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<H2><A NAME="SECTION000517000000000000000">
Geocentric Coordinates</A>
</H2>
The location of the observer on the Earth is significant in a
number of ways. The most obvious, of course, is the effect of latitude
on the observed <IMG WIDTH="66" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img28.gif"
ALT="$[\,Az,El~]$"> of a star. Less obvious is the need to
allow for geocentric parallax when finding the Moon with a
telescope (and when doing high-precision work involving the
Sun or planets), and the need to correct observed radial
velocities and apparent pulsar periods for the effects
of the Earth's rotation.
<P>
The SLALIB routine
sla_OBS
supplies details of groundbased observatories from an internal
list. This is useful when writing applications that apply to
more than one observatory; the user can enter a brief name,
or browse through a list, and be spared the trouble of typing
in the full latitude, longitude <I>etc</I>. The following
Fortran code returns the full name, longitude and latitude
of a specified observatory:
<P><PRE>
CHARACTER IDENT*10,NAME*40
DOUBLE PRECISION W,P,H
:
CALL sla_OBS(0,IDENT,NAME,W,P,H)
IF (NAME.EQ.'?') ... (not recognized)
</PRE>
<P>(Beware of the longitude sign convention, which is west +ve
for historical reasons.) The following lists all
the supported observatories:
<P><PRE>
:
INTEGER N
:
N=1
NAME=' '
DO WHILE (NAME.NE.'?')
CALL sla_OBS(N,IDENT,NAME,W,P,H)
IF (NAME.NE.'?') THEN
WRITE (*,'(1X,I3,4X,A,4X,A)') N,IDENT,NAME
N=N+1
END IF
END DO
</PRE>
<P>
The routine
sla_GEOC
converts a <I>geodetic latitude</I>
(one referred to the local horizon) to a geocentric position,
taking into account the Earth's oblateness and also the height
above sea level of the observer. The results are expressed in
vector form, namely as the distance of the observer from
the spin axis and equator respectively. The <I>geocentric
latitude</I> can be found be evaluating ATAN2 of the
two numbers. A full 3-D vector description of the position
and velocity of the observer is available through the routine
sla_PVOBS.
For a specified geodetic latitude, height above
sea level, and local sidereal time,
sla_PVOBS
generates a 6-element vector containing the position and
velocity with respect to the true equator and equinox of
date (<I>i.e.</I> compatible with apparent <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$">). For
some applications it will be necessary to convert to a
mean <IMG WIDTH="42" HEIGHT="29" ALIGN="MIDDLE" BORDER="0"
SRC="img3.gif"
ALT="$[\,\alpha,\delta\,]$"> frame (notably FK5, J2000) by multiplying
elements 1-3 and 4-6 respectively with the appropriate
precession matrix. (In theory an additional correction to the
velocity vector is needed to allow for differential precession,
but this correction is always negligible.)
<P>
See also the discussion of the routine
sla_RVEROT,
later.
<P>
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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