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.

Length of the months

According to the 13th century scholar 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:

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:30, 29, 30, 29, 30, 29, 30, 29, 30, 29, 30, and 29, totaling 354 days.

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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:

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:31, 29 (30), 31, 30, 31, 30, 31, 30, 31, 30, 31, and 30

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He then said Augustus changed this to:

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:31, 28 (29), 31, 30, 31, 30, 31, 31, 30, 31, 30, and 31

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giving us the irregular month lengths which we still use today, so that the length of Augustus would not be shorter than (and therefore inferior to) the length of Iulius.

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Although this theory is still widely repeated, Sacrobosco is almost certainly wrong. First of all, a wall painting of a Roman Republican calendar has survived http://www.personal.psu.edu/users/w/x/wxk116/RomanCalendar/Fasti4.gif

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which confirms the literary accounts that the months were already irregular before Julius Caesar reformed it:

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:29, 28, 31, 29, 31, 29, 31, 29, 29, 31, 29, and 29

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Also, one thing that was not changed by the switch from the old Roman calendar to the new Julian calendar was the dates of the Nones and Ides. In particular, the Ides are late (on the 15th rather than 13th) in March, May, July and October. This suggests that these months always had 31 days in the Julian calendar. Further, Sacrobosco's theory is explicitly contradicted by the third and fifth century authors Censorinus and Macrobius, and, finally, it is inconsistent with seasonal lengths given by Varro, writing in 37 BC, before the Augustan reform, and with the 31-day Sextilis given by the new Egyptian papyrus from 24 BC.

Related Topics:
Censorinus - Macrobius

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