sla at ucolick.org
Mon Jul 2 11:04:41 PDT 2007
On Mon 2007-07-02T18:57:48 +0200, Francois Ochsenbein hath writ:
> Yes this would be a good solution -- I mean to assign another name than
> MJD for a time which is a nomber of days elapsed since a reference date.
> DJD (differential julian date) could be a better wording than IJD.
I am going to plead that this discussion be stayed until Arnold Rots
returns from vacation. STC already has several representations for
reference date-time and offset date-time, and Arnold is also taking
the lead for the FITS WCS paper on time.
STC has a list of time scales which may have any sort of precision
meaning. For precision purposes its structure acknowledges the
situation that has been discussed over the past two decades by the
IAU Working Group on Relativity, Celestial Mechanics, Astrometry
and Metrology (RCMAM) which was (just last year at IAU Prague 2006)
elevated to full status as IAU Commission 52 Relativity in Fundamental
And I will caution that even when a time scale is specified there may
still be ambiguity not addressed by STC. For example, if the STC
option "LST" (or pre-1972 UTC) is chosen there is no way to
communicate whether that is based on calculations which were performed
using UT from pre-IAU-1956 Newcomb expressions, or UT1 from corrected
post-1956 Newcomb expressions, or post-1984 UT1 based on Aoki et al.,
or post 2003 UT1 based on Capitaine et al. Fully disambiguating
requires a complexity that envelopes almost all the historical
activity of IAU Comms 4, 19, and 31. This is beyond the level
which is reasonable to demand implementation in a standard. But
whatever standard is chosen should document these caveats lest
we end up with people basically defining their own time scale (as
Microsoft has done in .NET where the system time is 100 nanosecond
increments since 0001-01-01T00:00:00 UTC, a representation which
cannot be sensibly interpreted for instants before I was born).
So, please, let's wait on this until Arnold returns.
Steve Allen <sla at ucolick.org> WGS-84 (GPS)
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