One second will be added between 2008 and 2009 this New Year’s.  The reason is complicated but I will try to explain it.

The reason we need to add one second between 2008 and 2009 is because the Earth’s rotation is slowing down slightly.  Some reasons for this might be the sloshing of the oceans and Earth’s molten core.  This accounts for about 200 milliseconds to be added to each day.

The first “leap second” was added in 1972.  Since then, 24 leap seconds have been added.  Hmm.. sounds like this is starting to add up!!

Part of the need for “leap seconds” is because the definition of a second has changed been changed…more than once.  At first a second was 1/86400 of a day.  However the rotation of the Earth is not perfect.  Scientists found that there is a little extra time left over.  The definition of a second was changed to reflect the Earth’s Orbit around the sun, and then changed again to an atomic definition.  Now we just add an extra second whenever we have to.

You can read more about leap seconds from these articles.

Wikipedia – Leap Second