On Thu 2005-01-20T14:59:18 -0700, Rob Seaman hath writ: > Leap seconds are a perfectly workable mechanism. Systems > that don't need time-of-day should use TAI. Systems that do need > time-of-day often benefit from the 0.9s approximation to UT1 that UTC > currently provides. Let's stop pretending that *both* atomic time and > time-of-day are not needed. Instead, let's direct our efforts toward > implementing improved systems for conveying both of these fundamental > timescales to users of both precision and civil time.
On Sat 2005-01-22T20:43:51 -0500, Daniel R. Tobias hath writ: > Now, if a time standard is to be defined based solely on constant SI > seconds, with no reference to astronomy, then why even include all > the irregularities of the Gregorian Calendar, with its leap year > schedule designed to keep in sync with the Earth's revolutions? It > really makes no sense that TAI includes days, years, and so on at > all, and this will seem particularly senseless when the current date > by TAI is a day or more removed from Earth-rotational time, as will > happen in a few millennia. > > What is really needed is two different time standards: a fixed- > interval standard consisting solely of a count of SI seconds since an > epoch (no need for minutes, hours, days, months, and years), and a > civil-time standard that attempts, as best as is practical, to track > the (slightly uneven) motions of the Earth. Of course there are other units of "time" in civil history which have been converted from actual representations into conventional ones. Sailors have no qualms about calling out the next high tide in terms of local civil time (now practically based on UTC). They all know that the times shift by around an hour every day. The month lost its connection with the moon early in the Roman era. Everybody knows, and in general nobody cares, that the moon is not new at the beginning of a month in the Gregorian calendar. The Gregorian year is pretty good, but three millenia hence the vernal equinox will have drifted discernably from the original intent. In general nobody cares about the date of Easter that much, and (as seen in Duncan Steel's book) even some of the best astronomers have not understood the distinction between the tropical year (as popularly defined by Newcomb) and the "Vernal Equinox Year" that Pope Gregory's calendar actually aimed to match. Above Rob Seaman and Danial Tobias have echoed some of the issues discussed by Essen himself in his autobiographical work "Time for Reflection" which his son-in-law has reproduced at http://www.btinternet.com/~time.lord/ In particular, this footnote http://www.btinternet.com/~time.lord/TheAtomicClock.htm#_msocom_1 (and the entire chapter containing it) reveals that the tension between the physicists and the astronomers (notably Stoyko, who has largely been written out of history) was great enough that there almost became two SI units for "time", one being the second based on the day, and one being the "Essen" based on the cesium resonance. But Essen claims for himself (in both this autobiography and in Metrologia http://www.bipm.org/metrologia/ViewArticle.jsp?VOLUME=4&PAGE=161-165 ) the credit for recognizing that the existing systems of time distribution (and now presumably extended to time computation) basically cannot be expected to tolerate the existence of two kinds of time. I don't think this is really true anymore, but it is admittedly costly. It was the astronomers who first made the mistake of counting a truly uniform time scale using the calendrical/sexagesimal notation originally based on earth rotations (and now concisely communicated using ISO 8601). It was the physicists who pushed to continue the practice. Knowing the tides is a specialist operation, and has always been. Knowing the phase of the moon is a specialist operation, and has been in western culture for over two millenia. What we are being told by the Time Lords is that, starting from a date in the near future, knowing when noon is will also be a specialist operation. "Month" is entirely conventional in its meaning. "Year" is entirely conventional in its meaning. So soon "day" will be entirely conventional in its meaning. All of them become predictable, albeit upon examination silly, extensions of things which originally meant something else. The priesthood of astronomy has become irrelevant to the general populace, and the priesthood of the physicists has taken precedence. The trick will be to educate the general public that 12:00 means slightly less about where the sun is in longitude than the Gregorian calendar date means about where the sun is in latitude. Both of these schemes fail, it's just that atomic time fails by a full hour within 1000 or so years whereas the Gregorian calendar fails by a full day only after another 2000 or so years. I really like sundials, mean solar time, and the analemma. I think it is disingenuous to use the methods we see being used by the atomic clock keepers to accomplish this change. Realistically, anything worth wearing to accomplish the function of a wristwatch will soon have far more computational capability than is required to tell when noon will really be, if anyone cares. But the current strategy of retaining the name UTC creates one real and unresolvable problem that will persist indefinitely. It is very bad policy to corrupt the historical meaning of anything called "Universal Time" by redefining UTC to be something that has no relation to the rotation of the earth. This list has discussed mitigations for astronomers to use with legacy systems which amount to re-creating UTC with leap seconds. The effect of that is to create "old UTC" and "new UTC". It also leaves confusion over which form should be interpreted when reading historical documents and comments in software systems. We have already seen the chaos of such ambiguity when the British Admiralty demanded that the term GMT continue to be used when the beginning of the day was switched from noon to midnight. The 2003 conference in Torino made it quite clear that a new name should be used. Yes, there is extreme cost required to change the name of the most practically available civil time scale, but that cost is temporary. As seen with GMT, the cost of not changing the name of a time scale is also large. That cost is eternal, and eventually ends up demanding a name change anyway. The belief that a precisely-defined time scale can have a basic characteristic changed without eventually incurring the cost of also changing the name is a fantasy. -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 [EMAIL PROTECTED] Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93
