It's very interesting to see that 2008 will be an unusually long year, and that's not because of the fact that it's a Leap Year, which makes us add one day more to the month of February. This happens because of the way we now define time units.

Astronomers set the Earth's rotation as being 24 hours by definition, and the standard second was 1/86,400 of that, but once scientists started looking deeper into the heart of the atom, they discovered a better benchmark.

Atoms vibrate at rates that are amazingly steady and reliable and that helped researchers discover the Earth is very gradually slowing down. Furthermore, various shifts in the continents and within the interior make it speed up and slow down a little from year to year.

Nowadays, these changes are detectable and they make the Earth's rotation a poor time standard. In 1967, the world's timekeepers decided to switch to atomic time as their standard, as a second is now defined as 9,192,631,770 vibrations of a Cesium-133 atom. The Earth now runs slow about 2 milliseconds per day on the average, and this adds up to a little less than one second per year.

The International Earth Rotation Service (IERS) keeps track of things, and when it's necessary, they insert an extra second into a year. This leap second correction has been on since 1972 and it's done at midnight London time, sometimes at the end of June but usually at the end of December. The IERS predicts when a leap second will be necessary about six months ahead of time.

Normally, the time would change from 11:59:59 to 12:00:00, but on December 1, the sequence will be 11:59:59, 11:59:60, 12:00:00. This is not a regular process, as the previous leap second was in 2005, and that's because the Earth's rotation is a changeable thing.