[fitswcs] FITS WCS Time Paper

[Arnold Rots]

Thank you.
Responses below.

  - Arnold


patrick.wallace at stfc.ac.uk wrote:
[ Charset UTF-8 unsupported, converting... ]
> A few more comments on the paper.
> 
> --------------------------------------------------------------------
> 
> Section 2, penultimate paragraph:
> 
>    In addition to ?proper? time axes, we provide accommodation for
>    three types of time-related coordinates: Phase, Timelag, and
>    Frequency; see Section 5.5.
>  
> I don't think proper time is meant here.  I would write
> 
>    In addition to absolute time axes, i.e. proper or coordinate
>    time, we provide accommodation for three types of time-related
>    coordinates: Phase, Timelag, and Frequency; see Section 5.5.
> 

"proper" time is not the same as "proper time".
"proper" is meant in a more colloquial way and that's why it is in
quotation marks.
I think your phrasing is a bit cumbersome (and it still lacks relative
time - which is not the same as timelag).
real time?
Or are others not bothered by '"proper" time'?

> --------------------------------------------------------------------
> 
> Table 2:
> 
> You might want to update the "as of 2009-01-01" information to
> "as of 2012-07-01".
> 

Good point

> --------------------------------------------------------------------
> 
> Table 3:
> 
> I'd simply omit Pluto now that its membership of the golf club has
> been revoked.  Later on, when the possibility of including to minor
> planets is mentioned, you could also say dwarf planets.
> 

It was there because it was included in the JPL ephemerides.
We can drop it; it's unlikely to be used ;-)

> --------------------------------------------------------------------
> 
> Section 4.2.2, second sentence:
> 
> There should be a comma after "TREFPOS".
> 

Thank you.

> --------------------------------------------------------------------
> 
> Section 4.2.2, on p6 just after Table 4:
> 
>    "with respect to Earth"
> 
> This should be "the reference position" (which may not be the
> Earth).
> 

No, see my earlier message.

> --------------------------------------------------------------------
> 
> Section 4.2.2:
> 
> I am uncomfortable about the whole "reference position" concept.  A
> system of spacetime coordinates has a spatial origin, and a proper
> time is associated with a location in space.  But to think of, say,
> TDB as being "the time read by a clock at the SSB" is completely
> wrong.  Indeed, the whole purpose of a coordinate time system, which
> *all* of those listed except LOCAL are, is to provide a consistent
> system of time reckoning for widely separated spatial locations.
> You can label any event with any of them, and the choice depends
> simply on what makes the observed phenomenon easiest to describe.
> 
> I suggest taking the statement "The reference position...specifies
> the spatial location at which the time is valid" and trying to
> decide what is actually meant.  The only two meanings I can come up
> with are (i) the spatial origin for a system of spacetime
> coordinates and (ii) the topocentre, and these are different things.
> 

See the other email exchanges; this needs more thought.

> --------------------------------------------------------------------
> 
> 
> Patrick Wallace
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> STFC Rutherford Appleton Laboratory
> Harwell Oxford
> Didcot, Oxfordshire, OX11 0QX, UK       ??Patrick.Wallace at stfc.ac.uk
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> 
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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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