1 rotation as measured againsg what? The sun, the stars, the cmb? All of those are ever so slightly different too.

1 solar day is how long it takes between "high noon" (meridian transit) on two consecutive days.

1 sidereal day is about 4 minutes shorter and the time it takes for the stars to be in the same position.

As far as I know, unless you're doing certain types of astronomy, you're using solar days.

You also need sidereal days for computing the Coriolis parameter, and both oceanographers and meteorologists need to know that. So the astronomers are in good company (or bad, depending on your point of view).

Both. That's why there's UT1 and UTC for different purposes.

* That's why there's UT1 and TAI for different purposes.

UTC is a (strange) compromise between both: ticking SI seconds like TAI but being kept within 1 second of UT1.

As I see it, UTC is an excellent compromise for the purpose of civil timekeeping. But it would be easier in future if computers would use TAI internally and convert to UTC for display.

As the article points out, what is being broadcast to keep systems synchronized is UTC, and TAI only exists on paper, or as derived from UTC. (Note that even if you have an atomic clock, that isn't TAI. TAI is some average of atomic clocks distributed over the earth, due to gravitational and hence relativistic differences.) UTC is effectively our master clock, and changing that would require a large number of technical and legal changes. So it's much easier to just increase the DUT1 tolerance and stop having new leap seconds.

UTC solves nothing for civil timekeeping, compared to TAI.

Timezone bands are massive. If they were perfectly straight (they aren't) then civil time in any one place would be off by up to 30 minutes in either direction, depending on the location. In reality they line up with political entities instead, creating even larger discrepancies (with the extreme case being China, where some regions are hours off).

Compared to that, a second every couple of years is completely irrelevant.

Not to forget the political push to adopt offsets that don't even match correctly. Instead you know fixing the timetables and rules in single go, if that effect is wanted.

The first one under the Julian year paradigm, one rotation compared to the Sun under the Julian day paradigm. (Neither of which to be confused with the Julian calendar.)

What I do not understand, considering the above (Julian years and days have been used for a long time in respectively paleontology and astronomy, haven't they ?) as well as from TFA :

> Abolishing leap seconds will be helpful for users that have very tight accuracy requirements. UTC is the only timescale that is provided by national laboratories in metrologically traceable manner, i.e. in a way that provides real-time access, and allows users to demonstrate exactly how accurate their timekeeping is.

> TAI is not directly traceable in the same way: it is only a “paper” clock, published monthly in arrears in Circular T as a table of corrections to the various national timescales. (See the explanatory supplement for more details).

So, what are we waiting for to just have both scientific time and civil time being broadcast separately (and scientific time with more precision and frequency, not less !), and then you use one or the other as appropriate ?

> So, what are we waiting for to just have both scientific time and civil time being broadcast separately

That would require either doubling the radio frequencies (and other channels) used for time broadcast, or changing the transmission format to include both (so not really separate). Some systems actually broadcast the current leap second count along with UTC, so TAI can easily be derived. Of course, that presumes that UTC and TAI keep being related by a leap second count. Going to fractional seconds of delta would again require changing protocols and data formats.

In addition, we’d presumably still like a civil-time second to be the same as an SI second, and not go back to the “rubber seconds” of the 1960s. But how then would we handle the fact that the solar day isn’t exactly 86400 SI seconds [0]? Add/remove some fractional seconds at the end of each civil day? That might cause even more issues in software than leap seconds.

[0] https://www.timeanddate.com/time/earth-rotation.html

Right, you still might need computers having to deal with the fact those two don't align, which would be messy in a way that is acceptable for (pre)historical dating, but not for realtime systems.

Right, leap seconds it is then, but note that that and minutes/hours/days/years here is something that is only required for human interfaces, while the computer itself only cares about a monotonically increasing seconds counter on its internal clock, which is required to be stopped/started/re-adjusted once in a while anyway.

Oh, and conversion between scientific and civil time is then the job of local polities, which they do for timezones anyway.

Software also cares about which second is which when comparing times originating from different systems, or from earlier/later instances of the same system. A monotonically increasing local clock isn’t sufficient if you can’t tell the relation between time x from your clock and a time y from another clock. So you need some way to establish that relation. Similar for mapping to civil time. You need to know what actual time the source time is.

That relation is established by knowing which standard each clock uses, so I am not really sure what the problem is ?

Well, which standard do you use, and how do you ensure that your internal clock remains synchronized to that standard? For example, when starting your system, how do you determine the correct initial value for your clock, and how do you compensate for clock drift? Just having some monotonically increasing counter starting at some arbitrary value isn’t enough.

The current answer is everyone uses UTC, and they keep synchronized by observing the one universally disseminated time, which is UTC.

How is this relevant to this specific discussion ?

You use and synchronize to the same time standard as the world-metrologists do (TAI), the whole issue is that that's not the one that is being disseminated as the article and I already pointed out.

For civil purposes, I feel the second still makes more sense as 1/86400 of a solar day. Unless you have an atomic clock, you cannot accurately keep to the SI unit over weeks or months anyway, let alone the time between leap seconds.

Practically everyone has access to atomic clocks these days by the virtue of having dozens of them whizzing over our heads broadcasting their time and having handy-dandy receivers in our pockets. And even running your own atomic clock these days is not that far fetched, see open timecards for example.

And then you have sports, some of which are (or have been determined) on the level of 1/1000 of a second. I’m not sure how that level of precision goes hand in hand with that definition of seconds.

the chicken definitely came first!

> or is it one rotation of the Earth on its axis?

Or the amount of rotation required to point towards the sun again, which is slightly more than one rotation (remember, we orbiting while spinning, so noon to noon is a bit more than 360°). This is a solar day; your definition is called a "sidereal day" (and is a real thing, too).

(And the solar definition has issues, too, which leads to, e.g., mean solar time…)

https://en.wikipedia.org/wiki/Sidereal_time