NavList:

I wrote previously:
"Many people will tell you that it is because the Earth's rate of rotation is slowing down. But that isn't quite right. "

And Bill, that BBC article you referenced says:
"The Earth speeds up and slows down as it spins, which means that while one rotation is one day, some days end up being a few milliseconds longer or shorter than others."

That's the common error I was referring to. Leap seconds are not added because of variability in the rate of the Earth's rotation. That variability explains the variability in the scheduling of leap seconds. But their presence is determined primarily by a fixed mismatch between the definition of the SI second in 1960 and the actual rate of rotation of the Earth at that date. In part this is because the SI second was based at a fundamental level on the rate of rotation of the Earth something like eighty years earlier, but regardless, any fixed definition of the second would have had this problem.

-FER
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