6 ms·

"In other words, we want UTC noon to be within a second of mean solar noon on the prime meridian." Why? If I travel 1 mile east or west of the prime meridian,

by Vvector 2mo ago

"In other words, we want UTC noon to be within a second of mean solar noon on the prime meridian." Why? If I travel 1 mile east or west of the prime meridian, my solar noon now comes 2-3 seconds earlier/later. It's nearly impossible to have your local time match your local solar noon. For most of the population, solar noon is, on average, 30 minutes off of 12:00 noon. Plus, solar noon varies from day to day by 10-20 seconds. Check the charts out. https://www.timeanddate.com/sun/usa/new-york https://www.timeanddate.com/sun/usa/new-york

Perz1val 2mo ago
Probably there are things more important than your lunch that need time to be exactly synced with sun position
- oasisaimlessly 2mo ago
  For things that need much more precusion than my lunch, ±1 second probably still isn't good enough, so they need another layer of correction anyway. Given that exists, might as well push leap seconds into that layer too.
pdonis 2mo 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 2mo 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 2mo 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 2mo 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 2mo ago

by Vvector 2mo ago

Perz1val 2mo ago

Probably there are things more important than your lunch that need time to be exactly synced with sun position

oasisaimlessly 2mo ago
For things that need much more precusion than my lunch, ±1 second probably still isn't good enough, so they need another layer of correction anyway. Given that exists, might as well push leap seconds into that layer too.

pdonis 2mo 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 2mo 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 2mo 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 2mo 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 2mo ago