Re: Comparing time scales (second try!)

From: Steve Allen <sla_at_UCOLICK.ORG>
Date: Thu, 2 Feb 2006 19:50:29 -0800

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off from that indicated by any historical document produced by someone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
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                 <>                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     Hgt +250 m
Received on Thu Feb 02 2006 - 19:50:44 PST

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