On Tue, Jan 28, 2003 at 04:30:41PM -0700, Rob Seaman wrote: > Perhaps it would help our discussions to simply describe our professional > (or otherwise) connections to UTC and other precision timing issues.
Okay, I'll bite. I have no direct professional connection to UTC. I am a computer programmer who has, at various times, had to deal with a variety of different frustrations related to how time is kept, one of which is reminding other programmers that "because of leap seconds, not all days are 86400 seconds long, stop building that assumption into your programs" (a very poorly received warning, I might add). I have a strong appreciation of the value of a good standard, and likewise a strong disdain for any standard which I can recognize as poorly put together. While I have limited technical background with precision time, I do have a strong incidental personal interest in its issues. While my weak background makes me sincerely doubt I will be in a position to slice fine technical details in discussions of precision time, I do feel that I have ample background to understand the kinds of issues involved, and because I lack a certain amount of sophistication, I have some ability to notice when people are making claims that are based on "I'm used to thinking this way" because their arguments fail to enlighten me as to "why this way of thinking is correct and relevant". (Put another way: if you know you're right and yet a posting from me seems to miss the point, first check your assumptions, and then try explaining a different way.) [samples of Rob Seaman's job task descriptions snipped for brevity] > The notion of discarding the connection between UTC and GMT is not > just philosophically distasteful - it clearly would make my job more > difficult for zero benefit. Given my position as an intelligent-but-uninformed observer, it seems like UTC is a "good enough" standard for your uses. I see that UTC, as defined, is useful to your doing the job in the way that you currently do it, and certainly more useful than TAI-plus-constant-offset would be; if those are the only two options then of course you personally will favor UTC. I do not see that UTC is a maximally useful time reference for your work, though. When you look at pictures of stars as they vary over time, do you really find that a time reference based on the annual mean interval of earth rotations _relative_to_the_sun_ to be exactly what the doctor ordered? Ignore "in the real world where I work" for the moment; ignore "UTC" and "UT1" and "TAI" and dream a little: what would an *excellent* time standard look like for your specific niche? Granted there will be a need to compromise eventually in order to make everyone (almost) happy, but I think the discussion to date of (crudely) "UTC vs TAI" fails to take significant details into account, making any hypothetical consensus less optimal than it could be. Yes, civil time, an important consumer of any time standard, would much prefer approximation-of-earth-rotation-relative-to-the-sun than random-arbitrary-interval (e.g., 100,000 SI-second "days") or approximation-of-earth-rotation-relative-to-"fixed"-stars as its time standard. But civil time is a different consumer of a potential time standard than you-as-worker-for-NOAO-Data-Products-Program, and wouldn't be nice if someone smarter than both of us could, upon understanding the details of the issues involved, see a better solution than UTC which addresses both of these (and a vast array of other) uses? Without the details of what the various "ideal" standards would look like, such a hypothetical insight seems, to my mind anyway, impossible to come by. I realize that the astronomical community has evolved to a consensus that UT1 (approximated by UTC) is a highly useful way to mark time, with the additional feature that it is usable as a civil time standard, but there is so much of that evolution which is based on historical accident rather than purely technical requirements that I find it hard to believe that there would be no possible way to improve upon it, even after non-astronomical constraints are factored in, if only it were possible to start anew with a clean slate. As to FITS standards: well of course they are UTC based --- that is what the current time reference standard is; I wouldn't expect them to use anything else (even more so since much of this is data whose collection depends on earth-orientation-in-space, not merely SI-seconds elapsed between observations). And any proposal for a new time reference standard would need to have a transition plan which would enable straightforward translation between it and something resembling UTC for at least a couple of decades while legacy applications, standards, and on-line data either exceed their useful lifetime or get converted (archived off-line data would not get converted, though a means to make such a conversion on an as-needed should be defined, even if that conversion is (as would be the best case) a "no-op"). A meter is no longer the interval between two scratches on a specific platinum-iridium bar; the US federal encryption standard is now Rijndael (aka AES), not DES. Standards evolve, or are replaced wholesale, for various reasons such as improvement of accuracy, reproducibility of results with different equipment (including "sofware implementation" as a kind of equipment), or suitability for their ultimate purpose. Legacy applications are important, as is the economic impact of change, but neither is an adequate rationale for why things should not change if something which offers clear advantages can be devised. Is UTC really the best that can be done to meet the varied needs that currently make use of it (because it is "the" standard)? Maybe, but I sincerely doubt it. --Ken Pizzini