Keeping Time: Leap seconds, leap years, and why you should care


By Alexi White, Opinions Editor, MPP ‘13

The days are getting longer, and not because spring is nearly here. I mean the length of one solar day—the time it takes for the earth to spin around exactly once—is getting longer, and we have the moon to blame.

As we turn, the moon’s gravity pulls the oceans toward it, causing high and low tides. But because we’re spinning faster than the moon is orbiting, the tides are always slightly ahead of the moon’s gravity, and this creates enough drag to slow us down. The moon, meanwhile, gains energy through this process and moves a bit further away—about four centimeters a year—resulting in a slightly longer lunar month as well. Fast-forward a few billion years and this will eventually stop, but only after the length of a day and a month have converged to about 47 of our current days. Clearly, measuring time based on the rotation of the earth presents long-term challenges.

Before we get into the temporal consequences of these astronomical phenomena, let’s take a moment to go over the basics. Julius Caesar instituted the precursor to our modern calendar in 45 BC. His Julian calendar assumed that a year lasted 365.25 days, but he was a little over eleven minutes short. By the late 1500s the calendar was a full ten days behind.

In an effort to return Easter to its rightful time of year and to keep it there, Pope Gregory XIII eliminated ten days from the year 1582 and made important modifications to the rules defining leap years. In addition to the quadrennial leap year that was in already in place, his new Gregorian calendar eliminated this extra day for years divisible by 100—unless they were also divisible by 400, in which case they remained leap years. It’s confusing, but this is the reason that the year 1900 was not a leap year but 2000 was, even though both are divisible by four. It’s also why the Gregorian calendar is a big improvement on its predecessor. Of course, left-over seconds, a slightly longer solar day, and a gradual slowing of the earth’s journey around the sun will eventually put it a day behind, but not for at least another thousand years.

Of course, not everyone took to the Pope’s new calendar right away. As a poor peasant, how would you feel if your next rent payment was due ten days earlier? Protestant Europe took another hundred years to accept the change, and only after it became a tool to fuel tensions and precipitate religious violence.

From macro time to micro time, we now move to the story of the second, currently defined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” Pretty straightforward.

The definition dates back to 1967 when the Thirteenth General Conference on Weights and Measures decided it was time for a standard that was more uniform and precise than measurements of the solar system. They turned to the atomic clock, which was by then sufficiently advanced to take on this role. (Ten years later, atomic clocks were corrected to take into account elevation above sea level, since time passes a tiny bit slower at higher altitudes, as predicted by the theory of relativity.)

The 1967 definition of a second was designed to replicate the previous definition of a second, which was based on measurements of the average solar day between 1750 and 1892. Unfortunately, for reasons we have already seen, solar days are now 0.002 seconds longer on average. As International Atomic Time (calculated as the average of atomic clocks in laboratories around the world) has slowly diverged from mean solar time, the time we interact with in our lives—Coordinated Universal Time—has been kept up to date by periodically updating atomic time to match solar time through the use of leap seconds. The thirty-fifth such second will be added on June 30 of this year.

But disagreement is brewing within the international community about whether leap seconds are worth the trouble they cause. When they were first introduced, there were few activities that required precise timekeeping; in a world of computers, GPS and complex satellite networks, the addition of a leap second every few years is becoming a real headache. Because of leap seconds, comparing two instants of Coordinated Universal Time across a period of years requires consulting a table of when leap seconds have occurred. Similarly, predicting the precise time of future events more than six months in the future must somehow account for the possibility that leap seconds will be added.

In January of this year, the International Telecommunication Union debated the future of this tool, with the United States leading the anti-leap second group against the pro-leapers of the United Kingdom and their allies. In the end, a decision was postponed to the 2015 World Radio Conference.

Have your say. Contact your local representative and demand that they make time for you.