SED Data Model version 0.93

[Arnold Rots]

Just a few random comments on 0.93.

I would suggest to use BARYCENTER, rather than HELIOCENTER for defaults.

So, what do we do when the reference position is TOPOCENTER?
That renders the time information useless for certain applications,
lacking information on where the TOPOCENTER is exactly.

ISO 8601 cannot be prefixed with a minus sign.
We should stipulate that ISO8601 will always represent (extrapolated)
Gregorian dates, independent of date and location, but all values
prior to 0001-01-01 are undefined and need to be represented as JD -
as the next version of the STC WD will say.
I would suggest allowing absolute times in JD and MJD (i.e., without a
reference time) as well as reference times in JD and MJD.

  - Arnold

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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              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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