I hope you liked 2008. Because you're going to get an extra 0.0000031689% of it today.
That is, on top of the leap day we had on February 29, there's a leap second getting added tonight. 2008 is the year that keeps on giving.
OK, fair warning: this simple question will lead you down a maze of twisty passages all of which look alike. But first off, for those impatient readers, here's the simple answer: the length of the day based on the Earth's spin changes, but the length of the day based on atomic clocks doesn't, so every now and again we have to adjust them to make them match, adding in a second to align them better.
However, that's no fun. Jumping into the gory details; that's fun. And you know I love this stuff, so let's take a closer look. A much closer look.
The "second", though, is arbitrary; it's a human-designed length of time. We made it up. In general, you can think of a second as being 1/86,400th of a day. But scientists don't like to do that; for one thing, it's not sciencey enough. For another, the length of the day changes.
[UPDATE: What I wrote below is technically correct, but I decided to be a little more detailed in a followup post. Please read that when you finish this.]
If the Earth were a solid body with nothing nearby, it would merrily spin at the same rate forever. But it's not alone: the Moon and Sun are breathing down our necks, so to speak. The gravity of these two guys pulls on the Earth, stretching it and messing with its rotation (again, it's complicated, but fascinating). If the Earth were a solid sphere, the slowing caused by the Sun and Moon would be constant. However, the Earth is layered in solid and liquid and sorta-kinda-liquid sections (the mantle is more like a thick plastic than a liquid), and these change how the Earth spins. Instead of gradually slowing down, the rate jumps and skips. Even earthquakes can change the Earth's rotation; the huge seismic event in December 2004 that caused the tsunami in Indonesia shortened the length of the Earth's day by about 3 microseconds. That ain't much, sure, but it happened, and was measurable.
That's why scientists decades ago changed the basic unit from the day to the second, and then decided to base the length of the second on something that won't change over time. What they needed was a cosmic metronome, and light itself is a good one. It's a wave, so it has a frequency -- how many wavecrests pass through a point in space every second. If you're on the beach, and a wave crashes ashore twice per minute, then it has a frequency of once every 30 seconds, or 1/30 = 0.0333.
We can flip that definition over, and instead of saying the frequency of some wavelength of light is how many crests pass a point in space every second, we say the unit of time called the second is when a given number of wavecrests pass by that point in space. There's nothing magic here; it's just a definition. So let's do that.
Since cesium atom electron spin-flips are so very precise, we can measure them very accurately, and use them as a basis for our clocks (and when you hear the term "atomic clock", that's what it means). All we have to do is invert our measurement, and then say:
One standard second is the length of time corresponding to 9,192,631,770 periods of the radiation emitted by the hyperfine transition of an unionized cesium atom in the ground state.
Woohoo! OK, sure, that sounds complicated, but it's actually very precise and measurable and scientists get all charged up about it. All you have to do is nod your head and accept it. The bottom line is, we have a very precise definition of what a second is, and it's not based on the Earth's messy spin. This definition was ratified in 1967 during the Thirteenth General Conference on Weights and Measures. Yes, there's a meeting for that. I can imagine all the scientists and engineers sitting around during talks at that conference constantly checking their watches. That image makes me smile.
So anyway, now we're finally ready to understand why we have leap seconds! We have the superprecise scientific definition of a second, and we have the sloppy one we use in everyday life based on the Earth's spin. But the Earth's spin is changing! On average it's slowing down, and that difference builds up. [Don't forget to read my followup post!] Eventually, when the difference between the day measured by the Earth's rotation and the one based on the cesium atom gets to be more than 0.9 seconds, a leap second is added in to the day to match them up better.
So that's the tale of the leap second. Ironically, if you've read this far, you've blown several minutes of your life, and the extra second tonight won't make up for it. But now you know why you get that extra second, and you can savor it. Take an extra sip of champagne, or kiss that Significant Other a moment longer, or just take 9,192,631,770 oscillations of a cesium-133's atom to ponder what 2009 will be like... even if the coming year is a wee bit shorter than this one.
My, how time flies.