On Jan 8, 2006, at 9:09 AM, Poul-Henning Kamp wrote:
Doing so would once and for all have to divorce earth orientation from that unified time scale, leaving it to governments to align civil time with daylight as they see fit (just like today).
Without further debating the meaning of "civil time", consider the implications of this two stage system. The first stage conveys TAI or something related to it by a constant offset. The second stage at any location (correct me if I misunderstand you) would be a secondary clock disseminated at the direction of the local authorities. Governments and technical users would subscribe to the first stage clock. Businesses and civilians would subscribe to the second stage clock(s). Correct so far? For the sake of argument, let's discount the risks associated with confusing one stage's clock with the other. One imagines, however, that there won't be fewer safety critical, time dependent systems in the future. We might, in fact, suspect that every party to this conversation would both admit this and use it to argue for their own position :-) Those risks, however, represent only one issue falling under the umbrella of interoperability. It is one thing to say that any random local government can choose their own clock statutes. This is certainly true, but in practice the future international community will work together to reach joint decisions on evolving common clock practices (as you say, "just like today"). I won't belabor the many worldwide systems that must interoperate for the benefit of all. But these systems must interoperate not only between themselves, but with natural phenomena. Forgive me (or don't), but I am skeptical that phenomena of interest in the future will not continue to include the rising and setting of the sun. (And isn't claiming otherwise equivalent to saying that stage two is unnecessary?) The question is: how precisely does this differ from the situation now or in the past? Whether by fiat or not, some common worldwide "stage two" clock must exist. And some mechanism must exist for synchronizing (to some level of tolerance that we can continue to debate) that clock to diurnal cycles. It is this synchronization that is ultimately of interest to us, not leap seconds, per se. I have heard no response to my discussion of techniques for achieving synchronization - of the difference between naive "fall back" hours and 25 hour days. But how in practice is it envisaged that a scheme for migrating time zones versus TAI would work, precisely? Note, for instance, that nothing short of redefining the second can avoid the quadratic acceleration between the stage one and stage two clocks. Time zones (and the prime meridian?) would race more-and-more rapidly around the globe. Perhaps I've misunderstood, but this line of reasoning doesn't appear to resolve anything. Rob Seaman National Optical Astronomy Observatory