FITS standard

[Arnold Rots]

Jonathan McDowell wrote:
> 
> The new FITS standard draft seems pretty reasonable. I may not have been
> paying attention to the Y2K discussion enough,

You got that right! ;-)

> but I am puzzled by the
> statement in 5.4.2.1 that UTC shall be used for the DATE keyword
> 'for all data sets created 
> on earth'. Is it felt that there are practical difficulties in realizing
> UTC for computers not on earth? This seems unlikely to the accuracy
> required by the DATE keyword; it is also not a frivolous question;
> there are enough laptops flying on Shuttle and Mir missions which might
> in principle store their data in FITS. I suggest removing the 'on earth'
> qualification (or replacing it by 'in the solar system', since I can
> see there might be simultaneity issues for interstellar probes...)

You took the two easy examples: low earth orbit and outside the solar
system.  As you probably have guessed, the 'for all data sets created 
on earth' was added later.  It was argued (correctly, I believe) that
on the earth's surface, UTC is well defined and useful, but even on a
nearby planet like Mars it is not clear that UTC would be the system
of choice - TDB might be better.

What you see is the text as adopted by the IAU FWG displaying a
traditional FITS solution strategy: The convention is specific where
we know that it is sensible to do so.  In all other situations you are
free to follow the same convention if that works well; if not, then at
least we didn't paint ourselves into a corner - you can use something
that makes sense, as long as you document it in the comment field.

  - Arnold Rots

> 
>   - Jonathan McDowell
> 
> 

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Arnold H. Rots                                         AXAF Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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