[Talk] The Longest Year
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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