Why are leap seconds needed so often?

Question

In Coordinated Universal Time (UTC), leap seconds are added to account for the slowing down of Earth's rotation. But the slowing down is said to be of the order of milliseconds in a century. Then why there were more than 25 leap seconds added to UTC in the last few decades alone?

Answer 1

The problem here is not necessarily that Earth is slowing down. The problem is that Earth is slower, on average, than International Atomic Time (TAI). Basically Atomic clocks are so accurate with the rate of their 'ticks', that the Earth is slower on average and in order to coordinate atomic time with astronomical time, or the rate of Earth's rotation, we stand by until the Earth is about 1 second behind our atomic clocks, then essentially 'stop' the clock for a second and give the Earth time to catch up. We add a leap second. So, they are needed quite often because it is not the Earth's rotation itself slowing that is the problem, it is the fact that atomic clocks 'run fast' compared to the Earth and are actually more consistent than the Earth's rotation for keeping time.

I hope that clears up some confusion.