The Julian Calendar

The Julian calendar was a modification by Julius Caesar of the Republican calendar. As pontifex maximus, Caesar was responsible for the smooth operation of Rome's calendar, which previous neglect had allowed to fall behind the seasons. In the second year of Caesar's dictatorship (707 AUC = 47 BCE) the calendar was running seriously behind the solar year. Some scholars have argued it was 90 days behind, and Caesar began by ordering an ordinary (pre-Julian) leap year (with the extra month). Be that as it may, during his second consulship, 708 AUC, he inserted 67 days (exactly how isn't perfectly clear, but probably 2 months between November and December of 22 and 23 days, plus another intercalary month after February). So by the end of the year 708 (46 BCE), the calendar was pretty much back in sync with the seasons. As you might guess, these changes caused a great deal of confusion at the time. Later writers called it the annis confusionis.

In 709 AUC (45 BCE), the real Julian calendar begins. Not only did Caesar decree the leap year rule, but he lengthened several months by putting 10 more days into the regular year. The new leap day was inserted exactly where the old leap month was, i.e., after the 24th of February. Both the 24th and the leap day were counted as vi Mar., the second was called bis vi or the bisextile.

Day Changes in Julian Reform
Month Republican Julian
January 29 31
February 28 28
March 31 31
April 29 30
May 31 31
June 29 30
July 31 31
August 29 31
September 29 30
October 31 31
November 29 30
December 29 31

In adding the new days to the calendar, Caesar tried to disturb the separation between festivals as little as possible (relative to the Kalends system of dating), and the new days were actually added just before the last day of each month that was extended, except for April, where it was inserted between the 6th and 5th Kalends. The month of July (Julius, from earlier Quinctilis) got its name in 44 BCE by decree of the senate. Notice that while our general rule for converting days of the month still applies, the Kalends numbering in the lengthened months is different in the Julian and Republican calendars. E.g., December 25 = vi kal Jan. in the Republican, but viii kal. Jan. in the Julian.

Because the pre-Julian Roman calendar was not regular, the custom of historians is to use the Julian calendar proleptically for earlier dates. That is, dates before 45 BCE, which are naturally recorded in different calendars, are translated into the Julian calendar. This convention explains one seeming paradox: the first year of the Julian calendar should have been a leap year in the new sense, but one was not celebrated that year. Thus the Kalends of January that year actually fell on January 2, 45 BCE.

After Caesar's death, his new rules were faithfully followed. Unfortunately, the new pontifices do not seem to have understood his rules quite as they were intended. Caesar specified a leap year at four-year intervals, and since Romans typically counted inclusively (see the remarks on Olympiads), they took this to mean every three years. So leap years were observed in 42 (712 AUC), 39, 36, 33, 30, 27, 24, 21, 18, 15, 12, and 9 BCE. At this point, someone must have brought the problem to Augustus Caesar's attention, because he decreed that there should be no leap year at all for the next 12 years, and carefully rephrased the rule to say "intercalate at five year intervals," so dense Romans would get their counting right. The first correctly observed leap year was in AD 8. Augustus also took the opportunity to rename the month Sextilis after himself at the same time, which is how we wound up with August, but there is no evidence to support the story that he lengthened that month so that it would not be inferior to Julius's month. In fact, as indicated above, the month lengths were all changed by Julius Caesar.

The Start of the Calendar Year

The Romans took the start of the year to be January 1, just as we do. In examining other documents, however, particularly medieval ones, we need to be aware that such was not always the case. In the Byzantine empire, September 1 was the beginning of the new year. In the West, January 1 persisted, but alongside of that you also find year the beginning on December 25, March 25, or even Easter. Most of the early Western reluctance to use January 1 as the year's beginning stemmed from Christian disapproval of the traditional pagan ceremonies held on January 1. It is commonly thought that the institution of the Feast of the Circumcision was at least partly intended to give people a Christian holiday to celebrate on the first of January. Curiously, the calendars and tables the Church used to calculate Easter generally began in January, a tradition stretching back to antiquity. Eventually, January 1 regained its predominance in most of Europe. England persisted in using March 25 as the official new year until adopting the Gregorian calendar in 1752. Hence, a January-March 24 date will be look like it's a year before. That is, the day after, March 24, 1715 in Britain is March 25, 1716. Many private individuals in England, however, persist in calling January 1 the New Year (see, for example, Pepys's diaries).

Problems with the Julian Calendar

The Julian calendar was a major improvement over the older Roman calendar. The rule for leap years was simple enough that anyone could keep track of it, and the calendar stayed pretty much in time with the solar year. But just how accurate was the Julian calendar? With the quadrennial leap year, the mean calendrical year is 365.25 days. As we have seen, the actual length of the mean tropical year is closer to 365.2422 days. So the civil year on average is too long by about 11 minutes. Over a long period of time, this discrepancy means the seasons will slowly fall behind the calendar date. If we calculate a value Y, which equals the number of calendar years it takes for one day of "seasonal slip" to occur, we have a way to compare the relative accuracy of various calendars.

For those interested, the rough formula is Y = 1/(MCY - 365.2422), where MCY is the mean civil year length in days. To even more precise, we could take into account the gradually decreasing length of the tropical year. That requires we consider the calendar over a specific span of years and replace the approximate value for the tropical year with: 365.24231533 - (y1 + y2) * 3.06713e-8, where y1 is the start year and y2 the stop year in astronomical years (= Common Era, but for BCE dates, use negative number and add one, e.g., 1 BC = 0, 2 BC = -1, etc.). This kind of precision will be irrelevant unless we have precise astronomical observations in the relevant years against which to compare it. Otherwise, rounding the number to the nearest 10th gives a good value for comparison.

A similar value, M, could be generated to show the number of months it takes for a lunar calendar to slip 1 day, assuming the calendar works by rule rather than direct observation.

In the Egyptian calendar, Y = 4.1; clearly, no one who used this calendar was seriously concerned to keep it aligned with the seasonal year. Using the approximate formula, Y for the Julian calendar is 128.2; the precise formula, considered over the span -44 to 1582, gives 129.3.

These numbers should suggest why the Julian calendar remained unreformed for so long. Within any one person's lifespan, only an astronomer would notice the difference. Even if a culture keeps records over a long period of time, the change is not all that great: less than 8 days in a millennium. While an extremely long period of time (about 27,700 years) might change "the darling buds of May" into the darling buds of December (in the northern hemisphere, of course), it hardly seems like a pressing problem.

The fact is, however, that from the beginning of the 13th century, there was a constant call among leading intellectuals (most notably Roger Bacon in 1267) for a modification to correct this drift. The press of other urgent matters and general inertia delayed matters until the late 16th century, but eventually a change was made. To understand why, we first need to look at the calculation of Easter.