Julian calendar

The Julian calendar was introduced by Julius Caesar in 46 BC, taking force in 45 BC or 709 ab urbe condita. It was chosen after consultation with the Alexandrian astronomer Sosigenes 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 every four years, hence the average Julian year is 365.25 days. The calendar remained in use into the 20th century in some countries and is still used by many national Orthodox churches. However with this scheme too many leap days are added with respect to the astronomical seasons, which on average occur earlier in the calendar by about 11 minutes per year, causing it to gain a day about every 128 years. It is said that Caesar was aware of the discrepancy, but felt it was of little importance. In the 16th century the Gregorian calendar reform was introduced to improve its accuracy with respect to the time of the vernal equinox and the synodic month (for Easter). Sometimes the reference Old Style (OS), as opposed to New Style (NS) for the Gregorian Calendar, is used when there is a confusion about which date is found in a text.

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. 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.

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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.

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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.

Related Topics:
Kepler - Dio Cassius

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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 1 August 30 BC, were unaffected by his correction.

Related Topics:
Papyrus - Ephemeris - 1 August - 30

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Roman dates before 32 BC were 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 actually fell on 31 December 46 BC (Julian date). A curious effect of this is that Caesar's assassination on the Ides (15th day) of March in 44 BC fell on 14 March 44 BC in the Julian calendar.

Related Topics:
1 January - 31 December - 46 - 14 March - 44

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