You need, on average, to add more leap seconds than subtract because the earth is on average a little slower than the atomic reference. But that in itself does not mean Earth is slowing down.
Suppose that the Earth kept absolutely constant speed, without slowing down at all, then there would be a fixed speed difference with a corresponding number of leap seconds each year to be added or subtracted – just as when, when a clock needs to be adjusted by a minute each and every month, it means that its speed is constant – although slow (or fast). In fact, the Earth does slow down – and not just lately –, but this effect amounts to about 17 microseconds each year on average, and would only necessitate an extra leap second every sixty thousand years or so. The day-to-day fluctuations are much larger than this. All the best, Rudolf Hooijenga 52.5N 4.7E Brooke Clarke wrote: […] In order to keep the UTC1 correction below a second leap seconds can be added or subtracted as needed, but since the Earth is slowing down lately they seem to all be adding a second. […]
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