Julian calendar

• 1. Origins And Motivation

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  Julius Caesar returned to Rome in 46 BC after years of civil war and political maneuvering. He found the Roman calendar in a state of chaos due to political abuse by the pontifices. These priests controlled the insertion of intercalary months, often extending years when their allies held office or shortening them when opponents were in power. The ordinary year consisted of 355 days with an occasional 27-day Mensis Intercalaris inserted between February and March. This system allowed for roughly 380 days in some years but lacked consistency. Caesar sought to solve this problem permanently by creating a calendar aligned to the sun without human intervention. Plutarch records that he called in the best philosophers and mathematicians of his time to fix the issue. Sosigenes of Alexandria served as the principal designer of this reform. The new calendar combined old Roman months with the fixed length of the Egyptian calendar and Greek astronomical knowledge. Varro used the reformed calendar in 37 BC to fix dates for the start of four seasons. A century later, Pliny dated the winter solstice to the 25th of December because the sun entered the eighth degree of Capricorn on that date.

• 2. Implementation Mechanics

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  The first step of the reform was to realign the start of the calendar year to the 1st of January. This required making 46 BC a massive 445 days long to compensate for missed intercalations during Caesar's pontificate. The year had already been extended from 355 to 378 days by inserting a regular intercalary month in February. Caesar added another 67 days by inserting two extraordinary intercalary months between November and December. Cicero referred to these months as Intercalaris Prior and Intercalaris Posterior in letters written at the time. These months are sometimes incorrectly called Undecimber and Duodecimber, terms that arose over a millennium after the Roman Empire collapsed. The new calendar began operation after the realignment completed in 45 BC. The Julian months were formed by adding ten days to a regular pre-Julian Roman year of 355 days. Two extra days went to January, Sextilis, and December while one extra day went to April, June, September, and November. February remained unchanged at 28 days in ordinary years. The inserted days were all initially characterized as dies fasti. Macrobius states that extra days were added immediately before the last day of each month to avoid disturbing religious ceremonies relative to the Nones and Ides.

• 3. Drift And Discrepancy

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  The Julian calendar has an average length of 365.25 days due to its simple cycle of three normal years followed by a leap year. This exceeds the actual tropical solar year value of approximately 365.2419 days. Consequently, the calendar gains about three days every four centuries compared to observed equinox times. Greek astronomers like Hipparchus knew the tropical year was slightly shorter than 365.25 days a century before the reform but the calendar did not compensate for this difference. By 1582, the Julian calendar had gained a full day every 128 years against the natural solar year. The astronomical solstices and equinoxes advanced by 10.8 minutes per year against the Julian calendar year. As a result, the 21st of March moved ten days out of alignment with the March equinox by the time Pope Gregory XIII promulgated his reform. The discrepancy accumulated significantly over time even though it felt of little importance at the start in 46 BC. The Gregorian calendar modified the Julian rule by eliminating occasional leap days to reduce the average length from 365.25 days to 365.2425 days.

• 4. Augustan Correction

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  The pontifices initially added a leap day every three years instead of every four after Caesar's death. Solinus, Pliny, Ammianus, Suetonius, and Censorinus all provide accounts of this error. Macrobius records that priests inserted the first intercalation in February 42 BC when Caesar intended it for February 41 BC. They made eleven further intercalations after 42 BC at three-year intervals so that the twelfth fell in 9 BC. This created twelve years without leap years between 9 BC and AD 4. Emperor Augustus intervened to fix the triennial leap year error and restore the four-year cycle by AD 8. An inscription discovered in 1999 confirms that Egyptian astronomers used the correct Julian calendar in 24 BC. Alexander Jones concluded that the dates in the Alexandrian and Julian calendars were in one-to-one correspondence except for specific periods surrounding leap years. The reform ensured that the next leap year occurred in AD 8 and then followed every fourth year thereafter. The system of Scaliger, Ideler, and Bünting shows that leap years prior to suspension were BC years divisible by three while those after resumed as AD years divisible by four.

• 5. Naming Conventions

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  The Romans later renamed months after Julius Caesar and Augustus. Quintilis became Iulius (July) in 44 BC because it was the month of his birth. Sextilis became Augustus (August) in 8 BC to honor events in his rise to power including the fall of Alexandria. Caligula renamed September as Germanicus in AD 37 after his father. Nero renamed April as Neroneus, May as Claudius, and June as Germanicus in AD 65. Domitian renamed September as Germanicus and October as Domitianus in AD 84. Commodus uniquely renamed all twelve months after his own adopted names like Amazonius and Invictus. Tiberius rejected a senatorial proposal to rename September as Tiberius and October as Livius. Charlemagne introduced lasting Old High German names agriculturally in German according to Einhard. These included Wintarmanoth for winter month and Ostarmanoth for Easter month. These names continued with modifications until the late 18th century over 700 years after his rule. Eastern European languages retained older seasonal names into the 19th century including Belarusian, Bulgarian, Croatian, Czech, Finnish, Georgian, Lithuanian, Macedonian, Polish, Romanian, Slovene, and Ukrainian.

• 6. Global Adoption Patterns

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  The Julian calendar spread across Europe, Russia, and the Ottoman Empire alongside varying New Year dates. The Alexandrian calendar in Egypt started on the 29th of August while several local provincial calendars began on Augustus's birthday the 23rd of September. The Byzantine year using the Julian calendar began on the 1st of September due to the indiction. Vladimir I of Kiev adopted the Julian calendar in AD 988 numbering the year Anno Mundi 6496 beginning on the 1st of March. Ivan III realigned the start of the year to the 1st of September in 1492 so that AM 7000 lasted only six months from the 1st of March to the 31st of August. In Anglo-Saxon England the year most commonly began on the 25th of December which marked the start of the year in pagan times. From 1087 to 1155 the English year began on the 1st of January and from 1155 to 1751 it began on the 25th of March. Spain and Portugal adopted the Gregorian calendar in 1556 while Prussia and Denmark-Norway did so in 1559. Sweden delayed adoption until 1753 and Scotland until 1752. Russia finally switched to the Gregorian calendar in 1918 after using the Julian system for centuries.

• 7. Gregorian Replacement

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  Pope Gregory XIII promulgated the Gregorian calendar in 1582 to correct accumulated drift against the solar year. The reform eliminated occasional leap days to reduce the average length of the calendar year from 365.25 days to 365.2425 days. Most Catholic countries adopted the new calendar immediately while Protestant countries followed slowly over two centuries. Greece was the last country to adopt the Gregorian calendar as its civil calendar in 1923. Eastern Orthodox churches continue to use the Julian calendar for religious purposes but many adopted the Gregorian as their civil calendar in the early twentieth century. The Russian Revolution of the 7th of November 1917 New Style is known as the October Revolution because it began on the 25th of October Old Style. Dual dating became common during transition periods with O.S. or N.S. suffixes clarifying which system applied. The Revised Julian calendar was endorsed by a synod in Constantinople in May 1923 but all Orthodox churches refused to accept its lunar part so they continued celebrating Easter according to the Julian tradition. The Armenian Patriarchate of Jerusalem uses the Julian calendar while the rest of the Armenian Church uses the Gregorian calendar.