MJD - not acceptable according to IAU (?)

[Paul Schlyter]

In article <31D92E5B.41C6 at barnegat.gsfc.nasa.gov>,
Mike Corcoran  <corcoran at barnegat.gsfc.nasa.gov> wrote:
 
> Steve Allen 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.
> 
> also, MJD-OBS, defined as the "date of the observation" is ambiguous by
> definition - is this the start of the observation, the midpoint, etc?
> This difference is significant to the X-ray community, where a given
> image is generally composed of individual segments obtained over an
> interval of many days.
 
And this ambiguity has nothing to do with MJD -- it'll still be there
if you use JD, ordinary calendar dates, number of seconds since
1970-01-01, or whatever.
 
If the duration of the observation becomes significant, information
about this must of course be included.  The appropriate action is
then to give "start of observation" and "end of observation" or
"duration of observation".  If the observation is stopped and
restarted repeatedly, this too may be insufficient - in such
cases one simply must supply enough details about the observation,
for instance time of each start and stop.
 
-- 
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Paul Schlyter,  Swedish Amateur Astronomer's Society (SAAF)
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