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<H2><A NAME="SECTION00058000000000000000">
Parallax and Radial Velocity</A>
</H2>
For the utmost accuracy and the nearest stars, allowance can
be made for <I>annual parallax</I> and for the effects of perspective
on the proper motion.
<P>
Parallax is appreciable only for nearby stars; even
the nearest, Proxima Centauri, is displaced from its average
position by less than
an arcsecond as the Earth revolves in its orbit.
<P>
For stars with a known parallax, knowledge of the radial velocity
allows the proper motion to be expressed as an actual space
motion in 3 dimensions. The proper motion is,
in fact, a snapshot of the transverse component of the
space motion, and in the case of nearby stars will
change with time due to perspective.
<P>
SLALIB does not provide facilities for handling parallax
and radial-velocity on their own, but their contribution is
allowed for in such routines as
sla_PM,
sla_MAP
and
sla_FK425.
Catalogue mean
places do not include the effects of parallax and are therefore
<I>barycentric</I>; when pointing telescopes <I>etc.</I> it is
usually most efficient to apply the slowly-changing
parallax correction to the mean place of the target early on
and to work with the <I>geocentric</I> mean place. This latter
approach is implied in Figure 1.
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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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