http://www.nj.com/south/index.ssf/2008/12/the_longest_year.html By Keith Johnson Hasn't this seemed like a long year? It's no wonder: 2008 IS the longest year we've had since 1992. But we didn't know that would be the case until about six months ago. There are two reasons for this lengthy year. The first is a familiar one: 2008 is a Leap Year! It's also the easier of the two to understand. We need to insert an extra day into the end of February nearly every fourth year, because the Earth spins around 365 and (very nearly) a quarter times in a year. We need to insert an extra day into every fourth year, essentially to keep the seasons in their familiar places in the calendar. If we didn't do this, eventually spring would be starting in December! However, 2008 will be longer even than the typical Leap Year. We'll have to make New Year's Eve one second longer than usual. This happens because of the way we now define time units. We used to think that the rotation of the Earth was a good standard. It's been spinning around for millions of years, after all, and we didn't know of anything that ticked with a steadier beat than our home planet. So we set the Earth's rotation as being 24 hours by definition, and the standard second was 1/86,400 of that. Once we started looking deep into the heart of the atom, though, we found a better benchmark. Atoms vibrate at rates that are amazingly steady and reliable. We discovered that the Earth, on the other hand, is very gradually slowing down. Not only that, but various shifts in the continents and within the interior make it speed up and slow down a little from year to year. The changes are tiny, of course, but they are now detectable. And they make the Earth a poor time standard, almost like using a rubber ruler to define a foot. So in 1967 the world's timekeepers decided to switch to atomic time as their standard. A second is now defined as 9,192,631,770 vibrations of a Cesium-133 atom (you might ask your neighborhood physics major to explain what that means). --

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