5 ms·

> Why? Um, because it's the prime meridian and that's how UTC is defined? > It's nearly impossible to have your local time match your local solar noon. Which

by pdonis 3mo ago

> Why? Um, because it's the prime meridian and that's how UTC is defined? > It's nearly impossible to have your local time match your local solar noon. Which is why I specified on the prime meridian, which is the particular local meridian that UTC is defined as corresponding to. > solar noon varies from day to day by 10-20 seconds. Which is why I was careful to specify mean solar noon. I'm not quite sure what your issue is. Yes, we have time zones tied to specific meridians, and the actual sun's speed in the sky varies (which I mentioned in my post, so I'm not sure why you seem to think I'm unaware of it) so in most places local time by the clock doesn't match local time by the sun. Yes, a leap second adjustment to UTC is quite a bit smaller, taken in isolation, than the annual variation in actual solar time vs. mean solar time. But over time, if we didn't have leap seconds, the difference would accumulate. The accumulated difference now between UTC and TAI is 37 seconds--which is almost twice the maximum variation in actual solar noon from mean solar noon that you refer to. We humans have collectively decided that we don't want that, and that it's better to do the adjustments a little at a time rather than in bigger lumps.

Vvector 3mo ago
"But over time, if we didn't have leap seconds, the difference would accumulate. The accumulated difference now between UTC and TAI is 37 seconds--which is almost twice the maximum variation in actual solar noon from mean solar noon that you refer to." No, the 10-15 seconds I mentioned is the daily variation in solar noon. From the link I posted, in NYC, solar noon on 2026-01-01 is at 11:59am. On 2026-01-31, solar noon is at 12:09pm. In one month, it has drifted 10 minutes. That's much greater than the 37 leap seconds we have added in 60 years. "We humans have collectively decided that we don't want that, and that it's better to do the adjustments a little at a time rather than in bigger lumps." Yet we just reversed that decision. No more leap seconds after 2035. After trying it, we decided it was terrible.
- pdonis 3mo ago
  > the 10-15 seconds I mentioned is the daily variation in solar noon. Yes, but averaged over an entire year, it still comes out to zero. The difference between mean solar and atomic time does not. It accumulates over the years. > we just reversed that decision We paused it for 100 years after 2035. That doesn't change the physical fact that the Earth's rotation will continue to slow over the long term. We might eventually decide to just not care about that when it comes to civil timekeeping, but that's not what the decision you're referring to did. It just said we can afford to let the difference between UTC and TAI accumulate from 2035 to 2135 (by which time it is predicted to be about a minute) while we figure out what we want to do over the longer term.
Dylan16807 3mo ago
> Um, because it's the prime meridian and that's how UTC is defined? That's an explanation of how it is, not why we should care to preserve it. The definitions of hours minutes and seconds have changed before, and in recent history. > Which is why I was careful to specify mean solar noon. And "mean solar noon" is meaningless to people's lives. Even in the areas where time zones do follow meridians and not country borders that are many minutes off.
- pdonis 3mo ago
  > The definitions of hours minutes and seconds have changed before, and in recent history. In terms of what physical process we use to set the standard, yes. But those very changes were made to try to preserve the same time periods that were important to humans. In other words, to not change what hours, minutes, and seconds mean intuitively to us humans as we go about our daily lives.
  - Dylan16807 3mo ago
    I completely disagree. The intuitive meaning is that a day is 24 hours and you can divide that by 60 twice. But that makes the second vary by some parts per billion, so we nailed down the second to make the technical side easier at the expense of relatability.
    - pdonis 3mo ago
      > we nailed down the second to make the technical side easier at the expense of relatability. Would you also say that getting rid of leap seconds and allowing UTC to gradually drift away from the sun is making the technical side easier at the expense of relatability?
    - pdonis 3mo ago
      > The intuitive meaning is that a day is 24 hours and you can divide that by 60 twice. And that's exactly why the SI second has the length that it has--to be as close as possible in terms of how atomic clocks work to 1/86400 of an Earth mean solar day at a chosen epoch. (Note that the actual definition before the atomic clock one was adopted was in terms of the Earth's tropical year at that epoch--but the fraction of the tropical year that was chosen was to line up the second with 1/86400 of an epoch day.) If we didn't care about "relatability", nobody would have gone to all the trouble of trying to determine how many cesium clock oscillations there were in 1/86400 of an epoch day.

by pdonis 3mo ago

Vvector 3mo ago

"But over time, if we didn't have leap seconds, the difference would accumulate. The accumulated difference now between UTC and TAI is 37 seconds--which is almost twice the maximum variation in actual solar noon from mean solar noon that you refer to." No, the 10-15 seconds I mentioned is the daily variation in solar noon. From the link I posted, in NYC, solar noon on 2026-01-01 is at 11:59am. On 2026-01-31, solar noon is at 12:09pm. In one month, it has drifted 10 minutes. That's much greater than the 37 leap seconds we have added in 60 years. "We humans have collectively decided that we don't want that, and that it's better to do the adjustments a little at a time rather than in bigger lumps." Yet we just reversed that decision. No more leap seconds after 2035. After trying it, we decided it was terrible.

pdonis 3mo ago
> the 10-15 seconds I mentioned is the daily variation in solar noon. Yes, but averaged over an entire year, it still comes out to zero. The difference between mean solar and atomic time does not. It accumulates over the years. > we just reversed that decision We paused it for 100 years after 2035. That doesn't change the physical fact that the Earth's rotation will continue to slow over the long term. We might eventually decide to just not care about that when it comes to civil timekeeping, but that's not what the decision you're referring to did. It just said we can afford to let the difference between UTC and TAI accumulate from 2035 to 2135 (by which time it is predicted to be about a minute) while we figure out what we want to do over the longer term.

Dylan16807 3mo ago

> Um, because it's the prime meridian and that's how UTC is defined? That's an explanation of how it is, not why we should care to preserve it. The definitions of hours minutes and seconds have changed before, and in recent history. > Which is why I was careful to specify mean solar noon. And "mean solar noon" is meaningless to people's lives. Even in the areas where time zones do follow meridians and not country borders that are many minutes off.

pdonis 3mo ago
> The definitions of hours minutes and seconds have changed before, and in recent history. In terms of what physical process we use to set the standard, yes. But those very changes were made to try to preserve the same time periods that were important to humans. In other words, to not change what hours, minutes, and seconds mean intuitively to us humans as we go about our daily lives.
- Dylan16807 3mo ago
  I completely disagree. The intuitive meaning is that a day is 24 hours and you can divide that by 60 twice. But that makes the second vary by some parts per billion, so we nailed down the second to make the technical side easier at the expense of relatability.
  - pdonis 3mo ago
    > we nailed down the second to make the technical side easier at the expense of relatability. Would you also say that getting rid of leap seconds and allowing UTC to gradually drift away from the sun is making the technical side easier at the expense of relatability?
  - pdonis 3mo ago
    > The intuitive meaning is that a day is 24 hours and you can divide that by 60 twice. And that's exactly why the SI second has the length that it has--to be as close as possible in terms of how atomic clocks work to 1/86400 of an Earth mean solar day at a chosen epoch. (Note that the actual definition before the atomic clock one was adopted was in terms of the Earth's tropical year at that epoch--but the fraction of the tropical year that was chosen was to line up the second with 1/86400 of an epoch day.) If we didn't care about "relatability", nobody would have gone to all the trouble of trying to determine how many cesium clock oscillations there were in 1/86400 of an epoch day.