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 Astronomy.
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) UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855 University of California Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 mReceived on 2007-07-02Z20:05:13