[LEAPSECS] the big artillery
athenamadeleina at gmail.com
Sat Nov 1 16:50:57 EDT 2014
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.ferguson at gmail.com> wrote:
> On 30 Oct, 2014, at 12:12 , Richard Clark <rclark at noao.edu> wrote:
>> Well, for historical and archival purposes Julian date nearly always
>> 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
>> 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
> 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
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