7 ms·

How do we not have a way to predict leap seconds?

kelnos 5y ago
I believe we used to, but only recently have we began to understand that the Earth's rotation is not slowing down in a linear/predictable fashion. A comment on the bug says this has something to do with global warming, but I don't really have much context here.
- btilly 5y ago
  The Earth's rotation is complicated. See https://www.smithsonianmag.com/smart-news/global-warming-changing-how-fast-earth-spins-180957550/ https://www.smithsonianmag.com/smart-news/global-warming-cha... for how global warming is theoretically speeding up the Earth's rotation (melting ice causes land to rise at the poles, and moving rock turns out to be more important than water migrating from pole to equator) and https://phys.org/news/2015-12-scientists-reveal-rotation-earth-core.html https://phys.org/news/2015-12-scientists-reveal-rotation-ear... for how interactions of the core, mantle and crust are currently slowing the day down.
crote 5y ago
Leap seconds correct for the difference between the time as measured by atomic clocks and the time determined by solar observations. Turns out the rotation speed of Earth varies. Things like tides, earthquakes, and climate change can affect it. There is no formula for that, the only thing you can do is measure and issue a leap second when required.
oceanghost 5y ago
We're floating through space and that space doesn't have a uniform shape and then there's the n-body-problem. Interestingly enough we're finding out that a lot of our solar cycles are probably due to massive things in very long orbits in our solar system.
nonfamous 5y ago
The number of leap seconds required is determined by the Earth's rotation speed, which isn't constant. In the same way that an ice skater extending his arms slows down, shifts in mass on the Earth can alter its rotation speed. Earthquakes, icemelt, atmospheric warming, and even the filling of the Three Gorges Dam can have an effect at the scale required for GPS synchronization.
- pithon 5y ago
  This isn't even really a GPS thing, it's UTC time that bounces around because it wants to stay synchronized to the day/night cycle. GPS time is continuous.
packetslave 5y ago
Quoting from that thread: > I don't think gpsd has any reason to be predicting when a future leap-second is going to occu Until last year, leap seconds had been very predicable. The effect of global warming on earth rotational speeds was only very recently seen, or even predicted. But, yes, going forward, that needs to change. > And the code in question is clearly expecting only positive leap-seconds. Yes, because until 2020, the thought of a negative leap second was unthinkable. I would welcome you testing that and seeing what falls out.
swman 5y ago
It'd be cool if we could just invent a new standard time system that's independent from things like that which add variation or unexpected randomness. I mean sure it would be annoying but its a one time change and our generation has to endure the pain of upgrading our systems, but it would be worth it no?
- wmf 5y ago
  There's not really anything to invent or upgrade. Just never add any more leap seconds (the past ones can't be safely removed). We need to lobby the IERS harder.
  - sp332 5y ago
    We already have one of those. https://en.m.wikipedia.org/wiki/International_Atomic_Time https://en.m.wikipedia.org/wiki/International_Atomic_Time TAI is just UTC before the leap second adjustments.
    - wmf 5y ago
      Switching everyone from UTC to TAI is too much work and if you try to use TAI while everyone else is on UTC you'll run into off-by-37 bugs everywhere. It's better to keep using UTC but add no more leap seconds to it.
      - sp332 5y ago
        If people didn't need leap seconds, they would already not be using them. There's absolutely no use case for adding leap seconds for a few decades and then stopping. Either put them in or don't.
        wmf 5y ago
        Leap seconds are the right thing to do but people didn't realize all the bugs and costs they would trigger. Now that we understand these costs we can and should change our minds.
tialaramex 5y ago
We decided to have leap seconds to match UTC which otherwise needn't care and could be based on TAI, against UT, which depends on the gentle spinning of the large rock we all live on. The IERS https://www.iers.org/ https://www.iers.org/ is in charge of monitoring the spinning of the Earth. On the basis of their assessment a decision is made every six months whether to inject (or indeed remove) a leap second. If we decided not to match UTC to UT and thus we did not care precisely how quickly or slowly the Earth is spinning, we could abolish leap seconds. If you meant, "Why can't we precisely predict the motions of a vast rock floating in space years into the future" then I don't know what to tell you. We're not God?
- remexre 5y ago
  > If you meant, "Why can't we precisely predict the motions of a vast rock floating in space years into the future" then I don't know what to tell you. We're not God? IMO this feels like the more interesting one to explore: are leap seconds wholly from things that are within measurement error in existing rotation, or is the Earth's rotational rate actually changing? If we had leap minutes as the smallest increment, could we predict them out centuries in advance? etc
  - jhayward 5y ago
    No, they are not measurement error. The moment of inertia of the earth changes over time in irregular ways (e.g, melting glaciers, etc). Even a major earthquake can show up in the earth’s rotation. Leap seconds account for the accumulated difference in the rotational period from time to time.
- wumpus 5y ago
  Why not just go back to the Julian calendar? You'd avoid a lot of software bugs.
  - gerikson 5y ago
    You jest, but I like to use Julian days (fun fact: different Julian!) to calculate day ranges. It's built in to SQLite so it's really convenient.
foepys 5y ago
In the same vain there is no algorithm to precisely predict moon phases. A complete cycle takes proximately 27 days but not really. To find out you have to look into the sky which can also be a bit subjective.

I believe we used to, but only recently have we began to understand that the Earth's rotation is not slowing down in a linear/predictable fashion. A comment on the bug says this has something to do with global warming, but I don't really have much context here.

btilly 5y ago
The Earth's rotation is complicated. See https://www.smithsonianmag.com/smart-news/global-warming-changing-how-fast-earth-spins-180957550/ https://www.smithsonianmag.com/smart-news/global-warming-cha... for how global warming is theoretically speeding up the Earth's rotation (melting ice causes land to rise at the poles, and moving rock turns out to be more important than water migrating from pole to equator) and https://phys.org/news/2015-12-scientists-reveal-rotation-earth-core.html https://phys.org/news/2015-12-scientists-reveal-rotation-ear... for how interactions of the core, mantle and crust are currently slowing the day down.

crote 5y ago

Leap seconds correct for the difference between the time as measured by atomic clocks and the time determined by solar observations. Turns out the rotation speed of Earth varies. Things like tides, earthquakes, and climate change can affect it. There is no formula for that, the only thing you can do is measure and issue a leap second when required.

oceanghost 5y ago

We're floating through space and that space doesn't have a uniform shape and then there's the n-body-problem. Interestingly enough we're finding out that a lot of our solar cycles are probably due to massive things in very long orbits in our solar system.

nonfamous 5y ago

The number of leap seconds required is determined by the Earth's rotation speed, which isn't constant. In the same way that an ice skater extending his arms slows down, shifts in mass on the Earth can alter its rotation speed. Earthquakes, icemelt, atmospheric warming, and even the filling of the Three Gorges Dam can have an effect at the scale required for GPS synchronization.

pithon 5y ago
This isn't even really a GPS thing, it's UTC time that bounces around because it wants to stay synchronized to the day/night cycle. GPS time is continuous.

packetslave 5y ago

Quoting from that thread: > I don't think gpsd has any reason to be predicting when a future leap-second is going to occu Until last year, leap seconds had been very predicable. The effect of global warming on earth rotational speeds was only very recently seen, or even predicted. But, yes, going forward, that needs to change. > And the code in question is clearly expecting only positive leap-seconds. Yes, because until 2020, the thought of a negative leap second was unthinkable. I would welcome you testing that and seeing what falls out.

swman 5y ago

It'd be cool if we could just invent a new standard time system that's independent from things like that which add variation or unexpected randomness. I mean sure it would be annoying but its a one time change and our generation has to endure the pain of upgrading our systems, but it would be worth it no?

wmf 5y ago
There's not really anything to invent or upgrade. Just never add any more leap seconds (the past ones can't be safely removed). We need to lobby the IERS harder.
- sp332 5y ago
  We already have one of those. https://en.m.wikipedia.org/wiki/International_Atomic_Time https://en.m.wikipedia.org/wiki/International_Atomic_Time TAI is just UTC before the leap second adjustments.
  - wmf 5y ago
    Switching everyone from UTC to TAI is too much work and if you try to use TAI while everyone else is on UTC you'll run into off-by-37 bugs everywhere. It's better to keep using UTC but add no more leap seconds to it.
    - sp332 5y ago
      If people didn't need leap seconds, they would already not be using them. There's absolutely no use case for adding leap seconds for a few decades and then stopping. Either put them in or don't.
      - wmf 5y ago
        Leap seconds are the right thing to do but people didn't realize all the bugs and costs they would trigger. Now that we understand these costs we can and should change our minds.

tialaramex 5y ago

We decided to have leap seconds to match UTC which otherwise needn't care and could be based on TAI, against UT, which depends on the gentle spinning of the large rock we all live on. The IERS https://www.iers.org/ https://www.iers.org/ is in charge of monitoring the spinning of the Earth. On the basis of their assessment a decision is made every six months whether to inject (or indeed remove) a leap second. If we decided not to match UTC to UT and thus we did not care precisely how quickly or slowly the Earth is spinning, we could abolish leap seconds. If you meant, "Why can't we precisely predict the motions of a vast rock floating in space years into the future" then I don't know what to tell you. We're not God?

remexre 5y ago
> If you meant, "Why can't we precisely predict the motions of a vast rock floating in space years into the future" then I don't know what to tell you. We're not God? IMO this feels like the more interesting one to explore: are leap seconds wholly from things that are within measurement error in existing rotation, or is the Earth's rotational rate actually changing? If we had leap minutes as the smallest increment, could we predict them out centuries in advance? etc
- jhayward 5y ago
  No, they are not measurement error. The moment of inertia of the earth changes over time in irregular ways (e.g, melting glaciers, etc). Even a major earthquake can show up in the earth’s rotation. Leap seconds account for the accumulated difference in the rotational period from time to time.
wumpus 5y ago
Why not just go back to the Julian calendar? You'd avoid a lot of software bugs.
- gerikson 5y ago
  You jest, but I like to use Julian days (fun fact: different Julian!) to calculate day ranges. It's built in to SQLite so it's really convenient.

foepys 5y ago

In the same vain there is no algorithm to precisely predict moon phases. A complete cycle takes proximately 27 days but not really. To find out you have to look into the sky which can also be a bit subjective.