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Comment by fouronnes3 | Hacker News Reader

The slides just say "predictable speedup starts 2020" but don't really explain it, except for a vague reference to the "19 year metonic cycle".

The question I really wanted answered going in is: why is the Earth's rotation speeding up since 2020?

If we look at 19y intervals in the past at the UT1-UTC plot, we can see something unusual around 2000, but nothing really at 1980. And leap seconds started in the 70s! So this is a fascinating set of slides but the question is still frustratingly open for me.

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> The question I really wanted answered going in is: why is the Earth's rotation speeding up since 2020?

Wikipedia's chart of the length of the day explains this part [0]. Overall, the day has been getting shorter since 1970, though it oscillates somewhat due to the metonic cycle. (Which, as I understand it [1], is just the tides changing the shape of the Earth over the course of the Moon's orbit, causing a change in angular velocity, which adds up over the months.) It's only since 2020 that this has regularly gone under the 86400-second mark, making the integrated ΔT start to decrease. And the SI second itself was based on the year 1900; I assume even larger-scale effects caused the day to originally become longer since then.

Luckily, we seem to have already hit the low point of the current cycle around January 2023, so there shouldn't be much risk of a negative leap second in the next decade.

[0] https://en.wikipedia.org/wiki/Day_length_fluctuations?search...

[1] https://en.wikipedia.org/wiki/Tidal_acceleration

Here's a fun thought: As humans dig heavy materials out of the earth and build taller buildings, we increase the moment of inertia of the planet. This theoretically slows the rotation of the earth.

Of course, given the huge mass of the earth, the effect so far would be negligible. But maybe it would be noticed in the distant future if it were done on a much much greater scale than it is today.

As more food for thought, we are also changing the moment of inertia by putting so many planes in the sky, and by sending mass to space. If every plane were to land, the earth should rotate slightly faster.

Drift of Earth's Pole Confirms Groundwater Depletion as a Significant Contributor to Global Sea Level Rise 1993–2010

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GL10...

    Humans Have Shifted Earth's Axis by Pumping Lots of Groundwater. For decades, humans have been pumping so much water out of the ground that it has caused Earth's axis of rotation to shift, according to a new study published last week in the journal Geophysical Research Letters.

https://www.smithsonianmag.com/smart-news/humans-have-shifte...

You just have to build a big dam:

https://medium.com/coinmonks/three-gorges-dam-a-climate-chan...

But on the other side this contributes to global warming which melts of a lot of water-ice that is in high places and will (as water does) go to low places.

Someone knowledgeable should do the math

I think it's not answered in the slides. Instead they offer two hypotheses for the _characteristic_ of the observation of the declining length-of-day in the last 20 years, with all known effects removed such as the Metonic Cycle: Random walk & Linear.

They present the consequences of adopting one or the other prediction model, but only because one has to be adopted for practical decisions. Not because we necessarily know what's going on.

We can't know now which is the correct model. As always, only time and observation will tell.

I haven’t seen an explanation. I think it’s more surprising than the slides suggest, because if you look at the IERS charts there’s a clear change in the dX and dY plots in about 2019, corresponding to a change in gradient of UT1-UTC.

https://datacenter.iers.org/plots.php?id=6

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