All in Good Time

It's an odd tradition ... but a necessary one.

Every four years (more or less; more on that later), we tack on an extra day to the month of February. It's a variation on a tradition that's been around since the days of Julius Caesar (whose Julian calendar was introduced in 46 B.C.E.). The difficulties of accurate timekeeping were a problem even in Caesar's day, when it turned out that an actual, seasonal year (or "tropical year") is about a quarter of a day longer than the calendar year. A leap day was added every four years to correct this imbalance -- but inadvertently created another imbalance in its place, averaging out to a calendar year that was longer than the tropical one by approximately 0.0078 days. Not a problem in the short term, but by the sixteenth century A.D., the effects were adding up. Every century, the calendar year and the tropical year became off-kilter from each other by another 3/4ths of a day; by the sixteenth century, the beginning of spring had shifted from March 23 to March 11.

It was Pope Gregory XIII who instituted a new calendar in 1582 (the Gregorian calendar, if you notice a pattern). This new calendar was also intended to match the cycle of the seasons -- but with tropical years not consisting of whole days, the need for a "leap" system was still a necessity. But how to fix the Julian imbalance? The new Gregorian calendar instituted a new rule: years divisible by four would be leap years, except for centurial years that are not also divisible by 400. (That means that 1700, 1800, 1900, and 2100 aren't leap years; but 1600, 2000, and 2400 are.) Keeping the "leap days" in check allow for a more accurate correction to the calendar year; it's going to take 3,300 years before the Gregorian calendar falls even one day out of sync with the seasons.

But leap years aren't the only method in which we make adjustments in our timekeeping in order to keep our calendars and clocks in sync with nature. There's also leap seconds -- occasions when civilian time is adjusted in order to keep our atomic clocks more or less in sync with the actual rotational time of the Earth. (The USNO works to keep it within a 0.9 second difference.) With the "second" technically defined as the length of time it takes the Earth to make 1/86,400th of a rotation on the average solar day, it had been noted by the 1950s that the Earth's rotation was not at a consistent enough speed to hold against our standard of time. The "second" was redefined in 1956 by the International Committee for Weights and Measures, basing the unit instead on a fraction of the Earth's revolution around the Sun in a particular period. But to keep things even further in check, leap seconds allow for, well, the most minute of adjustments when necessary. (Our next "leap second" will be added on June 30 this year, the first in nearly four years.)

Coming as I do from a punctuality-challenged family, the history of timekeeping is something I find interesting. It is, in some respects, a somewhat recent invention ("recent," at least, by the standards of history!) -- and keeping time with manmade mechanics (instead of by the position of the sun and stars, or by other such natural means as water clocks) is an even newer phenomenon.

Timekeeping allows us not only to track our day-to-day tasks and activities -- but it also, in its small way, gives us a method of imposing a little order to an otherwise disorderly universe. (Even if keeping that order requires the occasional adjustment!)  So celebrate this most unusual of dates by checking out a few sites (and even a movie!) about the ins and outs of keeping time:

From the U.S. Naval Observatory:

- An Introduction to Calendars

- Leap Years

- Leap Seconds

- In a Leap Year, What a Difference a Day Can Make: from the Pittsburgh Post-Gazette

- A Walk Through Time: The Evolution of Time Management Through the Ages: from the National Institute of Standards and Technology

- The Leap Year: TimeAndDate.Com

- Leap Year: 10 Things About 29 February: BBC News Magazine

- Leap Year [DVD]: stars Amy Adams in this new romantic comedy about February 29th.


-- Post by Ms. B