Did it seem to you that 2008 would just never end? Not to me; the year just flew by. That’s interesting, because 2008 was longer than normal years for two reasons. First, it was a leap year, which means that February had a 29th, giving the year a total of 366 days instead of the usual 365. This happens because one rotation of the Earth doesn’t take exactly 24 hours, it’s a little under. To correct for this we’ll occasionally add a day to the calendar, usually every four years, though it’s really quite a bit more complicated than that. Click this link for a quite detailed description of leap years that might make your head spin.
In addition to messing with February, scientists decided that something else needed to be done to screw up 2008. Turns out that not only does it take not 24 hours for the Earth to rotate once, it doesn’t rotate at a constant rate! Things like the gravitational pull of the Moon and Sun change things. Those objects pull on huge bodies of water (oceans) and cause them to slosh back and forth like water in a bucket. This sloshing is the tide, and the friction between the water and the sea floor helps to slow the Earth’s rotation ever so slightly. It’s imperceptible over small periods of time, but after a while it adds up.
The result is that the rotation of the Earth, which is one way we have of measuring time, and atomic clocks, which is another (less variable) way of measuring time, are going at slightly different rates, so from time to time we’ll correct one of them. It’s a lot easier to change the calendar than to speed up the rotation of the Earth, so that’s what we do. At the very end of December 31, the clocks read 11:59:58….. 11:59:59…… 11:59:60…… then it rolled over to 01:00:00 on January 1.
My favorite science blogger The Bad Astronomer goes into a lot more detail about the extra 86,401 seconds in 2008 on this post and a (followup post), and you might find that interesting. He’s also the one who was so energetic about the pronunciation of kilometer that many of you chimed in on early in the year. (Wow, four pingbacks from a single post! Must be a record of some sort. I’m just trying to get my little blog noticed by the big boys.)
I don’t think the revelers noticed at Times Square, but then again they probably didn’t account for that when the ball was dropping. So I hope you used your extra second productively; I very much enjoyed my extra million microseconds of sleep (I was already in bed).