FITS standard

[William Thompson]

Jonathan McDowell wrote:

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

I'm a little confused by the phrase "accuracy required by the DATE
keyword".  My understanding of the Y2K proposal is that it does not impose
any limitations on the accuracy of date/time values, other than the
limitation imposed by the 80 character limit of a header line.

That said, I agree with Jonathan that limiting the use of UTC to the Earth
is a little odd.  I work with the SOHO satellite, which is ~5 light seconds
out from the Earth, and we have no trouble relating the times of
observations to UTC.  Obviously, one needs to be aware that different
observers will observe the same phenomena at different times, because of
the speed of light, but that's true for Earth-based observers as well at
the millisecond level.

Perhaps the writers wanted to leave open the possibility that times might
be converted to light arrival times at the solar system barycenter, or some
other such scheme?

Bill Thompson