Hopefully someone with more knowledge on this will correct any errors I make.
The rotation of the Earth is used as a basis for our time keeping. The day was divided into 24 periods and each of these was divided into 60 periods and each of these was further divided into 60 periods. The length of this solar day varies throughout the year so that this 1/86400 portion of a day is not consistent. Greenwich Mean Time (GMT) is calculated on an average of this length.
In the 1950s, as the accuracy of our measurements has improved, an atomic clock was developed at the National Physical Laboratory. It is more regular than any other clock..and more "regular" than the rotation of the Earth. In the 1960s an international agreement was made that specifically defined the lenth of the second in comparison to the atomic clock.
Gradually, the atomic clock becomes enough out of sink to the actual rotation that the International Earth Rotation Service decided to adjust the atomic clock to make it more in sync with the actual rotation.
I hope I didn't make many mistakes...and I hope this helps you understand the basis behind the leap second. It isn't that the Earth is necessarily slowing down it is that the average length of the rotation period isn't exact and the period measured by the atomic clock is. The leap second just brings them back into sync periodically.
[This message has been edited by Asgara, 04-02-2004]
Asgara
"Embrace the pain, spank your inner moppet, whatever....but get over it"
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