Leap Seconds and Earth’s Rotation

As most of you probably know, the “longest day of the year,” which should really be referred to as the “day of the year with the most sunlight,” is the summer solstice (the first day of summer, typically June 21st for the northern hemisphere of Earth). All days of the year are actually the same length…24 hours. But not in 2012. For those of you paying attention, there was a slightly longer day: June 30th. You probably didn’t feel the difference though—it was 86,401 seconds long, compared to the usual 86,400 seconds. That’s right, we had what’s called a leap second. But why?

Interestingly enough, it has to do with the Moon (shocked, right?). For those of you who learned about tides in school (yes Bill O’Reilly, we can explain them), you should know that the Moon is in the middle of a tug of war with the Earth, and as such, the part of Earth closest to the Moon feels the most tugging, causing the tides. Because the oceans are feeling this tugging against the continental shelves on Earth, a great amount of friction is felt—this is slowing down Earth’s rotation ever so slightly. It also causes the Moon to get farther away from Earth—it’s receding from us about 2 inches every year!

Think back to my last entry, where I talked about angular momentum. A spinning object will want to stay spinning unless something affects it. If the spinning object loses energy, it slows down. The Earth, in this case, is the spinning object. It rotates completely (roughly speaking) in 24 hours. However, the aforementioned friction is causing Earth to lose some of its energy to heat—in the same way your hands heat up when you rub them together and produce friction—which slows the Earth down. If the Earth spins slower, that means the length of the day gets longer. To be a little more precise, the Earth’s day is getting longer at a rate of about 1/500th of a second each century.

It may seem small compared to the lifetime of a human being, but think about how long the Earth has been around. Back when dinosaurs roamed the Earth, there was a time when a day on Earth was 23 hours long. But even in this timeframe, that of a human lifetime, the effects are still real.

For a long time, humans used the Earth as the most precise measurement of time. Sundials used the shadows caused by the Sun to tell the time of day, but wasn’t of much help at night. Early analog clocks and watches were very accurate and useful at all times, but were often a little too slow to keep up with the Earth’s more precise measurements, and so would have to be adjusted every now and then. With the invention of atomic clocks, everything changed. Suddenly, we had clocks more accurate than the Earth…after a while, the Earth slows enough that it gets to be almost a full second behind our atomic clocks.

That’s where leap seconds come in. Ever since the 1970’s, we’ve been adding “leap seconds” to compensate for the Earth’s slowing. Every so often, we add one second to the last day in either June or December to allow for the Earth to catch up to our atomic clocks. Neat, huh? There’s actually an organization known as the International Earth Rotation and Reference System Service that keeps track of the Earth’s rotation, and decides when to add a leap second to the year. So far, we’ve added 25 total leap seconds since the practice was introduced. Certainly an important job…but it seems a little boring to me. I’m glad I’m not watching the Earth rotate…that’s like watching the grass grow.

But this system does have some controversy. There are some problems with adding leap seconds, most of which are computing related. Proposals to end the practice have been submitted, and a decision is set to be made in 2015 at the World Radio Conference. Who knew a few extra seconds could make such a big deal? It goes to show you how much each moment matters.