WFC: Year-2000 issue (again)

[Steve Allen]

Don Wells wrote:
> Our interchange format design will be more robust if we revise the
> Bunclark proposal to specify that the DATExxxx timescale is (only) UTC.

Agreed, but I remain concerned for the validity of keywords describing
archival plate data when encoded as FITS.  While it is not simple, the
propsal from Arnold Rots acknowledges historical ignorance and
accommodates future (and non-earth-based) revision.

>             FITS reading and writing software can transform UTC to and
> from local timescale conventions with no ambiguity or loss of precision.

The simplicity of UTC is an admirable goal, but this statement is not
true for archival (pre-UTC) astronomical data.  A mandate for UTC will
cause ambiguity for the reader of any such existing FITS files and
dilemma for the writer of any such new FITS files.

The FITS standard could be amended to mandate that UTC be used for all
DATExxx keywords for earth-based observations subsequent to the
adoption date.  I would support that.  But if so it must also contain
explicit language indicating that the UTC mandate should not be
presumed for pre-1972 data.

This would permit future earth-based FITS data to be unambiguous, and
it could not invalidate existing archival data.  It would retain the
current level of time-system ambiguity for existing data, and for data
which cannot satisfy the mandate.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 408 459 3046     http://www.ucolick.org/~sla