Leap Second

If this June seems longer than usual, that’s because it is longer than usual. They’re adding a leap second to the end of June. I don’t know who this mysterious “they” are who tinker with our clocks from time to time, but when I’m in doubt I blame Bernie Horowitz.

Just kidding. The Time Lords who tinker with our clocks are actually the International Earth Rotation and Reference Systems Service, IERS. They measure Earth’s rotation and decide when a leap second is needed. The leap second is needed because the atomic clocks which keep our time are extremely accurate and consistent in their timekeeping, even as Earth’s rotational speed is slowing and the length of an actual day is getting longer.

An Earth day is officially 86,400 seconds long, but the last time an actual day was that length was around the year 1820. Due to gravitational tugs between the Earth, Moon, and Sun, Earth’s rotation is slowing by approximately 1.7 milliseconds per century. The mean length of a day is now 86,400.002 seconds. In one year the amount of “extra” Earth-day time compared to atomic clock time will theoretically accumulate to 365 multiplied by .002 seconds, which amounts to 0.73 seconds.

In reality, things are not so tidy. Earth’s rotation is affected by earthquakes, volcanoes, atmospheric disturbances, and even what is happening in Earth’s core. Leap seconds occur at irregular intervals and are unpredictable. When the IERS decides a leap second is needed, they will add a leap second to the end of a month. It can be any month, but June and December are preferred. The atomic clocks will stop for one second between the end of June and the beginning of July to allow actual (astronomical) time to catch up with atomic clock time.

Because of Earth’s variable rotation, in theory a leap second can be negative as well as positive. In other words, the atomic clocks could be programmed to skip a second. That has never happened in any of the 25 leap seconds added thus far, and with Earth’s rotation slowing it is unlikely to happen. 

The last leap second was added at midnight on June 30, 2012. A number of computers around the world crashed because their software wasn’t designed to handle the extra second. The Amadeus airline booking system, Hadoop, and Linux servers around the world were caught in the mini-Y2K bug. This is why space missions don’t launch on the day a leap second is added to international clocks. Scientists don’t want to gamble that a leap second might crash the multi-million dollar mission that is carrying their lifetime’s work.

Supposedly, the bug has been fixed. Supposedly, the problems won’t happen this time. Still, when June changes to July, I wouldn’t care to be flying on one of those new jumbo jets with a “fly-by-wire” control system where everything is handled by computers – just in case the software guys and gals overlooked something. It would be too weird to have my epitaph read, “Killed by a leap second.”