The One-second War (What Time Will You Die?)

Leap seconds make sure the sun is due south at noon by adjusting noon to happen when the sun is due south at the reference location. This very important job is handled by the IERS (International Earth Rotation Service).

Leap seconds are not a viable long-term solution because the earth’s rotation is not constant: tides and internal friction cause the planet to lose momentum and slow down the rotation, leading to a quadratic difference between earth rotation and atomic time. In the next century we will need a leap second every year, often twice every year; and 2,500 years from now we will need a leap second every month.

On the other hand, if we stop plugging leap seconds into our time scale, noon on the clock will be midnight in the sky some 3,000 years from now, unless we fix that by adjusting our time zones.

Good summary of the problem, but falls short in explaining why we have leap seconds. The common explanation is that the earth is slowing down, but that’s not quite right. You have leap seconds because the earth has slowed down. Even if tidal friction were to magically stop, you would still need to insert leap seconds. The earth, as a clock, is running slow compared to atomic time, so it lags farther and farther behind the longer you wait — which means adding leap seconds to get them to approximately agree. The fact that tidal friction will still be with us means this discrepancy is accelerating — that’s why leap seconds will have to be added at shorter and shorter intervals, should we decide to continue to make the adjustments.