In a Word: Bissextus: A Short History of Leap Years

Normally, I devote this column to exploring surprising roots of common words, but today my focus is on a word so uncommon that we only use it every four years. The word is bissextus, and it brings with it a lesson on the history of Roman calendars.

From near the founding of Rome (approximately the eighth century B.C.), its people relied on a local lunar calendar to keep track of seasons and religious ceremonies. A lunar calendar is one based on the phases of the moon instead of, like today’s calendar, on the Earth’s relationship to the sun. This ancient Roman calendar included ten months of 30 or 31 days each, and the new year began in March. (In the beginning, then, September, October, November, and December really were the seventh, eighth, ninth, and tenth months, as their names indicate.) The resulting calendar year contained only 304 days, which were followed by an uncounted winter season.

According to tradition, the second king of Rome, Numa Pompilius, decided he wanted wintertime on his calendar, so he added the months of January and February to the end of the year, creating a 354-day calendar. This was followed soon after with a similar Roman republican calendar that had 355 days.

To keep the dates in sync with the seasons, the people in charge of the calendar occasionally added weeks or even a whole month to the calendar to realign the dates. Unfortunately, those “people in charge of the calendar” were politicians, so new days were sometimes added sporadically and for the wrong reasons, including to extend one’s term in office.

In the mid-first century B.C., the calendars had become so misaligned with the seasons that the vernal equinox, usually in the last third of March, was falling in the calendar in the middle of May. Julius Caesar, by this time emperor of Rome, had had enough. He called on a top astronomer to offer a solution to the mess that was the Roman calendar.

The result — what we today call the Julian calendar — was a solar (or tropical) calendar, giving up all pretense to being guided by the moon’s phases. It recognized that a solar year was 365.25 days long (which is close to being accurate, but not spot on), and so it established that a regular calendar year would contain 365 days, and every fourth year would have one extra day added to it. It also realigned the calendar to make January 1 the beginning of the new year.

So we have arrived at the modern idea of the leap year, but the Romans didn’t call it a leap year, and that extra day isn’t the one you think it is. With the old lunar calendars, the days of the month weren’t simply numbered consecutively; they were named by counting backward from the next calends (the first of the month, coinciding with the new moon), ides (middle of the month, on the full moon), or nones (approximately nine days before the ides). This system wasn’t abandoned in the new Julian calendar.

When it came time to decide where to put that extra day every fourth year, Caesar and his astronomers didn’t stray from older calendar traditions: They decided to add that extra day where they had been inserting extra days for centuries — after the sixth day before the calends of March. That means, from a certain point of view, a second sixth day before March 1 was added to every fourth year. And this is where our word bissextus comes from.

Bissextus, or the bissextile day, comes from the Latin bis “twice” + sextus “sixth.” A leap year is also known as a bissextile year. We refer to February 29 as “leap day,” but to purists, that added bissextile day was actually last Monday, February 24.

The Julian calendar, with some later, minor adjustments (including a modern numbering system), sufficed for centuries. But a solar year is actually 365.242199 days long, not the nice round 365.25 that Caesar’s astronomer reckoned. By the mid-16th century, the Julian calendar was off by about 11 days, which was causing problems with the calculation of religious holidays.

A new solution was issued in 1582 as a papal bull from Pope Gregory XIII. The Gregorian calendar — which is what mall calendar kiosks are selling every November and December — eliminated 10 days from October of that year, thus realigning the spring equinox to March 21. It also established a new calculation for leap years: For centennial years, only those divisible by 400 would be leap years. The year 2000, then, was a leap year, but 2100, 2200, and 2300 will not be.

Protestant countries weren’t so keen on this new calendar because of its source, so it took time before it was widely adopted. England and its colonies didn’t adopt the Gregorian calendar until 1752, which is why George Washington, for example, appears to have two birthdays in 1731: February 11 according to the Julian calendar his parents would have used when he was born, and February 22 according to the Gregorian calendar we use today.

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