VOTable 1.4 Working Draft

[Arnold Rots]

Upon further consideration, I have to take that last comment back:
The timestamp of a photon's arrival HAS to mark the moment (in the
specified time scale) that the photon (would have) arrived at the specified
time reference position. Anything else would lead to an untenable situation.
Note that these issues are especially thorny for objects in the near field
(i.e., within the solar system).
This plays into the meaning of the spatial coordinates, too, and points to
the intertwining of space and time.
The intuitive interpretation is, at least for spherical coordinates
(especially equatorial and ecliptic), that the spatial location represents
the position of one's object as it is viewed or measured at the spatial
reference position at the time provided by the time stamp. Here one would
have to assume that the timestamps are identical at both the temporal and
the spatial reference positions (no path length corrections). But one
should be aware that this is not the way positions are kept in the
planetary and solar system ephemerides like DE-*nnn*, where the
(barycentric) positions are instantaneous locations at TDB time stamps in
the barycenter - not as *viewed *at those times from the barycenter.

  - Arnold

Arnold H Rots

Research Associate

SAO/HEAD

Center for Astrophysics | Harvard & Smithsonian

Email: arots at cfa.harvard.edu

Office: +1 617 496 7701 | Cell: +1 617 721 6756

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On Wed, Feb 20, 2019 at 11:57 AM Arnold Rots <arots at cfa.harvard.edu> wrote:

> This is about how space and time intertwine. They are not independent.
> It may be more about protecting users from themselves, though - so, it
> could be a strong recommendation.
> The hidden complication is that the time as given in the time column does
> not vary linearly at the spatial reference point.
>
> However, one would also be left with a significant ambiguity:
> Say, we have a photon detected at location S(Rs), labeled with time T(Rt),
> where Rs and Rt are the reference positions.
> Does T(Rt) represent the time stamp (at Rt) when the photon was detected
> at S(Rs), or does it represent the time it would have been detected at Rt
> (i.e., corrected for pathlength difference)? The pulsar community, for
> instance, would want to know.
> So, maybe this is an even stronger reason to consider requiring Rs=Rt.
>
>   - Arnold
>
> Arnold H Rots
>
> Research Associate
>
> SAO/HEAD
>
> Center for Astrophysics | Harvard & Smithsonian
>
> Email: arots at cfa.harvard.edu
>
> Office: +1 617 496 7701 | Cell: +1 617 721 6756
>
> 60 Garden Street | MS 69 | Cambridge, MA 02138 | USA
>
>
> cfa.harvard.edu | Facebook <http://cfa.harvard.edu/facebook> | Twitter
> <http://cfa.harvard.edu/twitter> | YouTube
> <http://cfa.harvard.edu/youtube> | Newsletter
> <http://cfa.harvard.edu/newsletter>
>
>
> On Fri, Feb 15, 2019 at 4:28 AM Francois Ochsenbein <
> francois.ochsenbein at gmail.com> wrote:
>
>> Hi Arnold,
>>
>> I'm not sure to understand the requirement of having the same spatial
>> reference position as the temporal one − measures of positions and time
>> are generally un-correlated from each other in different columns of a
>> table, where zero to many measures of positions / times can be reported
>> in one or several coordinate / time systems; in tables reporting very
>> specific events your requirement can be important, but it seems to me
>> that it's out of VOTable scope...
>>
>> Cheers, François
>>
>> >on 2019-02-14 at 15:00-0500, Arnold Rots wrote:
>>
>> >I note that there are no restrictions imposed on the
>> >timescale/reference position combination.
>> >That can really set you up for major trouble.
>> >And maybe one should just require that the spatial reference position
>> >is the same as the temporal one. Otherwise one runs the risk of time
>> >not moving linearly.
>> >
>> >  - Arnold
>> >
>>
>> --
>> ======================================================================
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>> ochsenbein at evc.net   ---   francois.ochsenbein at gmail.com
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>>
>
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