[welterde]

UT1 (and therefore UTC) is however a better starting point for calculating the sidereal time than TAI since it is connected to the rotation of the Earth, whereas TAI is not. This way the relation to calculate the (L)ST only depends on fixed constants and the time and date.

Also UT1 is actually measured based on distant celestial objects, because it is easier to measure those at very high precision than the precise transit time of the Sun.

[nullc]

That depends on if UTC alone is an accurate enough estimate of UT1 for you. If you only need to know UT1 to within +/- 0.9 seconds, then you can just use UTC and you're done.

If you'd like greater accuracy, UTC isn't a great starting point because UTC's leap seconds complicate getting an accurate estimate of UT1. Its hard to tell if your local idea of UTC has had the latest leap second applied or has had false leap seconds applied (or worse, accidentally received a leap smeared source, which can't be backed out to convert to UT1).

If you're going to produce a more accurate estimate of UT1 either using interpolations of IERS bulletin B predictions or a model based on the daily USNO ultra-rapid UT1 VLBI measurements the leap seconds don't do anything to help you, but they can mess up your calculations because you need to correctly and consistently back them out.

So essentially the practice of leap seconds helps applications that need UT1 to within better than an hour or two (otherwise just using TAI with offset or TAI plus a static linear correction is good for thousand of years), but hurts applications that need subsecond UT1 accuracy, hurts anyone that needs consistent or accurate duration, and creates a lot of software bugs including ones that can show up when there hasn't actually been a leap second.

I would argue that today the number of applications that need UT1 at all are much smaller and less significant than ones that need subsecond consistent durations or times. And that of the applications that want UT1 most either don't need leapseconds at all (e.g. TAI or TAI plus the simplest static linear correction is enough) or would prefer better than 1 second accuracy, where at best leapseconds don't help and in practice they add a lot of complexity and still sometimes cause failure.

[welterde]

We cannot predict Earths rotation to a sufficient degree into the future to get away with TAI + constant or TAI + some equation. This is the reason we don't know the precise time of all leap seconds decades into the future. Currently it looks like we won't have any leap second for the foreseeable future, since TAI+offset and UT1 tick quite closely.

However the less than 0.9s offset to UT1 is often good enough to allow accurate pointing for all except the highest accuracy observations (and for those you are essentially part of the pipeline that determines UT1 in the first place). A time difference of 1s corresponds to a error in position that is often equivalent to the pointing accuracy of the telescope/dish (or not too far off anyway). Whereas beyond 10s or so you might not even be able to see the target within your scope anymore.

Not sure about the points about the complications of calculating the true UT1, I don't really see how those would come up and at least for the bulletin B it is already included, so you don't have to back anything out.

[nullc]

> to a sufficient degree into the future

Depends on the application. If you need the lights to go down at sundown a constant is good for thousands of years, a rate correction is good for a fair bit longer.

> Currently it looks like we won't have any leap second for the foreseeable future

In fact, bootstrapping from recent residuals suggests there is a reasonable chance of a positive leapsecond in the next several years. Each ordinary leapsecond causes substantial disruptions and system outages, and there has never been a positive leap second so it's reasonable to expect substantial issues.

> Whereas beyond 10s or so you might not even be able to see the target within your scope anymore

If your pointing accuracy is 10% of your FOV your life will be hard, why make it harder by not using an accurate UT1? Besides, for accurate pointing you'll want the pole offsets that are also in circular b. Once you're using an accurate source of the UT1 offset, the leapseconds at best do nthing.

> Not sure about the points about the complications of calculating the true UT1

To calculate UT1 using circular b or USNO observations you need to have an accurate UTC. Leapseconds make UTC failure prone. It's extremely easy to miss leap seconds, gain false leaps seconds, or (as of recently) accidentally end up with leap-smeared time.

> so you don't have to back anything out

They need to be backed out to linearly interpolate between entries to avoid a discontinuity.

[welterde]

The pole offsets are in the order of hundreds of milli-arcseconds, whereas each second offset between UT1 and UTC produces an offset of (worst-case) ~14 arcseconds. Worlds of difference. Subtracting all leapseconds would yield an error of ~6 arcminutes, which can ruin your day if you are using any instrument with a small FoV (I know since I had that issue before where due to some hardware issues the telescope clock had lost a couple clock pulses and was 10-20s off compared to the observatory master clock).

> They need to be backed out to linearly interpolate between entries to avoid a discontinuity.

Ah.. if you want to apply it on the day of the leapsecond. Fair. I meant more in general. As in all other cases you can just more or less blindly apply it.

In practice I can't say I have been bitten by UTC+leap second issues before nor have I even heard from observatories being bitten by this. Now GPS epoch rollover - that I have been personally experienced issues with.

> In fact, bootstrapping from recent residuals suggests there is a reasonable chance of a positive leapsecond in the next several years. Each ordinary leapsecond causes substantial disruptions and system outages, and there has never been a positive leap second so it's reasonable to expect substantial issues.

We'll see if there will be a negative leapsecond in the future. But so far it also seems in line with no leapsecond at all.