On Jan 10, 2006, at 9:17 AM, Peter Bunclark wrote:
I'm not one to shy away from irony (see! just proved it again...), but I do think there is a real issue here. Was interested to read the pages Tom pointed us to. Both the IAU position and McCarthy's exposition of same are curiously silent about the issue of resolving ambiguities resulting from non-denumerable SI intervals and solar days. The IAU tells us: Which is to say that day number is (always) a solar unit and fraction of day (sometimes) an SI unit. In "practical" terms, a JD(TT) _expression_ would simply be calculated by running a count of TT seconds since some epoch through the obvious conversion mill, but we're then returned to the central issue of reconciling such a JD(TT) with a JD(UT1). A calculation would simply show a growing fractional difference between the two, of course. At issue is the unit jump in JDN. Which day is it? This ambiguity only holds for a bit over a minute a "day" in the current epoch. (UTC = TAI - 33s, TT = TAI + 32.184s) The ambiguity is growing. Perhaps the SI unit should have been called the "essen", rather than the "second", as Steve Allen has said. But whatever it is called, it has a clear definition. But what is the definition of a day? Am convinced we need to reach a consensus on this before leaping (irony again) into any changes to the current rules of civil/business/international/legal/historical date and timekeeping. You'll note that I omitted "technical" and "scientific" from that list. This is not now and has never been a discussion about resolving purely technical issues, although some of the implications strongly affect technical people. Rob |
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