What causes the unpredictability in this? I would have guessed we have earth's rotation and orbit down to many decimals. Does geological activity, weather, or something else cause rotation speed differences that we just can't predict?

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ahazred8ta 2 minutes ago | parent | next [-]

Among other things, turbulent currents of liquid iron in the Earth's core can make the core drift eastward or westward, which causes the crust and mantle to turn slower or faster. Same thing with the strength of the jet stream.

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_alternator_ 2 hours ago | parent | prev | next [-]

In short, yes, the weather, geology, and signicantly, human movement of water via aquifer draining and dam building, as well as glaicial and ice melts, all contribute to unpredictable changes in the earths rotational period, as well as the axis of rotation. The models for this are IIRC trigonometric polynomials of fairly low order, so even if we could model the unpredictability perfectly, truncation error would limit our ability to distribute the model at super high accuracy. The existing models are built in to, eg, satellites, so you can't just make them arbitrarily complex.

Fun fact: leap seconds will stop being a thing soonish. I think they phase out in 2035, with a delay because Russia needed time to update glonass satellites.

(Note: on mobile, this is from memory, details need checking ;))

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tialaramex 2 hours ago | parent | next [-]

2035 is the agreed drop dead date.

Everybody agreed that "Leap seconds" are a sufficiently bad idea that they should be replaced by 2035. Nobody has agreed how to fix it, and "Just turn them off" isn't technically legal. However, "What if there were Leap hours instead?" is technically legal and of course those hours would happen in the very distant future (likely after our civilisation is gone) so it's functionally identical to "Just turn them off" but without legal problems.

Now, I'm English, and England loves this sort of hack. You may have heard that controversial UK politician Nigel Farage "resigned" as a Westminster MP recently and that's not technically true because you can't resign, historically people hated that job and so you can't resign and we never changed that, but what you can do, and everybody does, is get assigned an "Office of profit" in which legally the King is paying you, an MP can't work for the King so you can't be an MP any more. The "Offices of profit" in question aren't real jobs† and don't pay real money, like this "Leap Hour" they'd be a legal fiction. So everybody says you "resigned" but in fact you legally can't do that...

† I mean, historically they were real jobs that made sense which is why the King paid somebody to do them, but England is very, very old so they haven't made sense for centuries and serve only as a legal fiction today.

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BoxOfRain an hour ago | parent | next [-]

On the subject of amusing British political legislation, should he defeat Nigel Farage in the resulting by-election Count Binface will not be able to wear his costume in Parliament; not only is business attire required in the House of Commons, it's specifically forbidden to wear a suit of armour there due to a law from the 14th century.

For those unaware, the major parties have declined to participate in the by-election triggered by Farage's resignation seeing the whole thing as a farce. As a result Farage will likely face only Count Binface, a space warrior from Sigma Six. He'd get my vote purely on the basis that he's promised to bring back Ceefax, and build at least one affordable house.

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prerok 39 minutes ago | parent | prev | next [-]

So, it's solving a real problem, why are we dropping it? I mean, why does everybody agree it's a bad solution?

	▲	tialaramex 5 minutes ago \| parent [-]
		Basically we guessed wrong. We thought knowing "Solar time" would be more useful than in it, and we thought these "Leap seconds" would be less trouble than they are. It's like you buy a cat to help with your rodent problem, figuring the cat will eat mice and isn't much trouble to look after, but after purchasing a cat you find that your problem was actually rats, your cat is terrified of these large dangerous creatures and sometimes gets bitten by them necessitating expensive vet bills and now you need to pay a lot of attention to the poor animal and also now need to buy cat food.

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hwc an hour ago | parent | prev | next [-]

just move the prime meridian. the one we use for timekeeping doesn't have to aligh with longitude forever.

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jeffrallen 2 hours ago | parent | prev [-]

Farage is such an ass, the King should make him feed donkeys or something.

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tialaramex an hour ago | parent [-]

"Vote Count Binface and Bin the Cunt" :D

There's a long tradition in the UK of having electoral candidates who don't expect to win but run because it's free publicity in a high profile race. "Count Binface" is a comedian who dresses up as a space alien whose outfit resembles well, having a Bin for a face. The serious political parties told Nigel to fuck off, if he wants to step down and then immediately contest the same seat they wouldn't run against him in this farce, but Binface isn't a serious politician so he is running in that by-election.

Nigel wanted to be able to do this whole thing about how the establishment is rotten and he (Wealthy public schoolboy who keeps lying to people and doesn't bother going to Parliament even though he was elected to do so) is a true man of the people and can put things right. It got him this far in life. But with the other candidate on your ballot being a space alien it's obvious which of these options is really "the establishment" and it's not the guy whose policies include "Building at least one house†" and who says he comes from a different planet...

† British political parties often insist they will build lots of housing because that's popular with voters. But, in practice they don't tend to really deliver because the various groups lobby not to actually build. So "at least one house" is a joke about this phenomenon, while conveniently also being technically possible, Binface could just build a house, that's a thing you can do.

	▲	overboard2 21 minutes ago \| parent \| next [-]
		Nigel Farage has decided to counter a scandal by throwing himself upon his constituents for judgement, the obviously establishment parties have backed off to allow Binface to run against him in a ~1v1, and you think Binface is more anti establishment than Farage?
	▲	f4c39012 40 minutes ago \| parent \| prev [-]
		I think it's more than publicity. Anyone can stand as a candidate, and anyone can vote for them. Money and connections and establishment and everything else don't matter, all candidates are equal on that stage. It's both weird and to be admired.

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thewebguyd 2 hours ago | parent | prev | next [-]

> as well as the axis of rotation

A frightening fact, the 2011 magnitude 9.0 Tohoku Earthquake shifted the position of the Earth's figure axis about 17 centimeters, making days about 1.8 microseconds shorter.

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lloeki 2 hours ago | parent | prev | next [-]

> Russia needed time to update glassnoss satellites

GLONASS maybe? or really glasnost era satellites?

	▲	_alternator_ an hour ago \| parent [-]
		Glonass is correct. On mobile, thanks for the correction.

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Xenoamorphous an hour ago | parent | prev [-]

A butterfly flaps its wings in Tokyo and we lose a leap second.

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entrope 2 hours ago | parent | prev | next [-]

Yes, all of those and more. Our measurement precision is much better than the year-to-year first and second derivatives of day length. https://datacenter.iers.org/singlePlot.php?plotname=Bulletin... has the most relevant plot to this; the vertical jumps reflect leap seconds. (IERS has other plots for other dimensions of rotation, but I like this one.)

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doctoboggan 2 hours ago | parent [-]

Very interesting, I wonder what happened in 2020 that causes the rotational speed to start drifting the other way?

Pandemic -> more people working from home -> less people in tall office buildings -> faster rotation (like a skater pulling in their arms).

Probably not remotely true but it would be funny.

	▲	wongarsu 2 hours ago \| parent [-]
		Seems like the seasonal change in June-October increased My best guess would be it's somehow related to water distribution? More water going into the atmosphere? Glaciers growing (unlikely)? Did multiple huge water reservoirs go into service and get filled up over the summer months?

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flohofwoe 2 hours ago | parent | prev | next [-]

Since I was checking the Wikipedia article anyway (for when the last leap second was inserted), it also has an answer for this:

"Because the Earth's rotational speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and not precisely predictable."

https://en.wikipedia.org/wiki/Leap_second

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moi2388 2 hours ago | parent | prev [-]

Yes. Geological activity, movement in the outer core, atmosphere, oceanic currents, melting ice, earthquakes, to name a few.

Earths rotation has been unusually fast lately. So there is not enough drift to warrant a leap second.