5 ms·

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 s

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?

flohofwoe 3mo ago
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 https://en.wikipedia.org/wiki/Leap_second
moi2388 3mo ago
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.
entrope 3mo ago
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=BulletinA_All-UT1-UTC&id=6 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.)
- doctoboggan 3mo ago
  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 3mo ago
    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?
    - recursivecaveat 3mo ago
      I know the Three Gorges Dam added ~0.06 microseconds on its own, so the effect of reservoirs definitely is noticeable. https://www.jpl.nasa.gov/news/nasa-details-earthquake-effects-on-the-earth/ https://www.jpl.nasa.gov/news/nasa-details-earthquake-effect...
_alternator_ 3mo ago
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 ;))
- thewebguyd 3mo ago
  > 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.
- tialaramex 3mo ago
  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.
ahazred8ta 3mo ago
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.
layer8 3mo ago
See https://en.wikipedia.org/wiki/Leap_second#:~:text=Other%20contributing%20factors https://en.wikipedia.org/wiki/Leap_second#:~:text=Other%20co...
fooqux 3mo ago
Even the migrations of animals can impact it, although they usually cancel out over the course of a year I think.
0ckpuppet 3mo ago
I predict there will be a news story or broadcast news pundit who blames this on climate change. bonus points if it winds up on dytime talk shows
- hdgvhicv 3mo ago
  Well melting ice certainly go tributes
ralferoo 3mo ago
Quite the opposite. The earth's rotation can vary by quite a few seconds each year. But over hundreds of years, the variations mostly cancel out, which is why I think trying to add or remove leap seconds are a bad idea. The only people who care about such things are space agencies, and they can apply whatever offset they want, it doesn't need to affect everyone else. Arguably, the only real-world impact this drift has on normal people is GPS, but GPS already transmits an offset from its own clock so that receivers can correct for that. The GPS clock is different both from UTC and TIA.
- chippiewill 3mo ago
  > the variations mostly cancel out I don't think we know this do we? we haven't been measuring accurately enough for long enough to be confident that it does in fact cancel out. In fact for the period of time where we've been applying leap seconds they were happening with significant frequency and always in the same direction. It's only been very recently that there's been the realistic suggestion it might drift in the other direction. If anything people have been suggesting that if we do get rid of leap seconds that we can just wait until the offset is enormous instead (say an hour) because it would take a very long time to happen. But even still, that does actually affect everyday people (because people would surely notice when solar noon is an hour later/earlier). Although pragmatically the obvious solution there is to change timezone instead.

flohofwoe 3mo ago

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

moi2388 3mo ago

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.

entrope 3mo ago

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=BulletinA_All-UT1-UTC&id=6 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.)

doctoboggan 3mo ago
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 3mo ago
  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?
  - recursivecaveat 3mo ago
    I know the Three Gorges Dam added ~0.06 microseconds on its own, so the effect of reservoirs definitely is noticeable. https://www.jpl.nasa.gov/news/nasa-details-earthquake-effects-on-the-earth/ https://www.jpl.nasa.gov/news/nasa-details-earthquake-effect...

_alternator_ 3mo ago

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 ;))

thewebguyd 3mo ago
> 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.
tialaramex 3mo ago
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.

ahazred8ta 3mo ago

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.

layer8 3mo ago

See https://en.wikipedia.org/wiki/Leap_second#:~:text=Other%20contributing%20factors https://en.wikipedia.org/wiki/Leap_second#:~:text=Other%20co...

fooqux 3mo ago

Even the migrations of animals can impact it, although they usually cancel out over the course of a year I think.

0ckpuppet 3mo ago

I predict there will be a news story or broadcast news pundit who blames this on climate change. bonus points if it winds up on dytime talk shows

hdgvhicv 3mo ago
Well melting ice certainly go tributes

ralferoo 3mo ago

Quite the opposite. The earth's rotation can vary by quite a few seconds each year. But over hundreds of years, the variations mostly cancel out, which is why I think trying to add or remove leap seconds are a bad idea. The only people who care about such things are space agencies, and they can apply whatever offset they want, it doesn't need to affect everyone else. Arguably, the only real-world impact this drift has on normal people is GPS, but GPS already transmits an offset from its own clock so that receivers can correct for that. The GPS clock is different both from UTC and TIA.

chippiewill 3mo ago
> the variations mostly cancel out I don't think we know this do we? we haven't been measuring accurately enough for long enough to be confident that it does in fact cancel out. In fact for the period of time where we've been applying leap seconds they were happening with significant frequency and always in the same direction. It's only been very recently that there's been the realistic suggestion it might drift in the other direction. If anything people have been suggesting that if we do get rid of leap seconds that we can just wait until the offset is enormous instead (say an hour) because it would take a very long time to happen. But even still, that does actually affect everyday people (because people would surely notice when solar noon is an hour later/earlier). Although pragmatically the obvious solution there is to change timezone instead.