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<div class="moz-cite-prefix">Hi all,</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">We did some digging with Baptiste and
Mireille before ADASS / Interop on this geo_app<br>
term, and I take this VEP as an opportunity to report on what we
found.</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">TL/DR : geo_app most likely corresponds
to the <font face="monospace">GAPPT</font> FITS keyword, for<br>
the geocentric apparent equatorial coordinate
system.</div>
<div class="moz-cite-prefix">And VEP-017 looks good !<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Detailed search (for those who might
dig into this 25 years from now) :</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">- <font face="monospace">geo_app</font>
has been a possible value of <font face="monospace">COOSYS</font>
in VOTable IVOA Rec since the beginning:<br>
<a class="moz-txt-link-freetext" href="https://www.ivoa.net/documents/VOTable/20031017/PR-VOTable-1.0-20031017.html">https://www.ivoa.net/documents/VOTable/20031017/PR-VOTable-1.0-20031017.html</a><br>
and even before IVOA, in the first draft (v0.1 18-Nov-2001 ) : <br>
<a class="moz-txt-link-freetext" href="https://cds.unistra.fr/doc-cds/VOTable/">https://cds.unistra.fr/doc-cds/VOTable/</a><br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">- <font face="monospace">geo_app</font>
was also present as a possible value for <font face="monospace">coosys</font>
element in XML precursors of<br>
VOTable, namely :</div>
<div class="moz-cite-prefix"> - astrores (discussions from ADASS
1999 to January 2001) :<br>
<a class="moz-txt-link-freetext" href="https://vizier.cds.unistra.fr/vizier/doc/astrores.htx">https://vizier.cds.unistra.fr/vizier/doc/astrores.htx</a></div>
<div class="moz-cite-prefix"> - AML (Astronomical Markup
Language), in Damien Guillaume's PhD, p. 112 (Jan. 2000) :<br>
<a class="moz-txt-link-freetext" href="https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7b45eb756c6887b9cdb967998369f44c7a660024">https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7b45eb756c6887b9cdb967998369f44c7a660024</a></div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">(Internet Archive Wayback machine if
some links disappear :<br>
<a class="moz-txt-link-freetext" href="https://web.archive.org/web/20000526191744/http://vizier.u-strasbg.fr/doc/astrores.htx">https://web.archive.org/web/20000526191744/http://vizier.u-strasbg.fr/doc/astrores.htx</a>
)<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">- The exact origin of <font
face="monospace">coosys</font> and its allowed values are hard
to trace, there<br>
are few relevant links :<br>
<a class="moz-txt-link-freetext" href="https://web.archive.org/web/19990901020112/http://pioneer.gsfc.nasa.gov/xml/internal/mail/mhonarc/xml/msg00005.html">https://web.archive.org/web/19990901020112/http://pioneer.gsfc.nasa.gov/xml/internal/mail/mhonarc/xml/msg00005.html</a><br>
<a class="moz-txt-link-freetext" href="http://lists.xml.org/archives/xml-dev/">http://lists.xml.org/archives/xml-dev/</a><br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">- Most likely, <font face="monospace">geo_app</font>
corresponds to the <b><font face="monospace">GAPPT</font></b>
FITS keyword in Table 2 of<br>
Calabretta & Greisen 2002 "Representations of celestial
coordinates in FITS" :<br>
<a class="moz-txt-link-freetext" href="https://www.aanda.org/articles/aa/pdf/2002/45/aah3860.pdf">https://www.aanda.org/articles/aa/pdf/2002/45/aah3860.pdf</a></div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">
<blockquote type="cite"><font face="monospace">GAPPT</font> :
geocentric apparent place, IAU 1984 system<br>
<br>
Geocentric apparent equatorial and ecliptic coordinates<br>
(GAPPT) require the epoch of the equator and equinox of date.<br>
This will be taken as the time of observation rather than<br>
EQUINOXa. <br>
</blockquote>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">- It is also described in the
documentation of the Starlink AST WCS library<br>
<a class="moz-txt-link-freetext" href="https://starlink.eao.hawaii.edu/docs/sun211.htx/sun211ss454.html">https://starlink.eao.hawaii.edu/docs/sun211.htx/sun211ss454.html</a><br>
</div>
<div class="moz-cite-prefix">
<blockquote type="cite"><code>"</code> GAPPT<code>"</code> , <code>"</code>
GEOCENTRIC<code>"</code> or <code>"</code> APPARENT<code>"</code>
: The geocentric apparent equatorial coordinate
system, which gives the apparent positions of sources relative
to the true plane of the
Earth<code>’</code> s equator and the equinox (the coordinate
origin) at a time specified by the
qualifying Epoch value. (Note that no Equinox is needed to
qualify this coordinate
system because no model <code>"</code> mean equinox<code>"</code>
is involved.) These coordinates give the
apparent right ascension and declination of a source for a
specified date of
observation, and therefore form an approximate basis for
pointing a telescope. Note,
however, that they are applicable to a fictitious observer at
the Earth<code>’</code> s
centre, and therefore ignore such effects as atmospheric
refraction and the
(normally much smaller) aberration of light due to the
rotational velocity of the
Earth<code>’</code> s surface. Geocentric apparent coordinates
are derived from the standard
FK5 (J2000.0) barycentric coordinates by taking account of the
gravitational
deflection of light by the Sun (usually small), the aberration
of light caused by
the motion of the Earth<code>’</code> s centre with respect to
the barycentre (larger),
and the precession and nutation of the Earth<code>’</code> s
spin axis (normally larger
still).
</blockquote>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Cheers,<br>
</div>
<div class="moz-cite-prefix">Sébastien<br>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Le 20/11/2024 à 15:02, Markus
Demleitner via dm a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:20241120140247.fikeohti4jubwcvm@victor">
<pre class="moz-quote-pre" wrap="">Dear Semantics, Apps, and DM WGs,
In our refframe vocabulary, we have a term geo_app that we have
inherited from VOTable's COOSYS/@system. When we created the
vocabulary, nobody could quite remember what it was supposed to mean,
in particular because the "geo" part made us suspect it would have
been a frame for Earth.
At the recent Interop, JJ Kavelaars recognised what this was supposed
to mean: geocentric apparent positions. Even though the term is
deprecated (and we will not un-deprecate it for reasons mentioned
below), we should fix the ugly label and description. This is what
VEP-017 tries:
Vocabulary: <a class="moz-txt-link-freetext" href="http://www.ivoa.net/rdf/refframe">http://www.ivoa.net/rdf/refframe</a>
Author: Markus Demleitner <a class="moz-txt-link-rfc2396E" href="mailto:msdemlei@ari.uni-heidelberg.de"><msdemlei@ari.uni-heidelberg.de></a> and J.J. Kavelaars
Date: 2024-11-17
New Term: geo_app
Action: Modification
Label: Geocentric Apparent Positions
Description: Positions given as observed by a fictitious observer at the
Earth’ s centre for the equator of observation.
Relationships: narrower than #EQUATORIAL
Used-in: VOTable legacy
Rationale:
When the legacy VOTable terms were added to the refframe vocabulary
(which was necessary to keep validity criteria when VOTable changed
COOSYS/@system to being controlled by this vocabulary), nobody rememberd
what geo_app might have meant. It recently turned out that it must
have been “Geocentric Apparent”, a frame used primarly in geodesy and
solar system science. It is documented to some extent in
<a class="moz-txt-link-freetext" href="https://starlink.eao.hawaii.edu/docs/sun211.htx/sun211ss454.html">https://starlink.eao.hawaii.edu/docs/sun211.htx/sun211ss454.html</a>.
This VEP replaces label, description, and relationship for the concept.
At this point, this is a deprecated concept, and since it is mixing
refposition and frame ("geocentre" vs. "apparent positions at equator of
epoch"), we quite certainly won't want anyone to use it. We still want
to update the concept metadata and hence ask for exemption from the
“must be used somewhere” rule.
<a class="moz-txt-link-freetext" href="https://github.com/ivoa-std/VEPs/blob/d79185c3a36bf704b84fe439ba4513b8cfb5304e/VEP-017.txt">https://github.com/ivoa-std/VEPs/blob/d79185c3a36bf704b84fe439ba4513b8cfb5304e/VEP-017.txt</a>
Comments? Thoughts? I'd say this is reasonably harmless, and so I'd
suggest to bring this to the TCG in about two weeks if nobody
protests here.
Thanks,
Markus
</pre>
</blockquote>
<p><br>
</p>
<pre class="moz-signature" cols="72">--
((( Sebastien Derriere <a class="moz-txt-link-abbreviated" href="mailto:sebastien.derriere@astro.unistra.fr">sebastien.derriere@astro.unistra.fr</a>
(. .) Observatoire de Strasbourg Phone +33 (0) 368 852 444
(( v )) 11, rue de l'universite Telefax +33 (0) 368 852 432
---m-m--- F-67000 Strasbourg France
</pre>
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