## Saturday, July 4, 2015

### Leap Second

On June 30th we had a leap second added to the year. A leap second? That's right, a second. We're all familiar with the leap day we add to the year every 4 years (next year will be one). That day is added to keep our calendars aligned with the stars. When someone asks how many days in a year, we automatically say 365. And that's pretty close.

But it actually takes us about 365.25 days. That extra quarter of a day gets added together every four years into a leap day. (Actually it's not really a quarter of a day so the rules are actually more complicated but most of us won't see years that don't follow this basic rule.) And why do we do this? To keep the seasons at about the same point in the calendar. Otherwise given enough time the seasons would get out of line and we'd have spring in winter.

Well, a leap second is similar. In terms of seconds, there should be 86,400 seconds in a day. That is the amount of time it takes the Earth to rotate once on its axis, or one day. But you see, that isn't true. Right now the Earth takes 86,400.002 seconds to rotate once. Or an extra two one-thousandths of a second over the “proper” number. And again, given time that extra fraction of a second can build up and get astronomical events out of sync with when they should be. So we add an extra second every now and then.

That's right, unlike leap days that occur every four years (sort of) we never really know when a leap second will be added. And that's because there are many things that affect how fast the Earth is rotating.

Most of the change is due to the Moon. As you know the Moon is a cause of tides in the ocean. Well those same tidal forces drag on the Earth and is actually slowing down how fast the Earth rotates. But this value isn't constant. In the past the Moon was closer to the Earth and the rate was different. Even now the Moon is still moving away from the Earth, so the rate changes.

But there are other things that affect the rate the Earth spins. And some of those are geological. Movement of the tectonic plates affects how fast the Earth rotates. And earthquakes can make big changes all at once. In 2004, the earthquake that generated the Indonesian tsunami shortened the day by over two and a half microseconds.

Since there are random events, like earthquakes, that affect the speed at which the world turns, we never know when a leap second will be added. They aren't that common since there have been only 26 between 1972 (when they started) and now. I hope you made the best of yours.