So days may come and go, but UTC with or without leap seconds meets its definition just fine - for those who just think of it as a universally agreed-upon time reference that's coordinated by timing labs. It is not amibuguous if this universal reference coincides with UT1 to .9 seconds until 2020 and then less closely thereafter - that's just the way it would work out.
On 11/1/14, Dennis Ferguson <dennis.c.fergu...@gmail.com> wrote: > > On 30 Oct, 2014, at 12:12 , Richard Clark <rcl...@noao.edu> wrote: >> Well, for historical and archival purposes Julian date nearly always >> means >> traditional days, as in solar days. But for astronomical uses a fixed >> unit, the apocryphal atomic day is implied. This means needing to know >> delta T if you need to relate it back to a civil date or time. >> >> The term 'day' has an awful lot of linguistic baggage that clearly >> implies that the solar day is meant. But now the use of 'day' can be >> at the speaker's and listener's risk. >> >> The minute, hour, day, year... these are not SI units. We need to >> start considering it sloppy to use them as if they are. >> >> Do we mean 'atomic day'? If so we need to: >> 1. say so, and >> 2. make it official by defining, rather than just implying one. >> >> Perhaps hectosecond would be better. At least it doesn't invite >> confusion. >> >> Yeah, and now to convince anyone to do this. > > I agree with that. > > While it is true, though, that the minute/hour/day are not SI units > they are accepted by the CIPM for use with the SI with dimensions > of time using their traditional (dating back to Ptolemy?) defined > relationships to each other when the second used is the SI version. > Table 6 here > > http://physics.nist.gov/Pubs/SP330/sp330.pdf > > lists the CIPM definition, to which TT as a JD seems to conform. > Solar days/hours/minutes/seconds don't conform, but since their > dimensions are now rotational angle rather than time this is a > use of the units which is off label with respect to SI even if > it is a traditional and original use of units with those names. > > I guess the point is that while "day", like many traditional > units, is ambiguous and in need of a qualifier to know exactly > which kind is being referred to, a definition of "day" as a > standard time unit which is 86400 SI seconds long already exists > and is in use. This requires no new invention. > > Dennis Ferguson > _______________________________________________ > LEAPSECS mailing list > LEAPSECS@leapsecond.com > https://pairlist6.pair.net/mailman/listinfo/leapsecs > _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs