MJD - not acceptable according to IAU (?)

[Paul Schlyter]

In article <4r98r6$e6r at darkstar.ucsc.edu>,
Steve Allen <sla at umbra.ucolick.org> wrote:
 
> In article <31D80C21.7662 at ast.cam.ac.uk>, Guy Rixon  <gtr at ast.cam.ac.uk> wrote:
> [about MJD-OBS]
>> In fact it's  one of the few keywords that's unambiguous already
>> without  needing an offical definition.
> 
> MJD is ambiguous by about a minute depending on whether you mean
> ET, UT, TAI, or some other timescale.
 
I've never seen JD or MJD used with TAI as the fundamental time scale.
One implicitly assumes UT as the basic time scale, which still leads
to a small ambiguity of course since "UT" can be either of UTC, UT0,
UT1 or UT2.....
 
Astronomers sometimes use JD based on ET (nowadays TDT) -- to show
that, they talk about "JED" in such cases.
 
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There are more serious ambuiguities involved here though.  MJD is
often used, by various people, as any day count, starting at any day
they feel is suitable.  And even JD is subject to this: the NORAD
people, who keep track of all our artificial satellites, have their
own definition of JD.  Today's date is 1996-07-03, let's assume it's
15:00 UT.  This will become:
 
   Astronomer's JD:  2450268.125
   NORAD "JD":           185.625
 
The NORAD "JD" is simply a count of the number of days into the year, and
it's reset to 1 at the beginning of each new year.
 
I once complained to T.S. Kelso (he's the one who use to post
satellite orbital elements in sci.space.news) about this usage of
"JD".  He responded that he understood my point of view, but that
this usage now is so widespread within NORAD that there's no hope in
changing it.
 
-- 
----------------------------------------------------------------
Paul Schlyter,  Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40,  S-114 38 Stockholm,  SWEDEN
e-mail:  pausch at saaf.se        psr at home.ausys.se