name to Augustus) and December, and one extra day was added to Aprilis, Iunius, September and November. Februarius was not changed in ordinary years, and so continued to be the traditional 28 days. Thus, the ordinary (i.e., non leap year) lengths of all of the months were set by the Julian calendar to the same values they still hold today. (See Debunked theory on month lengths below for stories purporting otherwise).

Macrobius states that the extra days were added immediately before the last day of each month to avoid disturbing the position of the established religious ceremonies relative to the Nones and Ides 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 2nd 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 AD 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.

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.

Leap year error

Although the new calendar was much simpler than the pre-Julian calendar, the pontifices initially 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; perhaps the earliest recorded example of a fence post error. After 36 years, this resulted in three too many leap days. Augustus remedied this discrepancy by restoring the correct frequency. He also skipped three leap days over 12 years in order to realign the year. Once this reform was complete, the Roman calendar was the same as the Julian proleptic calendar.

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. Scaliger established in 1583 that the Augustan reform was instituted in 8 BC. He inferred that leap years had occurred in every third year from 42 BC to 9 BC before the reform, and that the first reformed leap year had been AD 8. This proposal is still the most widely accepted solution, although Ideler and Mommsen, among others, argued 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.

Several other solutions have been proposed since the 16th century.

In 1590, Bunting assumed the same phase for the triennial cycle as Scaliger, but differed with him on the date of its start (45 vs 42 BC) and end (12 vs 9 BC) and on the date at which the correct cycle was resumed (AD 4 vs 8).

Other scholars proposed a different phase. Also in 1590, Christmann argued that Caesar had intended leap years to be accounted from 46 BC, the year of Caesar's decree, and not 45 BC. He inferred that leap years had occurred in every third year from 42 BC to 9 BC before the Augustan reform. By using Scaliger's reconstruction of the method for backing out the three extra leap years, he supposed that the Julian cycle had resumed in a year equivalent to AD 7, but other scholars followed the same line of argument while correcting for this. Harriot considered that the reformed cycle started in AD 4, while in 1614 Kepler discussed a resumption in AD 8. However Scaliger's solution became generally adopted.

The question was reopened in 1883, when Matzat reviewed a passage in Dio Cassius which mentions a leap day inserted in 41 BC, "contrary to the (i.e. Caesar's) rule", in order to avoid having a market day on the first day of 40 BC. Dio stated that this leap day was compensated "later". Matzat proposed this was done by omitting a scheduled leap day in 40 BC, rather than by omitting a day from an ordinary year, and inferred a solution in which leap years had occurred in every third year from 44 BC to 11 BC, resuming in AD 4. In 1889, Soltau suggested a modified version of this, in which 45 BC was a leap year and the triennial cycle ran from 41 BC to 11 BC, with leap years resuming in AD 8.

In 1960 Radke argued the reform was actually instituted when Augustus became pontifex maximus in 12 BC. This suggested a sequence of triennial leap years starting in 45 BC and ending in 12 BC, with leap years resuming in AD 4 – the same solution as the one Bunting had proposed in 1590.

In 1999, an Egyptian papyrus was published that gives an ephemeris table for 24 BC with both Roman and Egyptian dates. While the Roman dates match the proleptic Julian calendar, they do not match any previously proposed solution for the triennial cycle. One suggested resolution of this problem, which matches the data of the papyrus, is a new triennial sequence, in which the triennial leap years started in 44 BC and ended in 8 BC, with leap years resuming in AD 4. This sequence is very close to that proposed by Matzat, differing only in the date of the last triennial leap year.

The various proposed solutions are summarised in the following table. The table shows for each solution the implied proleptic Julian date for the first day of Caesar's reformed calendar (Kal. Ian. AUC 709) and the first Julian date in which the Roman calendar date matches the proleptic Julian calendar after the completion of Augustus' reform.

Scholar	Date	Triennial leap years (BC)	Leap year resumes	First Julian day	First aligned day
Scaliger	1583	42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12, 9	AD 8	2 Jan. 45 BC	25 Feb. AD 4
Bunting	1590	45, 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12	AD 4	1 Jan. 45 BC	25 Feb. 1 BC
Christmann	1590	43, 40, 37, 34, 31, 28, 25, 22, 19, 16, 13, 10	AD 7	2 Jan. 45 BC	25 Feb. AD 4
Harriot	after 1610	43, 40, 37, 34, 31, 28, 25, 22, 19, 16, 13, 10	AD 4	1 Jan. 45 BC	25 Feb. 1 BC
Kepler	1614	43, 40, 37, 34, 31, 28, 25, 22, 19, 16, 13, 10	AD 8	2 Jan. 45 BC	25 Feb. AD 4
Ideler	1825	45, 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12, 9	AD 8	1 Jan. 45 BC	25 Feb. AD 4
Matzat	1883	44, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11	AD 4	1 Jan. 45 BC	25 Feb. 1 BC
Soltau	1889	45, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11	AD 8	2 Jan. 45 BC	25 Feb. AD 4
Radke	1960	45, 42, 39, 36, 33, 30, 27, 24, 21, 18, 15, 12	AD 4	2 Jan. 45 BC	25 Feb. 1 BC
Bennett	2003	44, 41, 38, 35, 32, 29, 26, 23, 20, 17, 14, 11, 8	AD 4	31 Dec. 46 BC	25 Feb. 1 BC

Month names

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. 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.

The Romans later renamed months after Julius Caesar and Augustus, renaming Quintilis as Iulius (July) in 44 BC and Sextilis 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, occurred in that month.

Other months were renamed by other emperors, but apparently none of the later changes survived their deaths. In AD 37, Caligula renamed September as Germanicus; in AD 65, Nero renamed Aprilis (April) as Neroneus, Maius (May) as Claudius and Iunius (June) as Germanicus; and in AD 84 Domitian renamed September as Germanicus and October as Domitianus. 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. The emperor Tacitus is said to have ordered that September, the month of his birth and accession, be renamed after him, but the story is doubtful since he did not become emperor before November 275. Other name changes were proposed but were never implemented. Tiberius rejected a senatorial proposal to rename September as Tiberius and October as Livius, after his mother Livia. Antoninus Pius rejected a senatorial decree renaming September as Antoninus and November as Faustina, after his empress. Similar honorific months were implemented in many of the provincial calendars that were aligned to the Julian calendar.

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, over 700 years after his rule, and continued with some modifications until the late 18th century in Germany and in the Netherlands. The names (January to December) were: 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).

The calendar month names used in western and northern Europe, in Byzantium, and by the Berbers, were derived from the Latin names. However, in eastern Europe older seasonal month names continued to be used into the 19th century, and in some cases are still in use, in many languages, including: Belarusian, Bulgarian, Croatian, Czech, Finnish, Georgian, Lithuanian, Macedonian, Polish, Romanian, Slovene, Ukrainian. When the Ottoman empire adopted the Rumi calendar, the month names reflected Ottoman tradition.

Year numbering

The principal 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, the start of the calendar year, and Julius Caesar did not change the beginning of either year. Thus this consular year was an eponymous or named year. In addition to consular years, the Romans sometimes used the regnal year of the emperor, and by the late 4th 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 and his regnal year, in addition to the indiction and the consul, while also allowing the use of local eras.

In 309 and 310, and from time to time thereafter, no consuls were appointed. When this happened, the consular date was given a count of years since the last consul (so-called "post-consular" dating). After 541, only the reigning emperor held the consulate, typically for only one year in his reign, and so post-consular dating became the norm. Similar post-consular dates were also known in the West in the early 6th century. The system of consular dating, long obsolete, was formally abolished in the law code of Leo VI, issued in 888.

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 3rd 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.

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 local eras for the Julian calendar or its local equivalent, 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 associated with the Alexandrian calendar and often used by the Alexandrian Christians to number their Easters during the 4th and 5th centuries, and continues to be used by the Coptic and Ethiopian churches, as well as influencing the modern Ethiopian calendar.

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 Etos Kosmou, used throughout the Byzantine world from the 10th century and in Russia until 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.

The Julian calendar was also used in some Muslim countries. The Rumi calendar, the Julian calendar used in the later years of the Ottoman Empire, adopted an era derived from the lunar AH year equivalent to AD 1840, i.e. the effective Rumi epoch was AD 585. In recent years, some users of the Berber calendar have adopted an era starting in 950 BC, the approximate date that the Libyan pharoah Sheshonq I came to power in Egypt.

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 local provincial calendars were aligned to start on the birthday of Augustus, 23 September. The indiction caused the Byzantine year, which used the Julian calendar, to begin on 1 September; this date is still used in the Eastern Orthodox Church for the beginning of the liturgical year. When the Julian calendar was adopted in AD 988 by Vladimir I of Kiev, the year was numbered Anno Mundi 6496, beginning on 1 March, six months after the start of the Byzantine Anno Mundi year with the same number. In 1492 (AM 7000), Ivan III, according to church tradition, realigned the start of the year to 1 September, so that AM 7000 only lasted for six months in Russia, from 1 March to 31 August 1492.

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, approximating the vernal equinox), or even Easter, as in France (see the Liturgical year article for more details).

In England, even before 1752, 1 January was sometimes treated as the start of the new year – for example by Pepys – while the "year starting 25th March was called the Civil or Legal Year". To reduce misunderstandings on the date, it was not uncommon in parish registers for a new year heading after 24 March, for example 1661, to have another heading at the end of the following December indicating "1661/62". This was to explain to the reader that the year was 1661 Old Style and 1662 New Style.

Most Western European countries 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 16th century. The following table shows the years in which various countries adopted 1 January as the start of the year. Eastern European countries, with populations showing allegiance to the Orthodox Church, began the year on 1 September from about 988. The Rumi calendar used in the Ottoman empire began the year on 1 March.

Country	Year starting 1 January	Adoption of new calendar
Republic of Venice	1522	1582
Holy Roman Empire	1544	1582
Spain, Portugal	1556	1582
Prussia, Denmark/Norway	1559	1700
Sweden	1559	1753
France	1564	1582
Southern Netherlands	1576	1582
Lorraine	1579	1760
Holland, Zeeland	1583	1582
Dutch Republic except Holland and Zeeland	1583	1700
Scotland	1600	1752
Russia	1700	1918
Tuscany	1721	1750
British Empire excluding Scotland	1752	1752
Serbia	1804[citation needed]	1918
Ottoman Empire (Turkey)	1918	1926

Debunked theory on month lengths

The Julian reform set the lengths of the months to their modern values. However, a 13th century scholar, Sacrobosco, proposed a different explanation for the lengths of Julian months 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 3rd and 5th 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.

From Julian to Gregorian

Main article: Gregorian calendar

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. 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. As a result, the calculated date of Easter gradually moved out of phase with the March equinox. 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 at the time of the Julian reform. However, it accumulated significantly over time: the Julian calendar gained a day about every 134 years. By 1582, it was ten days out of alignment from where it supposedly was in 325 during the Council of Nicaea.

The Gregorian calendar was soon adopted by most Catholic countries (e.g. Spain, Portugal, Poland, most of Italy). Protestant countries followed later, and the countries of Eastern Europe adopted the "new calendar" 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 Gregorian calendar in 1753, but Russia remained on the Julian calendar until 1918 (1 February became 14 February), after the Russian Revolution (which is thus called the "October Revolution" though it occurred in Gregorian November), while Greece continued to use it until 1923 and Turkey till 1926.

Since the Julian and Gregorian calendars were long used simultaneously, although in different places, calendar dates in the transition period are often ambiguous, unless it is specified which calendar was being used. In some circumstances, double dates might be used, one in each calendar. 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 to clarify dates from countries which continued to use the Julian calendar after the Gregorian reform, such as Great Britain, which did not switch to the reformed calendar until 1752, or Russia, which did not switch until 1918.

Throughout the long transition period, the Julian calendar has continued to diverge from the Gregorian. This has happened in whole-day steps, as dropped leap-years on certain centennial years in the Gregorian calendar continued to be leap years in the Julian calendar. Thus, in the year 1700 (after Feb. 29, 1700) the difference increased to 11 days; in 1800, 12; and in 1900, 13. Since 2000 was a leap year according to the Gregorian calendar, the Julian calendar remained in step with it: 29 February 2000 (Julian) fell on 13 March (Gregorian). This difference will persist through the last day of February, 2100 (Julian), which is not a Gregorian leap year, but is a Julian leap year. Monday 1 March 2100 (Julian) falls on Monday 15 March 2100 (Gregorian), a full two-week discrepancy.

Eastern Orthodox usage

Russian icon of the Theophany (6 January), the highest-ranked feast which occurs on the fixed cycle of the Eastern Orthodox liturgical calendar.

Although all Eastern Orthodox countries (most of them in Eastern or Southeastern Europe) had adopted the Gregorian calendar (or in the case of Greece, possibly the Revised Julian calendar) by 1927, their national churches had not. The "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 Pascha (Easter) astronomically at Jerusalem. All Orthodox churches refused to accept the lunar part, so almost all Orthodox churches continue to celebrate Pascha according to the Julian calendar (with the exception of the Estonian Orthodox Church and the Finnish Orthodox Church).

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, Serbia, Macedonia, Georgia, Ukraine, and the Greek Old Calendarists and other groups continue to use the Julian calendar, thus they celebrate the Nativity on 25 December Julian (which is 7 January Gregorian until 2100). The Russian Orthodox Church has some parishes in the West which celebrate the Nativity on 25 December Gregorian. Parishes of the Orthodox Church in America Bulgarian Diocese, both before and after the 1976 transfer of that diocese from the Russian Orthodox Church Outside Russia to the Orthodox Church in America, were permitted to use the 25 December Gregorian date. Some Old Calendarist groups which stand in opposition to the state churches of their homelands will use the Great Feast of the Theophany (6 January Julian/19 January Gregorian) as a day for religious processions and the Great Blessing of Waters, to publicize their cause.^{[citation needed]}

The Oriental Orthodox Churches generally use the local calendar of their homelands. However, when calculating the Nativity Feast, most observe the Julian calendar. This was traditionally for the sake of unity throughout Christendom. In the West, some Oriental Orthodox Churches either use the Gregorian calendar or are permitted to observe the Nativity according to it. The Armenian Apostolic Orthodox Church celebrates the Nativity as part of the Feast of Theophany according to its traditional calendar.^{[citation needed]}

See also

• Berber calendar
• Computus
• Conversion between Julian and Gregorian calendars
• Dominical letter
• Easter
• Julian day
• Julian year (astronomy)
• Mixed-style date
• Proleptic Gregorian calendar
• Proleptic Julian calendar
• Revised Julian calendar
• Rumi calendar
• Week

Bibliography

• Blackburn, Bonnie; Holford-Strevens, Leofranc (2003, reprinted with corrections). The Oxford Companion to the Year. Oxford University Press. 
• Nautical Almanac Offices of the United Kingdom and the United States of America (1961). Explanatory Supplement to the Astronomical Ephemeris and the American Ephemeris and Nautical Almanac. London: Her Majesty's Stationery Office. 

External links

• 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