Last week’s Vocabula Mira post was about the intercalary month called “Mercedonius”, which the Romans irregularly added after February to bring their very ancient lunar calendar into line with the solar year. This month was abolished in the reform instituted by Julius Caesar in 46 B.C., which established a regular leap day every four years; however, Caesar’s leap day was added according to a different system from the one we use today.

In the Roman calendar, each month has three days which are called the Kalends, Nones, and Ides; the first of these three is the first day of each month. In March, May, July, and October, the Nones are on the 7th, and the Ides on the 15th; in all other months, they are on the 5th and 13th. These designations probably arose, like most features of most calendars, from some sort of religious observances fixed to those days, perhaps connected to the primitive lunar calendar, but we know nothing for certain about their origin.

The Romans named the days of each month by counting backward from these three points. Thus, Caesar was killed on the day which we call March 15, but which they called “the Ides of March”; their name for the 14th was therefore “the day before the Ides of March.” As every Latin student knows, this system becomes difficult to keep track of because the Romans counted inclusively, not exclusively; therefore, the day we call “March 13” was called “three days before the Ides of March” (not “two days before”), including the day itself, the day before the Ides, and the Ides themselves. We can only assume that this is not an example of complexity created for complexity’s sake, and that it served as a way of preparing for whatever religious observances were connected to the three points.

When the Julian Calendar was instituted, the leap day was added by counting February 24th, “the sixth day before the Kalends of March”, twice. From this, the Latin term for “leap year” is “annus bisextilis”, meaning “a year in which the sixth day (before the Kalends of March) occurs twice.” This term has survived into all the Romance languages, as in Italian “anno bisestile”, even though Western Catholic nations long ago passed over to the medieval system of numbering the days of each month in straight order, and the leap day is simply added to the end of February. This term was even adopted by the Greeks (“δίσεκτο έτος” in the modern language), even though the ancient Greeks had their own very different calendar. (The Romans had an idiom “ad kalendas graecas – until the Greek kalends”, meaning “postponed forever,” since there were no kalends in the Greek calendar; it was a favorite expression of Emperor Augustus, and also survives in the Romance languages.)

Pictured below: the first page of the liturgical calendar of a Missal copied out in Paris in the 13th century, in which the Roman dating system is still being used.

The true solar year, however, is actually about four minutes shorter than the 365¼ days given by Caesar’s calendar; over time, the accumulation of these minutes causes the solstices and equinoxes to drift backward. In 325 A.D., the Council of Nicea had fixed the vernal equinox, the starting point for the calculation of the date of Easter, to March 21st, but by the later 16th century, the four-minute discrepancy had accumulated to ten days, pushing it back in real astronomical terms to March 10th or 11th. (The problem of the calendar’s drift had, of course, been noticed and discussed much earlier.)

In 1582, Pope Gregory XIII, issued the bull Inter gravissimas, announcing a two-fold correction of the calendar; today is the anniversary of its official promulgation, not by coincidence. First, he decreed that in the following October, the 4th would be followed immediately by the 15th, removing the extra ten days, and thus putting the spring equinox back on March 21st. (St Theresa of Avila, the foundress of the Discalced Carmelite Order to which our magister communis Fr Reginald Foster belonged, died during the night between these two days.) Secondly, he established a new regular interval for leap years, for the sake of which we now call the common calendar “Gregorian.” In this new system, only one centennial year (i.e. a year evenly divisible by 100) in four is a leap year; therefore, 1600 was a leap year, but 1700, 1800, and 1900 were not, and 2000 was a leap year, but 2100 will not be, etc. It is so accurate that if it were left without further correction, it would not require the removal of a full day until sometime in the early 47th century.

The Catholic nations of Europe all adopted the Gregorian calendar within a year or two of its promulgation. Most of the Protestant nations did not do so until the 18th century (Voltaire famously quipped that the English thought it better to be wrong with the sun than right with the Pope), and the Orthodox countries of Eastern Europe until the early 20th century. Most of the Orthodox churches, however, continue to use the Julian calendar, which drifts a full day every 128 years, for religious purposes, and thus, for example, currently celebrate Christmas six days after the civil New Year. In other words, by the time the Gregorian calendar will be due for a one-day correction, Julian Christmas will be on January 27th.

Pictured below, is a close-up of the cenotaph of Pope Gregory XIII in St Peter’s Basilica, which shows the astronomers he had employed to calculate the correction of the calendar presenting their findings to him. Image from Wikimedia Commons by SailkoCC BY-SA 3.0