The Roman Calendar

Before the Julian Reform

Year Lengths

In the time before the introduction by Caesar of the Julian calendar the Roman calendar was in almost complete disorder. The calendar in use was mainly solar, but the leap year rule was similar to that of a lunisolar calendar as there was a whole month intercalated in leap years. Every other year of a four year cycle were supposed to be leap years, giving four years a total length of 355 + 378 + 355 + 377 = 1465 days, which was in fact about four days longer than four tropical years (1460.969 days). The leap month, designated Intercalaris, was inserted after February, the last day of which was 23 February in leap years. Intercalaris had 27 or 28 days.

Month Names and Lengths

The names, lengths, and sequence of the months in Roman common years ans leap years are shown in the follwing table.

Although numbering the days sequentially seems to be the easiest way to designate days within a month, the Romans used an intricated pattern to name their days. There were some special days, called Kalendae, Nonae, and Idus. The first day of each month was called Kalendae, or calends, while the fifth day or, in months of 31 days, the seventh day, was called Nonae, or nones. Finally the 13th or 15th day, respectively, was named Idus, or ides, marking more or less the middle of the month. In between these kind of lables, the Romans counted the days backwards, including the "lable days". With this system, the second day of Ianuarius was designated "day IIII before the nones of Ianuarius" (ANTE DIEM IIII NONAS IANVARIAS). In addition, the day preceeding calends, nones, or ides was simply called "pridie" instead of "day II before ...".

With all these rules, a common year of the Roman calendar looked like this.

In a leap year, the month Intercalaris began with the day after 23 February, the latter day having been called Terminalia. Some sources say, that Intercalaris had only 22 or 23 days and was inserted between 23 February and 24 February, but that seems unlikely, as the days after the ides of Intercalaris were to be referred to as days "before day VI before the calends of march". More likely, the days after the ides of February were counted as shown in the table below, with the days of Intercalaris counted according to the other months.

Numbering Years

During the Roman Republic the years were designated by the names of the consuls then in office(1). Even if a consul left office before the regular end of his term - throug death or resignation - the remaining year kept its designation. Though seldom during the republican period, resignations became frequent in the Empire when the consuls had lost most of their power in the Empire. The short terms of some consuls were commented with the question "Under which consuls was he consul?" After the division of the Empire into East and West each part nominated one consul. These candidates were not always agreed upon, leaving the office vacant. In such cases, years were called "year ... after the consulate of ... " The earliest more or less reliable lists of consuls date from the third century BCE.

In 537, emperor Justinian abolished consular years and introduced regnal years. Beginning with Justinian's successor, Justin II., each emperor took the office of consul on 1 January of the year following the year of accession. The years were then counted as "post-consular" years.

For chronological purposes the era of the "founding of the city" (i. e. Rome) was used from about the first century CE on. According to Marcus Terrentius Varro that era was supposed to start in 753 BCE. But another era, starting a year later, was also used sometimes.

Problems of the Calendar

With the calendar year one day longer than the tropical year, the difference between the calendar and the seasons soon became obvious. In trying to keep the calendar aligned with the sun's movement deliberate intercalations were carried out. In some years, even until the ides of February one did not know whether the current year should stay a common year or become a leap year. In this case no one knew when the next calends (of Intercalaris or of march) took place. So the days after the ides of February were counted backwards to the Terminalia (23 February), 20 Februar becoming ANTE DIEM IIII (quartum) TERMINALIA. The insertion of 27 or 28 days in leap years made it impossible to keep the calendar synchronized with the moon's phases, too.

There have been several laws trying to find a leap year pattern keeping the calendar aligned to the seasons, but eventually intercalations were made by the pontifices whithout a fixed pattern at all. The pontifices were allowed to intercalate at will, and sometimes Intercalaris was inserted for political reasons.

During the Second Punic War no intercalations had been made which led to the calendar being almost four months ahead of the sun after the war. In 168 BCE the difference was still 72 days and by the time of the introduction of the Julian calendar the Roman calendar was 90 days ahead of the sun.

Calendar Reform

The old Roman calendar was replaced by the Julian calendar worked out by the astronomer Sosigenes from Alexandria. Its leap year rule, eras, and beginnings of the year are described in the Julian calendar section. What's left to mention is the adaption of the designation of days of the old Roman calendar to the new Julian calendar. The days were designated according to the system described above, but, with some months lengthened by one or two days, the numbers of the days after the ides changed a bit. In addition, instead of a whole month, a single day had to be added in a leap year. This day, for some reason, was placed after 24 February, which the Romans called ANTE DIEM VI KALENDAS MARTIAS. The leap day was then referred to as ANTE DIEM BIS VI KALENDAS MARTIAS, or second day VI before the calends of March(2).

The new leap year rule was misinterpreted by the pontifices, which intercalated every three years rather than every four years. Caesars successor, Augustus, corrected the calendar by not intercalating until the error was eliminated. From 8 CE on the Julian calendar was used according to the rules defined by Sosigenes.

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