content, format, ctype, or xtype ?

[Steve Allen]

On Fri 2009-05-15T09:54:36 -0400, Arnold Rots hath writ:
> A single Time Scale is not feasible, but I think it is reasonable to
> restrict it to TT, UTC, and TDB (and their historic predecessors).

Note that this is the subset which has already been specified for use
with the VOEvent protocol, see section 3.4.1 of
http://www.ivoa.net/Documents/REC/VOE/VOEvent-20061101.html
(with an exception in place for solar observations, sec 3.4.4)

The use cases are basically justified as follows

UTC
This is the practical solution for all terrestrial observations
because by international agreement it is the name of the time scale
currently specified for use in all broadcasts of time signals.
This is for terrestrial observations where the provider either does
not care to convert to any other system, or where the provider is
not even sure the clock was set correctly.  That is to say,
events expressed in this time scale may not be rigorously precise.
Note that UTC is based on TAI which is defined to have the
characteristics of TT, and as such it is only rigorously defined
in the vicinity of the surface of the earth.
In many cases historic observations originally expressed in GMT or UTx
can be transmitted in the VO context as UTC with inconsequential loss
of rigor.

TT
This is the practical solution for all observations made in
the vicinity of the earth which
1) demand a rigorously-defined time scale which has a
specific meaning that is not subject to arbitrary re-definition
2) are not sensitive to the motion of the earth
(Note that Hipparcos and the IERS use TT, not TCG.  The IERS briefly
switched from TT to TCG, and then reverted to TT.)
In most cases historic observations originally expressed as TDT or ETx
can be transmitted in the VO context as TT with inconsequential loss
of rigor.

TDB
This is the practical solution for all observations which
1) demand a rigorously-defined time scale which has a
specific meaning that is not subject to arbitrary re-definition
2) are sensitive to the motion of the earth
(Note that JPL and most other ephemerides use TDB, not TCB.)
In most cases historic observations originally expressed as ETx can be
transmitted in the VO context as TDB with inconsquential loss of
rigor.

body-centric or system-centric times
This is the bag of worms of which solar observations are the most
evident exception to the above 3 cases.  It also applies to
oscillations of one star in a binary system.  For these cases there
is no standards body which has defined a time scale.

For the practical question it is worth investigating whether solar
observations, and all other observations likely to be encountered in
the VO, are adequately expressed in one of the above 3 named scales.
For rigorous cases the data provider and data user will always have
responsibilities for characterizing the time which supersede what
the VO mechanisms can easily represent.

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99855
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