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?
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 glassnoss satellites.
(Note: on mobile, this is from memory, details need checking ;))
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.)
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."
The changes in Earth's rotational speed that leap seconds help account for affect the whole globe. Why shouldn't the effects be noted in the global time standard?
Leap days, February 29th, are not at the level of time zones. Different time zones do not disagree as to when March 1st will occurs immediately after February 28th.
god that would be awful. Can you imagine time zones being one second off from each other. Or two or three? ah yes, india is GMT+4:30:03, where europe is GMT+0:59:58
It’s a huge problem. The most common approach to address it is called smearing; the duration of each second for a 24 hour period ahead of the “leap” is adjusted. For strict ordering systems this works as each device maintains time sync with the global clock, the duration of a clock cycle is just slightly different. I think this was in the original Spanner paper, actually.
Some rare systems use monotonic oscillator seconds and ignore the earth rotation second, but if you ever have to translate those to real time, you get an accumulating disaster over time and it’s generally regarded as not a good idea.
Clocks used to be able to use the 60Hz cycle to track time, and grid providers would run slightly slow or fast ("time error correction") to get back into sync. A leap second would just be part of this.
I believe in the US this error correction has been discontinued in the East and in Texas, but is still done in the West for some kind of non-clock "inadvertent interchange" reasons I don't understand.
Leap seconds are not added on a regular schedule like leap days, they depend on physical measurements of Earth. So high reliability systems with comprehensive timekeeping would not be perturbed by these choices, I would think.
There's an opportunity to insert or remove a leap second twice a year. They only decide about 6 months in advance of each opportunity what to do (leap second, skipped second, or do nothing).
Not anymore forever. We’re just not adding one for this year. We might need one next year, we might not. It all depends on the Earth’s rotation and orbit
In practice it will never affect anyone because it's a legal fiction, but even if you pretend to believe we would actually introduce this "leap hour" it would be in the distant future long after we're all dead and if there are still humans who have any idea the year 2026 happened they're not sure which of Donald Trump, Taylor Swift, Tony Stark and John McClane were real people.
"To authorities responsible for the measurement and distribution of time" is just the best preamble ever.
The only better thing is the organization being called "International Earth Rotation Service"
Oh boy :) I think that would come with IERS Tax.
You have to go to the ends of the earth to cancel.
For many years, the title of the leadership role over the various precise time products at the USNO was "Director of the Directorate of Time"
Sounds like something out of a Douglas Adams novel.
Or XKCD. I love patch day.
"Director Earth Orientation Center of IERS Observatoire de Paris, France"
Even the titles are sci-fi.
They should call themselves Time Lords
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?
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 glassnoss satellites.
(Note: on mobile, this is from memory, details need checking ;))
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.)
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.
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
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.
Hear me out. We can just mount jet engines along the equator and rotate them 180 to gain or lose time. And then connect them to my snooze button.
They should have a global holiday to celebrate the people who maintain time/date related code in OS kernels that keeps the world from imploding.
Lol. Exactly!
> The difference between Coordinated Universal Time UTC and the International Atomic Time TAI is :
>
> from 2017 January 1, 0h UTC, until further notice : UTC-TAI = -37s
This means the atomic clock is behind the solar clock by 37 seconds? I also don’t understand the reference to 2017.
Apparently December 2016 was the last time a leap second was inserted, at least that's what Wikipedia says:
https://en.wikipedia.org/wiki/Leap_second
we were ought to insert a negative leap second, but cowards got too afraid it would break code
> I also don’t understand the reference to 2017.
My guess is that is when they last changed the offset, so the -37s has been in effect since then.
As one HN comment said years ago: I feel leap seconds have always lived in the wrong abstraction layer.
They should live in the same abstraction layer that does leap days and daylight savings: the time zones.
The changes in Earth's rotational speed that leap seconds help account for affect the whole globe. Why shouldn't the effects be noted in the global time standard?
Leap days, February 29th, are not at the level of time zones. Different time zones do not disagree as to when March 1st will occurs immediately after February 28th.
god that would be awful. Can you imagine time zones being one second off from each other. Or two or three? ah yes, india is GMT+4:30:03, where europe is GMT+0:59:58
What happens to systems such as Spanner under these circumstances?
Is it a headache or a non-issue
It’s a huge problem. The most common approach to address it is called smearing; the duration of each second for a 24 hour period ahead of the “leap” is adjusted. For strict ordering systems this works as each device maintains time sync with the global clock, the duration of a clock cycle is just slightly different. I think this was in the original Spanner paper, actually.
Some rare systems use monotonic oscillator seconds and ignore the earth rotation second, but if you ever have to translate those to real time, you get an accumulating disaster over time and it’s generally regarded as not a good idea.
I wonder if that's what electricity producers do? If you are selling 50 or 60 Hz service, an extra second here or there must really mess things up.
Clocks used to be able to use the 60Hz cycle to track time, and grid providers would run slightly slow or fast ("time error correction") to get back into sync. A leap second would just be part of this.
I believe in the US this error correction has been discontinued in the East and in Texas, but is still done in the West for some kind of non-clock "inadvertent interchange" reasons I don't understand.
Leap seconds are not added on a regular schedule like leap days, they depend on physical measurements of Earth. So high reliability systems with comprehensive timekeeping would not be perturbed by these choices, I would think.
I enjoy how Chrome asks me if I want to auto translate from German to English. Where did it get German from? It's French!
Probably from the "Content-Language: de" header
Ok but that makes even less sense
It's a mix of french and english... so it's german.
A mix of English and French is just called... English
This book would probably disagree :-) https://www.fnac.com/a19325120/Bernard-Cerquiglini-La-langue...
Frenglish
Franglais?
Does this mean the negative leap second isn't happening anymore?
There's an opportunity to insert or remove a leap second twice a year. They only decide about 6 months in advance of each opportunity what to do (leap second, skipped second, or do nothing).
Not anymore forever. We’re just not adding one for this year. We might need one next year, we might not. It all depends on the Earth’s rotation and orbit
and Earth's rotation was too fast for last several years
we were all waiting for the negative leap second to finally happen - but cowards got too afraid
Notice they only said leap second.
Meanwhile....
International timekeepers to vote on changing the leap second to a leap hour
https://www.scientificamerican.com/article/international-tim... (https://archive.ph/GnQUj https://news.ycombinator.com/item?id=48842329)
really, my I just don't have the time to keep up with this.
A leap hour wouldn't affect you.
In practice it will never affect anyone because it's a legal fiction, but even if you pretend to believe we would actually introduce this "leap hour" it would be in the distant future long after we're all dead and if there are still humans who have any idea the year 2026 happened they're not sure which of Donald Trump, Taylor Swift, Tony Stark and John McClane were real people.
We can change that!