Julian Calendar (46 BC): How Caesar Reformed the Roman Year

The Julian calendar was the reform of the Roman calendar introduced by Julius Caesar in 46 BC and in force from 1 January 45 BC. It replaced a confused lunar-based system with a solar year of 365 days and an extra day every fourth year. With one later correction, it remained the calendar of Europe for more than 1,600 years and is still used by several Orthodox churches today.

The old Roman calendar

Before Caesar, the Roman year had 355 days, divided into twelve months of 29, 31 or 28 days. Since the solar year is about 365 and a quarter days, the calendar fell behind the seasons by roughly ten days a year. To correct this, an extra month of 22 or 23 days, called intercalaris or Mercedonius, was inserted in February in some years.

The decision to add this month lay with the college of pontiffs, Rome’s senior priests. Because the length of the year affected the terms of magistrates and governors and the dates on which debts and contracts fell due, intercalation became a political weapon. Pontiffs could lengthen the year of an ally or shorten that of an opponent.

During the turbulent decades of the late Republic, and especially during the civil war that began when Caesar crossed the Rubicon in 49 BC, intercalation was often neglected. By the 40s BC the official calendar was running about three months ahead of the solar year, so that festivals meant for the harvest fell in the wrong season.

Caesar and Sosigenes

Caesar had been pontifex maximus, head of the college of pontiffs, since 63 BC, so the calendar was formally his responsibility. After his victory at the Battle of Thapsus in April 46 BC he returned to Rome with the power to carry out wide reforms, and the calendar was among the most lasting of them.

According to Pliny the Elder, Caesar relied on the advice of Sosigenes, an astronomer from Alexandria. Egypt had long used a civil year of 365 days, and Greek astronomers there knew that the solar year was close to 365 and a quarter days. In 238 BC the Ptolemaic king Ptolemy III had even ordered a leap day every four years in Egypt, though the change did not take hold. Caesar had spent many months in Egypt in 48 and 47 BC, and the Egyptian model clearly influenced the reform.

The year of confusion

Before the new calendar could start, the year had to be realigned with the seasons. Caesar therefore made 46 BC exceptionally long. As well as the regular intercalary month of 23 days already due in February, he inserted two extra months, totalling 67 days, between November and December.

The result was a year of 445 days, according to the third-century writer Censorinus. Macrobius later called it the “last year of confusion”, meaning the final year of the old disorder rather than a year of chaos in itself. On 1 January 45 BC the new calendar came into force, with the start of the year falling roughly where it should in relation to the winter solstice.

How the new calendar worked

Caesar kept the twelve Roman months and their traditional names but added ten days to the year to make 365. Two days each went to January, Sextilis (later August) and December, and one day each to April, June, September and November. The result was the pattern of month lengths still used today: 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31.

Every fourth year an extra day was added in February. The Romans did not number days consecutively but counted backwards from fixed points in the month. The leap day was inserted by repeating the sixth day before the Kalends (first day) of March, roughly our 24 February. Because this day was counted twice, a leap year came to be called bissextile, from the Latin for “twice sixth”.

The new calendar kept 1 January as the start of the civil year, the day on which Roman consuls had taken office since 153 BC. Religious festivals kept their traditional dates, which now stayed in their proper seasons.

Early errors and Augustus’ correction

After Caesar’s assassination in March 44 BC, the pontiffs misapplied the leap-year rule. Counting inclusively, as Romans often did, they added a leap day every third year instead of every fourth. Over about 36 years this produced three more leap days than intended.

Augustus corrected the error by suspending leap years for a period, according to Macrobius, until the excess had been absorbed. The exact years of the suspension are debated by scholars, but the calendar was working as Caesar intended by the early first century AD.

The months also gained new names in this period. In 44 BC Quintilis, the month of Caesar’s birth, was renamed Iulius, giving us July. In 8 BC Sextilis was renamed Augustus in honour of the emperor, giving us August.

A small error that grew

The Julian year averages 365.25 days, but the solar year, measured from one spring equinox to the next, is about 365.2422 days. The difference of roughly 11 minutes a year meant that the calendar slipped by about one day every 128 years.

By the 16th century the spring equinox fell around 11 March instead of 21 March, the date assumed by the Church when calculating Easter. In 1582 Pope Gregory XIII introduced the Gregorian calendar, which dropped ten days and refined the leap-year rule so that century years are leap years only if divisible by 400.

Spain, Portugal and most Italian states adopted the change at once and other Catholic states within a few years, but Protestant and Orthodox countries followed slowly. Britain and its colonies switched in 1752, omitting eleven days in September. Russia changed only in 1918 and Greece in 1923. Several Orthodox churches still follow the Julian calendar for their festivals, which is why their Christmas falls on 7 January in the Gregorian reckoning.

Myths and facts

A popular story claims that Augustus took a day from February and added it to August, so that his month would be as long as Julius Caesar’s July. There is no ancient evidence for this. The tale is usually traced to the 13th-century scholar Johannes de Sacrobosco, and Macrobius’ ancient description of Caesar’s reform shows that Sextilis, the future August, already received 31 days in 46 BC.

Another common assumption is that the Julian calendar introduced the numbering of years from the birth of Christ. In fact the Romans usually identified years by the names of the two consuls in office, and sometimes counted from the legendary founding of Rome. The Anno Domini system was devised by the monk Dionysius Exiguus in AD 525 and spread slowly over the following centuries, long after Caesar’s reform.

Finally, the Julian calendar is sometimes confused with the Julian day count used by astronomers. That is a separate system for numbering days continuously, proposed in 1583 by the scholar Joseph Scaliger, and it has nothing to do with Caesar’s reform beyond its use of the Julian year.

Timeline

• 63 BC: Caesar is elected pontifex maximus.
• 46 BC: Caesar’s reform; the “year of confusion” is lengthened to 445 days.
• 45 BC, 1 January: The Julian calendar comes into force.
• 44 BC: The month Quintilis is renamed Iulius (July) in Caesar’s honour.
• 8 BC: The month Sextilis is renamed Augustus (August).
• 1582, October: The Gregorian calendar replaces the Julian in Catholic countries; 4 October is followed by 15 October.
• 1752, September: Britain and its colonies adopt the Gregorian calendar.
• 1918: Soviet Russia adopts the Gregorian calendar.

Why it matters

The Julian calendar gave the Roman world a stable, predictable year that was no longer at the mercy of priests and politicians. Its months, their lengths and the idea of a leap year every four years survive almost unchanged in the calendar used worldwide today, making it one of Caesar’s most enduring achievements.