How Julius Caesar Reformed the Old Roman Calendar

Although most people take their calendar for granted, there is nothing natural about it. Most of the world has now adopted the Gregorian calendar which closely approximates the solar year, but it does not match it exactly. Therefore, there is little reason to believe that our 365 day years with occasional leap years are “normal.” Indeed, for most of human history, other calendars have been used. One of the most popular calendars in Western Civilization prior to the introduction of the Gregorian calendar was the Julian calendar, introduced by Julius Caesar in 46BC. Although it was not perfect, it was a marked improvement over the calendars which had come before it.

Tradition says that the first Roman calendar was instituted by the legendary founder of Rome, Romulus. Supposedly, each year under that old system had only 304 days in ten months. It is also said that Numa Pompilius, a king of Rome, reformed the calendar in 713BC by adding two more months and by changing the lengths of some of the months. This reformed calendar allowed for 355 days per year. Obviously, that is more than ten days short of a solar year, so the calendar date easily drifted away from the solar year. Over time, the calendar could easily become weeks or months different than the solar year. Imagine if the weather typical of November started as early as July or August! To correct the errors in the calendars, the Romans put extra days or months into certain years. This was done not on the basis of any regular pattern, but simply by order of whoever happened to be in power at the time.

Obviously, the old Roman calendar needed extensive reforms. When Cleopatra introduced Julius Caesar to the Egyptian astronomer Sosigenes, Sosigenes helped Caesar reform the calendar by making it more closely match the sun. Before he could mandate a new calendar, however, he had to correct the errors in the old calendar which was two months off in 46BC. To that end, he called for two extra months between November and December and added three weeks between February and March to end 46BC where it “should” be. The result was a year of 445 days! In a time when communication was difficult, many Roman citizens had no idea what the date was at any given time during that year. Thus, 46BC was known as the “year of confusion” to the Romans.

The new Julian calendar made January, not March, the first month of the year. It also called for a 365 day year with an extra day every four years. While this calendar is not perfect, it closely approximates the solar year and is still in use by some today. Interestingly, the introduction of the new calendar made the names of some of the months inaccurate and this inaccuracy remains in our own calendar today.

Although the names of the months September and October indicated that they are the seventh and eighth months, they are really the ninth and tenth months. Under the old Roman calendar which began in March, however, they were part of a series of numbered months which included Quintilis (five) and Sextilis (six). The names of those months were eventually changed to July and August, however, to honor Julius Caesar and Caesar Augustus. Nevertheless, it is interesting that the names for the seventh and eighth months were never changed after they were moved back relative to the first month of the year.