The Roman Calendar

Author: Bolton, R., (2017), “The Roman Calendar”, first published in The Imperial Courier, Volume 12, Issue 9, THE RMRS, pp. 4.

Introduction The most well-known Roman calendar is of course the Julian calendar, introduced by Gaius Julius Caesar himself in 45 BC. This calendar survived for over 1,500 years before being replaced by the subtly modified Gregorian calendar. But what preceded the Julian calendar?

Early days It seems that the early Roman calendar consisted of ten months of either 30 days or 31 days with a total of 304 days. The difference to an actual year was taken up by an unorganised winter period, although it is possible that this gap was filled by “intercalation”, the process of inserting extra days or months to bring the year up to the proper length.

The year began in March (Martius, named for the god Mars) and continued to December (the tenth month). It is this ten-month year that gave rise to the Roman names for the fifth to tenth months inclusive. Four of these names (September to December) survive in our modern calendar, with Quintilis and Sextilis having been renamed in honour of Julius Caesar and Augustus respectively (July and August, obviously).

Whatever next? By the later republican era the calendar had been reformed into a twelve-month year, with the months being 28, 29 or 31 days long (see table for details). This gave a 355-day year. The shortfall was made up by an “intercalary” month inserted every other year. The additional month alternated between 22 and 23 days long, so over a four year cycle the average year length was (355 x 4 + 22 + 23) / 4 = 366.25 days. Somewhat bizarrely the intercalary month, Mercedonius, was not inserted between two months. Instead it was inserted after the 23rd day of February. Even more confusingly, the last five days of February proper (i.e. after the intercalary month) were then treated as belonging to Mercedonius, not to February. So, in an intercalary year February was reduced to 23 days and was followed by Mercedonius which was 27 or 28 days long.

There were two significant problems with this system. Firstly, the year was just over a day longer than an actual (tropical) year. Macrobius, writing in the early 5th-century AD, set out a system to resolve this using a 24-year cycle. This had two 8-year periods containing the four 'standard' intercalary months (alternating between 22 days and 23 days as above). The third 8-year period contained only three intercalary months all of 22 days. This gave an average year length of 365.25 days over the 24-year cycle. It is highly unlikely that this system was ever implemented, however.

The second problem was that the pontifices were responsible for inserting intercalary months. As politicians they often omitted them to shorten the term of office of their opponents or inserted extra months to favour their allies. In addition, the decision to insert intercalary months was often taken late which only added to the confusion. The worst example of this occurred during the years of civil war prior to the Julian reform. For example, the year 46 BC, the last before the Julian calendar was introduced, included three intercalary months to bring the calendar back into line with the seasons. With a total of 445 days it was known, appropriately, as the “last year of confusion”. The Julian calendar, being fixed, brought this confusion to an end in 45 BC.

How long was a year? All these adaptations to match the calendar year the seasonal one meant that:

• On average the Republican year was 366.25 days long (ignoring Macrobius's 24-year cycle).

• The Julian year averaged 365.25 days or 11 minutes 15 seconds too long.

• The modified Gregorian year was on average 365.2425 days, albeit still 27 seconds too long.

• And the actual (tropical) year is calculated to be 365.24219 days (365 days 5 hours 48 minutes 45 seconds).