[LEAPSECS] Straw men

Rob Seaman seaman at noao.edu
Mon Jan 9 13:45:36 EST 2012

Ian Batten wrote:

> And you accuse others of erecting straw men? Are you saying that if UTC were redefined tomorrow, in a hundred years time there would still be equipment in use that would be mis-behaving?

Convenient way to forget the word "decades". Yes, I expect issues to continue to arise decades hence in currently deployed systems and processes. There are, for instance, ongoing Y2K-related issues. And since time-of-day will fundamentally remain mean solar time, issues due to confusion will continue to arise indefinitely in new code, new systems, new "equipment".

> We get that you say you've got instruments that rely on a steady diet of UTC which either assume that DUT1 is negligible for the purpose at hand (in which case it's not clear why |DUT1|<0.9s is fine while |DUT1|<1.1s is the end of the world),

Were you paying attention to Y2K?

> or are designed such that although they can add DUT1 to the received UTC, they can only do that if |DUT1|<0.9s. In either case, it sounds like your problem is that you need UT1.

We need Universal Time. It was reasonable to expect something called "Coordinated Universal Time" to remain Universal Time. What it "sounds like" to you appears to not be a very close analogue to the real world.

> But presumably what you _really_ want is local apparent solar time, or some such, in order to point instruments, in which case you need to combine UT1 with both the equation of time and the longitude of your location.

No. Mean solar time is sidereal time adjusted by one day per year for lapping the sun. It is UT that we need and want.

Apparent solar time does come in handy for the solar astronomy community - that is, the folks who deal with space weather reports and their impact on space and ground-based infrastructure (such as telecommunications) that society relies on. Apparent solar time is derived from mean solar time, not the other way around.

> It really defines belief that in systems which combine UTC, DUT1, EoT and position it will take centuries to add in an additional offset.

And more straw. I did not say that. The fundamental issue is that time-of-day and atomic time are two different things. A coherent engineering model of real-world issues will not result if we pretend otherwise. For astronomical and aerospace applications the dependencies are very obvious. The existence of obvious dependencies in these bellwether communities does not in some bizarre way imply the absence of dependencies in other communities. If anything, the opposite. I don't disagree with Warner's point about "Earth-facing" applications - I just think that in applications designed and deployed on Earth and near Earth that there are very likely to be widespread such dependencies that will only become apparent as the error grows. And nobody has looked. It is the decided disinterest in looking that is the problem.

And for the thousandth time, correcting an error offset at some future time is not the only issue here. The rate will mismatched to Earth rotation at all epochs in between.

And in any event the proposal doesn't provide new access to atomic time, it rather removes access to Universal Time.

Rob Seaman
National Optical Astronomy Observatory

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