On Thu 2006-02-02T17:47:24 -0800, James Maynard hath writ: > Would you be kind enough to review it, to see if I've got it right?
Any use of the term GMT before around the year 1935 is subject to a 12 hour ambiguity when compared with contemporary records. In particular: To use GMT for the epoch of MJD is to produce a time which is 12 hours off from that indicated by any historical document produced by someone who deduced time using procedures derived from the British astronomical almanac. Ditto when using GMT for the epoch of NTP. In both cases it is safest to use the term UT. That term was not in use at those epochs. The meaning of UT is therefore unambiguous, albeit anachronistic. The meaning of UT prior to 1901 is subject to the mean solar time expressions in use by the observatory providing the local time. The meaning of UT from 1901 through 1983 is based on Newcomb. The meaning of UT1 from 1984 through 2002 is based on Aoki et al. The meaning of UT1 from 2003 is based on Capitaine et al. The phrase "Universal Coordinated Time" is incorrect. The term MJD by itself is ambiguous. As noted by the IAU resolution on its use, MJD needs to be qualified by attaching a particular time scale. This may be TAI, UT, or any of the other astronomical time scales. In the same sense as NTP time and POSIX time are ambiguous, it is not obvious whether there is meaning if MJD is associated with the time scale UTC. It is probably reasonable to footnote that PTP is functionally equivalent to POSIX time. Did you get it right? That's a good question. I suggest that we all rent a copy of Rashomon. After viewing that we could at least discourse on Kurosawa's mastery of cinematography. -- Steve Allen <[EMAIL PROTECTED]> WGS-84 (GPS) UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99858 University of California Voice: +1 831 459 3046 Lng -122.06014 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m oc