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Section ‘Slowing down of the Earth’ of the Wikipedia article Leap second has the following paragraphs:

A mathematical model of the variations in the length of the solar day was developed by F. R. Stephenson and L. V. Morrison, based on records of eclipses for the period 700 BCE to 1623 CE, telescopic observations of occultations for the period 1623 until 1967 and atomic clocks thereafter. The model shows a steady increase of the mean solar day by 1.70 ms (± 0.05 ms) per century, plus a periodic shift of about 4 ms amplitude and period of about 1,500 yr. Over the last few centuries, rate of lengthening of the mean solar day has been about 1.4 ms per century, being the sum of the periodic component and the overall rate.

It is a mistake, however, to consider leap seconds as indicators of a slowing of Earth's rotation rate; they are indicators of the accumulated difference between atomic time and time measured by Earth rotation. The plot at the top of this section shows that in 1972 the average length of day was approximately 86400.003 seconds and in 2016 it was approximately 86400.001 seconds, indicating an overall increase in Earth's rotation rate over that time period. Positive leap seconds were inserted during that time because the annual average length of day remained greater than 86400 SI seconds, not because of any slowing of Earth's rotation rate.

So it seems that, at least over the last half century, Earth’s rotation is speeding up. Why am I seeing everywhere on the Web articles claiming that Earth’s rotation is slowing down?

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There are few competing processes regarding Earth's rotation. The major two are

  1. Tidal friction - Moon and Sun tides keep transferring angular momentum from the Earth's rotation into the orbital rotation of the Moon and the Earth, respectively. The process is very slow and detectable only long-term. This slows down the rotation.

  2. The mass of the Earth migrating towards the center. This constantly happens as substances in more or less liquid parts of the Earth segregate by density and the Earth as a whole cooling down and shrinking. This effect speeds up the rotation as it decreases the Earth's moment of inertia.

In general, the first effect prevails. On the other hand, the second effect is not really gradual. There are major earthquakes that cause rapid shifts of the moment of inertia down and the angular velocity up. That's why slowing down is not exactly monotonic.

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  • $\begingroup$ Expanding this further, since creating a planet is outside our current technological capabilities, the only thing we can really do is observe its behavior. We most likely do not have the full picture of all the forces in play. $\endgroup$
    – Nelson
    Jun 3, 2021 at 2:03
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    $\begingroup$ As an interesting aside, pulsars (rapidly rotating neutron stars that emit pulses of radio waves) can suddenly increase their frequency. This is believed to be due to collapse of material towards the core, reducing the moment of inertia. It's called a starquake! $\endgroup$
    – Oscar Bravo
    Jun 3, 2021 at 7:47
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    $\begingroup$ @OscarBravo I was thinking of the pulsars myself. The difference is, the pulsar primary slowing down mechanism is not tides - unless there is another pulsar in pretty much close proximity. The "starquake" term is derived from our earthquakes because of the similar result. $\endgroup$
    – fraxinus
    Jun 3, 2021 at 7:59
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    $\begingroup$ The main mechanism of pulsars slowing down is considered magnetic. There is unknown contribution of gravitational waves, as pulsar models suggest they are inhomogenous enough (by density). The rate of this slowdown is pretty much measurable for new, milisecond-period pulsars. $\endgroup$
    – fraxinus
    Jun 3, 2021 at 9:19
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    $\begingroup$ @Maggyero about the Moon: yes, it is locked and will not slow down further, except by staying locked when its orbital period increases. And yes, the plot shows exactly this. $\endgroup$
    – fraxinus
    Jun 5, 2021 at 11:57

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