6 ms·

Will we have a negative leap second? [pdf] (2022)

89 points by fanf2 2y ago

fouronnes3 2y ago
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.
- LegionMammal978 2y ago
  > 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?searchToken=4lwcvvfvoo8zj0syhpo1xqzwi#/media/File%3ADeviation_of_day_length_from_SI_day.svg https://en.wikipedia.org/wiki/Day_length_fluctuations?search... [1] https://en.wikipedia.org/wiki/Tidal_acceleration https://en.wikipedia.org/wiki/Tidal_acceleration
- evv 2y ago
  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.
  - rrr_oh_man 2y ago
    Someone knowledgeable should do the math
  - defrost 2y ago
    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/10.1029/2023GL103509 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-shifted-earths-axis-by-pumping-lots-of-groundwater-180982403 https://www.smithsonianmag.com/smart-news/humans-have-shifte...
  - zer00eyz 2y ago
    You just have to build a big dam: https://medium.com/coinmonks/three-gorges-dam-a-climate-change-catalyst-and-earths-rotational-slowdown-56acb0ae8bf4 https://medium.com/coinmonks/three-gorges-dam-a-climate-chan...
  - atoav 2y ago
    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.
- grandchild 2y ago
  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.
- fanf2 2y ago
  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 https://datacenter.iers.org/plots.php?id=6
Sniffnoy 2y ago
What's mts?
- schoen 2y ago
  The math in his presentation seems to work out if you interpret it as milliseconds (e.g., the day is getting shorter by an average of 2.2 milliseconds every century). I'm not sure why that particular abbreviation. I tried searching Google to find people using "mts" to refer to milliseconds, and it seems like it could be "millisecond timestamp" or "metric time series"??
  - croemer 2y ago
    Maybe a mistake? I would usually read "mts" as "minutes".
- croemer 2y ago
  It looks like minutes but it can't be. It's milliseconds if you look at the numbers: "1.4 mts/day/century (0.5 sec/year/cty) historical slowdown"
sed3 2y ago
Dealing with leap seconds is very expensive. What happens if countries like China, Russia or Iran just decide to ignore leap seconds? We will have a clusterfuck of time zones divergent by a few seconds! I am really into astronomy. But dealing with this, just so stars pass local meridian exactly at 00:00:00.000 is simply not worth it! And one funny note, astronomers still use use Julian calendar (one made by Ceasar without Gregorian corrections in 16th century) to avoid similar issues. They avoid their own inventions!
- fanf2 2y ago
  Leap seconds are in the process of being abolished. The ITU-R confirmed their part of the agreement at the world radiocommunication conference last year (the ITU-R’s triennial treaty update); the next stage is the CGPM in 2026 (the BIPM’s triennial treaty update).
  - arijun 2y ago
    Will there be another solution for the desynchronization of SI days and solar days? It looks like there’s been about 25 seconds difference in the past 30 years or so.
    - fanf2 2y ago
      UT1 is available from the IERS if you need it.
    - eequah9L 2y ago
      An option if it gets way off would be to just express it as TZ shift. Current rate seems to be about a second every two years, so we have some hundreds of years before we need to even shift by 15 minutes. An amount of divergence that I suspect would not really be even noticeable, especially as it develops over hundreds of years. The TZ code is used widely and regularly and systems tend to know how to handle it. Leap seconds are always a bit of an adventure from what I understand. (I'm not a sysadmin, so I don't know the details.) Like -- I don't know, but is it actually important that the solar day is tied to "wall clock day" so snugly? So what if it's a couple minutes off...?
icytotem 2y ago
This may be totally dumb, but if the variation is attributed to the tides, is it possible that global warming affected it? With the ice cap melting, there is more sea water sloshing around instead of just floating at the top.
- mianos 2y ago
  It's not. They are statistically insignificant compared the rotation of the core mass.
  - sigmoid10 2y ago
    Wrong. For rotation you don't look at mass, but at moment of inertia. That scales like ~M*R^2 or ~rho*R^5 for a density of rho and distance R from the axis of rotation. So even if the core is much denser, it contributes a lot less to the earth's total moment of inertia. Earth's rotation is even measurably affected by plants changing throughout the seasons. Glacial melts and earthquakes also have measurable effects that need to be compensated for. See here for the plot: https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/Deviation_of_day_length_from_SI_day.svg/2560px-Deviation_of_day_length_from_SI_day.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/De... And here for a thorough explanation: https://www.sciencedirect.com/science/article/pii/S1674984717300666 https://www.sciencedirect.com/science/article/pii/S167498471...
    - nextaccountic 2y ago
      M*R^2 is understandable but what's this raised to 5th power stuff in rho*r^5? And is this lowercase r the same as the uppercase R?
      - sigmoid10 2y ago
        Mass scales like density times radius cubed. The first M would technically be a function of position. So you would integrate over slices of varying M not just because of varying density.
    - 2y ago
bluenose69 2y ago
This following book may be of interest to some people in this discussion thread. It's a re-issue of a book published decades prior, so of course it is not up-to-date. But it is deeply researched and really quite fascinating. Munk, Walter, and Gordon J. MacDonald. The Rotation of the Earth: A Geophysical Discussion. Digitally printed version. Cambridge Monographs on Mechanics and Applied Mathematics. Cambridge: Cambridge University Press, 2009.
acimim_ha 2y ago
"The Earth’s spin is slowing down. This is annoying." https://what-if.xkcd.com/26/ https://what-if.xkcd.com/26/
deleted 2y ago
[deleted]
dfc 2y ago
The author of the PDF was the head of Time Services at USNO for decades.
gmiller123456 2y ago
"Add an entire day to the calendar, no one bats an eye. And one second and everyone loses their minds". I think the main issue with dealing with leap seconds is really a lack of awareness. Our calendars are already full of idiosyncrasies, but we've learned to deal with them just fine. When UTC was created, so was Atomic Time (TAI). The overwhelming majority of systems are unaffected by leap seconds, for the few that are, it would simply be a matter of using TAI instead of UTC, and having software to readily convert between the two.
- phicoh 2y ago
  The problem with leap seconds is that any conversion between TAI and UTC for a time stamp more than 6 months in the future is undefined. If we did the same thing with time zones or leap days, that some committee would only announce changes a few months in advance then people would get very upset.
  - Mountain_Skies 2y ago
    It does happen, just not all that frequently. The US and Canada changed daylight saving time's dates as recently as 2007 and time zone rules are changed in various parts of the world now and then. It happens frequently enough that there's a database maintained to track it and let systems programmed to use the database to adjust to the changes.
    - phicoh 2y ago
      Think about if it happens (or does not happen) every 6 months and you can never plan ahead. And daylight saving is a local problem. In my country the rules have not changed for more than 4 decades. The other problem is that most systems use time zones and daylight saving mostly as a presentation format. Only time that is explicitly for human consumption deals with time zones. Internally computers mostly use UTC. The problem with leap seconds is that computers often represent time as some number of seconds since an epoch. That requires a stable conversion between year-month-day-hour-minute-second to seconds since epoch. And that's where leap second are a problem. Because future leap seconds a undefined, this conversion is undefined for dates in the future. That then results in a collection of hacks to let it work most of the time. Where I live, civil time is currently off by about one hour and 41 minutes, compared to solar time. It is safe to say that essentially nobody cares about solar time with more precision than one hour. If you want to know about sunrise or sunset, you just look it up. Noon is mostly irrelevant.

89 points by fanf2 2y ago

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.

LegionMammal978 2y ago
> 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?searchToken=4lwcvvfvoo8zj0syhpo1xqzwi#/media/File%3ADeviation_of_day_length_from_SI_day.svg https://en.wikipedia.org/wiki/Day_length_fluctuations?search... [1] https://en.wikipedia.org/wiki/Tidal_acceleration https://en.wikipedia.org/wiki/Tidal_acceleration
evv 2y ago
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.
- rrr_oh_man 2y ago
  Someone knowledgeable should do the math
- defrost 2y ago
  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/10.1029/2023GL103509 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-shifted-earths-axis-by-pumping-lots-of-groundwater-180982403 https://www.smithsonianmag.com/smart-news/humans-have-shifte...
- zer00eyz 2y ago
  You just have to build a big dam: https://medium.com/coinmonks/three-gorges-dam-a-climate-change-catalyst-and-earths-rotational-slowdown-56acb0ae8bf4 https://medium.com/coinmonks/three-gorges-dam-a-climate-chan...
- atoav 2y ago
  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.
grandchild 2y ago
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.
fanf2 2y ago
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 https://datacenter.iers.org/plots.php?id=6

Sniffnoy 2y ago

What's mts?

schoen 2y ago
The math in his presentation seems to work out if you interpret it as milliseconds (e.g., the day is getting shorter by an average of 2.2 milliseconds every century). I'm not sure why that particular abbreviation. I tried searching Google to find people using "mts" to refer to milliseconds, and it seems like it could be "millisecond timestamp" or "metric time series"??
- croemer 2y ago
  Maybe a mistake? I would usually read "mts" as "minutes".
croemer 2y ago
It looks like minutes but it can't be. It's milliseconds if you look at the numbers: "1.4 mts/day/century (0.5 sec/year/cty) historical slowdown"

sed3 2y ago

Dealing with leap seconds is very expensive. What happens if countries like China, Russia or Iran just decide to ignore leap seconds? We will have a clusterfuck of time zones divergent by a few seconds! I am really into astronomy. But dealing with this, just so stars pass local meridian exactly at 00:00:00.000 is simply not worth it! And one funny note, astronomers still use use Julian calendar (one made by Ceasar without Gregorian corrections in 16th century) to avoid similar issues. They avoid their own inventions!

fanf2 2y ago
Leap seconds are in the process of being abolished. The ITU-R confirmed their part of the agreement at the world radiocommunication conference last year (the ITU-R’s triennial treaty update); the next stage is the CGPM in 2026 (the BIPM’s triennial treaty update).
- arijun 2y ago
  Will there be another solution for the desynchronization of SI days and solar days? It looks like there’s been about 25 seconds difference in the past 30 years or so.
  - fanf2 2y ago
    UT1 is available from the IERS if you need it.
  - eequah9L 2y ago
    An option if it gets way off would be to just express it as TZ shift. Current rate seems to be about a second every two years, so we have some hundreds of years before we need to even shift by 15 minutes. An amount of divergence that I suspect would not really be even noticeable, especially as it develops over hundreds of years. The TZ code is used widely and regularly and systems tend to know how to handle it. Leap seconds are always a bit of an adventure from what I understand. (I'm not a sysadmin, so I don't know the details.) Like -- I don't know, but is it actually important that the solar day is tied to "wall clock day" so snugly? So what if it's a couple minutes off...?

icytotem 2y ago

This may be totally dumb, but if the variation is attributed to the tides, is it possible that global warming affected it? With the ice cap melting, there is more sea water sloshing around instead of just floating at the top.

mianos 2y ago
It's not. They are statistically insignificant compared the rotation of the core mass.
- sigmoid10 2y ago
  Wrong. For rotation you don't look at mass, but at moment of inertia. That scales like ~M*R^2 or ~rho*R^5 for a density of rho and distance R from the axis of rotation. So even if the core is much denser, it contributes a lot less to the earth's total moment of inertia. Earth's rotation is even measurably affected by plants changing throughout the seasons. Glacial melts and earthquakes also have measurable effects that need to be compensated for. See here for the plot: https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/Deviation_of_day_length_from_SI_day.svg/2560px-Deviation_of_day_length_from_SI_day.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/5/5b/De... And here for a thorough explanation: https://www.sciencedirect.com/science/article/pii/S1674984717300666 https://www.sciencedirect.com/science/article/pii/S167498471...
  - nextaccountic 2y ago
    M*R^2 is understandable but what's this raised to 5th power stuff in rho*r^5? And is this lowercase r the same as the uppercase R?
    - sigmoid10 2y ago
      Mass scales like density times radius cubed. The first M would technically be a function of position. So you would integrate over slices of varying M not just because of varying density.
  - 2y ago

bluenose69 2y ago

This following book may be of interest to some people in this discussion thread. It's a re-issue of a book published decades prior, so of course it is not up-to-date. But it is deeply researched and really quite fascinating. Munk, Walter, and Gordon J. MacDonald. The Rotation of the Earth: A Geophysical Discussion. Digitally printed version. Cambridge Monographs on Mechanics and Applied Mathematics. Cambridge: Cambridge University Press, 2009.

acimim_ha 2y ago

"The Earth’s spin is slowing down. This is annoying." https://what-if.xkcd.com/26/ https://what-if.xkcd.com/26/

deleted 2y ago

[deleted]

dfc 2y ago

The author of the PDF was the head of Time Services at USNO for decades.

gmiller123456 2y ago

"Add an entire day to the calendar, no one bats an eye. And one second and everyone loses their minds". I think the main issue with dealing with leap seconds is really a lack of awareness. Our calendars are already full of idiosyncrasies, but we've learned to deal with them just fine. When UTC was created, so was Atomic Time (TAI). The overwhelming majority of systems are unaffected by leap seconds, for the few that are, it would simply be a matter of using TAI instead of UTC, and having software to readily convert between the two.

phicoh 2y ago
The problem with leap seconds is that any conversion between TAI and UTC for a time stamp more than 6 months in the future is undefined. If we did the same thing with time zones or leap days, that some committee would only announce changes a few months in advance then people would get very upset.
- Mountain_Skies 2y ago
  It does happen, just not all that frequently. The US and Canada changed daylight saving time's dates as recently as 2007 and time zone rules are changed in various parts of the world now and then. It happens frequently enough that there's a database maintained to track it and let systems programmed to use the database to adjust to the changes.
  - phicoh 2y ago
    Think about if it happens (or does not happen) every 6 months and you can never plan ahead. And daylight saving is a local problem. In my country the rules have not changed for more than 4 decades. The other problem is that most systems use time zones and daylight saving mostly as a presentation format. Only time that is explicitly for human consumption deals with time zones. Internally computers mostly use UTC. The problem with leap seconds is that computers often represent time as some number of seconds since an epoch. That requires a stable conversion between year-month-day-hour-minute-second to seconds since epoch. And that's where leap second are a problem. Because future leap seconds a undefined, this conversion is undefined for dates in the future. That then results in a collection of hacks to let it work most of the time. Where I live, civil time is currently off by about one hour and 41 minutes, compared to solar time. It is safe to say that essentially nobody cares about solar time with more precision than one hour. If you want to know about sunrise or sunset, you just look it up. Noon is mostly irrelevant.