This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Is there a more suspicious day than the Ides of March? The day on which Julius Caesar was assassinated? It's a day associated with betrayal—a day to worry about the loyalties of those closest to you because everyone has a price. The Ides of March occur in the middle of the month (from the Latin iduare, meaning "to divide") and is marked on March 15th. The Ides are a regular occurrence then since all months have a middle. However, March 15th took on special meaning when the term became overwhelmingly tied to Caesar. I'm sure your Facebook feed or Twitter stream were filled with warnings that you should beware because the Ides had come.
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But what if I told you that it was all for naught? That you were warned and suspicious of friends named Brutus for no reason? Why? Because the Ides had not come. The Ides would not arrive for roughly another two weeks—in fact, the Ides, according to the Julian calendar, falls on March 27th.
People have marked time differently throughout the ages, struggling to reconcile solar and lunar cycles with the changes that occurred around them. The Roman calendar started the year in March and was 10 months long—six months had 30 days and four months had 31. It was 304 days long, with some 61 odd days left over that accounted for winter which were not assigned to any month in particular. It's likely it was a lunar calendar as the Romans divided the month into three segments:
The Kalends signified the start of the new moon cycle and was the first day of the month.
The Nones were related to the half moon, and were typically observed eight days before the Ides.
The Ides were believed to be the day of the full moon. It occurred on the 15th day in March, May, July, and October, and the 13th day in the other months.
Days were counted by their relationship to these markers. So, March 11th would have been "Five Ides" to the Romans because it is four days before the Ides.
The year began on the vernal equinox (so in addition to a Happy Spring, I should wish you a Happy New Year!) The Romans fiddled with the calendar—Numa Pompilius added January and February, and shifted days around to lengthen the calendar to 354 days. And then they added a leap month to try and keep the calendar aligned to a tropical year (equinox to equinox). Leap years were 377 days long!
Time is not a concrete thing, and it can be made to serve to various purposes. Roman politicians understood this quite well. Because their terms were defined by the calendar, they could extend or shorten the year to serve their political needs as they saw fit. The Roman calendar, therefore, lacked consistency.
Julius Caesar changed that.
He realigned the calendar year to the tropical calendar by making 46 BC 445 days long and starting the year on January 1. This accounted for leap years that had been missed during some turbulent years. In 45 BC, he further restructured the Roman calendar by doing away with the leap month and introducing a leap day every four years. His calendar combined the old Roman months, the Egyptian calendar, and Greek astronomy. However, the leap day appears to have been incorrectly applied initially by Roman leaders who marked it every third year. After 36 years, things were out of sync again and Caesar stepped in. It took 12 years, during which he skipped three leap days, but he realigned the calendar.
Still, his calendar doesn't quite match the solar year—it's just a bit longer. So, this generated an error of one day every 128 years, shifting the tropical year backwards, and moving the Easter observance around. That did not sit well with the Roman Catholic Church, so in the 16th-century Pope Gregory XIII made a few changes. However, you shouldn't overlook that the Julian calendar was used well after the Roman Empire had fallen (and in some cases, it's still used today!).
Gregory's calendar combined the Julian calendar with the lunar calendar of the church. He reduced the number of leap years, and changed their structure: Leap years would occur every four years, except for years that are divisible by 100—though centurial years divisible by 400 would still be still leap years, which is the pattern we follow today. He also reduced the year by roughly 10 minutes to realign time to the vernal equinox. His calendar actually began on March 21st—skipping ahead from March 11th to reestablish the date of the equinox—and Easter.
The Gregorian calendar wasn't immediately adopted by all. The Protestents—their Reformation well underway—thought the calendar was an insidious plot and resisted. And other countries adopted the calendar at different times. And even today, Jewish and Islamic calendars mark time differently, as does the Chinese calendar. Time is what we make of it.
All of this to show that what we think of as a concrete, definite thing is actually a flexible creation. With all of these calendar changes, dates were in a state of flux. Birthdays changed! (Though Caesar kept his.) So beware the Ides of March. And you might want to pass on that lunch with Brutus today.
H/T to Prof. Kevin Birth for pointing out the correct day of caution on (the Gregorian) March 15th.
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