[fitswcs] FITS WCS Time Paper

[patrick.wallace at stfc.ac.uk]

A couple of points to be going on with.

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A.6. GPS time

 "Note that GPS time is defined in terms of UTC(USNO). That is to
  emphasize that GPS time is not defined in terms of TAI. GPS time
  is only required to match UTC to within a microsecond."

  This is potentially misleading to ordinary readers.  It reads as
  if the author is in denial that GPS time is *the* de facto
  distribution mechanism for TAI, at least for applications that can
  make do with 100ns accuracy.

  Anyone reading the statement "GPS time is only required to match
  UTC to within a microsecond" is going to think that a GPS time
  readout and a UTC readout will agree, which is clearly not true
  (except for the fractional part).
  
  GPS time is really defined in terms of a specific UTC epoch, while
  UTC is itself defined in terms of TAI.  And in practice GPS time
  is steered to TAI.  So to say only that "GPS time is not defined in
  terms of TAI" may be formally correct but it is not helpful.

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A.9. UT

 "UT was initially defined by Newcomb’s fictitious mean sun Tables of
  the Sun, Astr. Papers Amer. Ephemeris 6, 1895 The name Universal
  Time was established as the subdivision of the Universal Day by
  Commission 4 of the IAU at the IIIth General Assembly in 1928 at
  Leiden (IAU, 1928 13 )."

  This needs some brackets and other punctuation.  Perhaps

  ...by Newcomb’s "fictitious mean Sun" (Tables of the Sun, Astr.
  Papers Amer. Ephemeris 6, 1895). The name... 

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Patrick Wallace
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Space Science & Technology Department                +44-1235-531198
STFC Rutherford Appleton Laboratory
Harwell Oxford
Didcot, Oxfordshire, OX11 0QX, UK         Patrick.Wallace at stfc.ac.uk
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