Chronologies and Julian days

Joseph Justus Scaliger

One of the fundamental concerns of civilized man is the study of the past, natural or human. For this, it is necessary to be able to fix the date on which each event took place. This is the goal of a science called chronology.

If the exact date of an event is known, we can define it by giving the day, month and year in which it took place. For example, we can say that the Second World War began on September 1, 1939. We have no problem with the day and the month, but how are the years numbered? Obviously we must take an origin or starting point that everybody will agree to use.

This dating system causes a curious effect: the numbers assigned to the years after the origin grow towards the future, while the previous years grow towards the past. Thus, the year 2000 of our era came after the year 1000, but the year 2000 B.C.E. came before 1000 B.C.E. (see below the meaning of these acronyms). The years prior to the origin work as negative numbers.

The Roman calendar

Commemorative coin in honor of
Numa Pompilius

According to Plutarch, the Roman calendar was established by the second king of Rome, Numa Pompilius (753-674 BC), who at first divided the year into ten months, beginning in March, and gave numerical names to the fifth to tenth months, but later added two extra months (January and February), and moved the beginning of the year to January 1st. The months of the early Roman calendar, therefore, were these: Ianuarius, Februarius, Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November and December. It will be noted that, by adding two months at the beginning, the numbers of the fifth to tenth months became seventh to twelfth, but the names were already fixed and nobody bothered to correct them and adapt to the new situation. Plutarch comments on the origin of the month names:

The first month, consecrated by Romulus to Mars, was called Martius, and the second Aprilis, named after Aphrodite, who is Venus, because in this month sacrifices are made to this Goddess... The next month is called Maius, after Maia, as it is devoted to Mercury [son of Maia]; and Iunius is named after the goddess Juno. But there are some who argue that they take their denomination from the oldest and the youngest; because the eldest are called maiores, and the youngest iuniores... The first, Ianuarius, comes from Janus [the god of the doors].

The Roman months were lunar, alternating 28 and 29 days. As twelve lunar months fall short of the year by more than 11 days, from time to time an additional month was added (the thirteenth month), but a regular system was not established for the addition, as they did in Babylon and Greece. The decision to add the additional month was taken by the pontifex maximus, the main religious authority. But this position was political and fell under the party game, which was especially virulent in the last years of the republic. As the political magistracies lasted a year, the additional month was inserted when the pontifex wished to prolong the government of the party holding power, and omitted it when the magistrates belonged to the opposite party. The result was chaotic. By mid-first century B.C., the total error amounted to eighty days, almost a season.